UNITED STATES DEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY

GENESIS OF A ZONED GRANITE STOCK, SEWARD PENINSULA, ALASKA

BY

Travis Hudson

Th i s r e p o r t i s p r e l i m i n a r y and has n o t been e d i t e d o r rev iewed f o r conforrni ty w i t h Geo log i ca l Survey s tandards

TABLE OF CONTENTS

Page

. . . . . . . . . . . . . . . . . . . . . . . . . LIST OF FIGURES . " v i

. . . . . . . . . . . . . . . . . . . . . . . . L I S T OF PLATES i x

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . x

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . x i

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . x i v

Chapter

. . . . . . . . . . . . . . . . . . . . . I . INTRODUCTION

. . . . . . . . . . . . . . . . . . . . . . . L o c a t i o n Methods o f Study . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . Phys i ca l S e t t i n g . . . . . . . . . . . . . . . . . . . . Prev ious Work

. . . . . . . . . . . . . . . . . . . . . . . GEOLOGY

. . . . . . . . . . . . . . . . . . Metamorphic Rocks . . Gne i ss i c 'Rocks . . . . . . . . . . . . . . . . . . . .

Paragneiss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Orthogneiss

Graphi ti c Metasedi ments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Banded H o r n f e l s

Dark-Gray H o r n f e l s . . . . . . . . . . . . . . . P h y l l i t i c S c h i s t . . . . . . . . . . . . . . . . M e t a s i l t i t e . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . " Mica-Quar tz S c h i s t . . . . . . . . C h l o r i te-Muscovi t e - Q u a r t z S c h i s t s

. . . . . . . . . . . . . . . . . . . Carbonate Rocks Carbonate Rocks W i t h i n G r a p h i t i c Metasediments . . Marb le i n I s o l a t e d Masses . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . Thermal Metamorphism . . . . . . . . . . . . . . . . . . . . . . S t r u c t u r e

. . . . . . . . . . . . . . . . . . . . . . . Folds . . . . . . . . . . . . . . . . . . . . . . . F a u l t s

Age o f S t r u c t u r a l Fea tu res . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IgneousRocks

. . . . . . . . . . . . . . . . The Grani t e Complex Age . . . . . . * . . . . . . . . . . . . . . . Contac t R e l a t i o n s . . . . . . . . . . . . . . . .

iii

Chapter Page

. . . . . . . . . . . . . . Depth of Emplacement Peripheral Di kes . . . . . . . . . . . . . . . . R o c k s o f Z o n e l . . . . . . . . . . . . . . . .

Faci es 1A-- Medi um-grained Equi granul a r Granite . . . . . . . . . . . . . . . . . . .

Facies 1 B-.Coarse-grained, Approxi . . . . . . . . . mately Equigranul a r Granite

Facies 1C.. Semiporphyritic Granite . . . . . Rocks of Zone 2 . . . . . . . . . . . . . . . .

. . . . . . . . Faci es 2-. Porphyri t i c Grani t e Rocks of Zone 3 . . . . . . . . . . . . . . . .

Facies 3A-- Se r i a t e Granite . . . . . . . . . Facies 39--Composit e-textured Granite . . . .

Rocks of Zone 4 . . . . . . . . . . . . . . . . Faci es 4A-- F i ne- l o Medi urn-grained . . . . . . . . . . . . Equi granul a r Granite Faci es 40-. Leucocrati c Grani t e . . . . . . .

Microscopic Textures . . . . . . . . . . . . . . Subsolidus Albite . . . . . . . . . . . . . . Myrmekite . . . . . . . . . . . . . . . . . . Fringed B io t i t e . . . . . . . . . . . . . . . Graphic Intergrowths . . . . . . . . . . . .

. . . . . . . . . . . . . Megascopic Structures Fol i a t i ons . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . Dikes . . . . . . . . . . . . . . . . . . Cavi t i e s

. . . . . . . . . . . . . . . . . . Inclusions . . . . . . . . . . . . . . . . . . . Jo in t s

Deuteric Features . . . . . . . . . . . . . . . . . . . . . . . . . . . Pl ag i ocl ase Compos i t i on

Modal Compos i ti on . . . . . . . . . . . . . . . Variation of Modal Averages . . . . . . . . . . Chemical Composition . . . . . . . . . . . . . .

. . . . . . . . . . Variation of Major Oxides . . . . . . . . . Variation of Trace Elements

. . . Variation of Trace Elements i n B io t i t e Trace Elements in Pocket Muscovite . . . . . Comparison w i t h Average Composi t ions of Granite and B io t i t e . . . . . . . . . . .

. . . . . . . . . . . Crystal Popul a t ions : . . . . . . . . . . . . . . Geometry o f the Complex

. . . . . . . . . . . . . . . . External Form . . . . . Crys ta l l i za t ion of the Granite Complex

. . . . Stage 1-- Crys ta l l i za t ion of Facies 1A Stage 2.. Crystal1 i zation of Facies 1B .

. . . . . . . . . . . . . . . . . l C , and 2 Stage 3-. Emplacement and Crysta l l iza t ion

o f Facies 3A . . . . . . . . . . . . . . . . . Stage 4-- Emplacement and Crystal l i zat ion

. . . . . . . . . . . . . . . . of Facies 3B Stage 5.- Crystal1 i z a t i on of Fac ies 4A

. . . . . . . . . . . . . . . . . . . and 4B

Chapter Page

. . . . . . . . . . A1 t e r n a t i ve I n t e r p r e t a t i o n s . . . . . . . . . . . . . . . . . . . Econorni c Geology . . . . . . . . . . . . . . . . Geochemical Surveys . . . . . . . . . . . . . . . A l l u v i a l M a t e r i a l s . . . . . . . . . . . . . . . Bedrock M a t e r i a l s . . . . . . . . . . S p a t i a l and S t r u c t u r a l Cont ro ls . . . . S i m i l a r i t i e s t o Other T i r . - M i n e r a l i z e d Areas . . . . . . . . . . . . . O r i g i n of M inera l l z a t i on

I11 IMPLICATIONS OF THE STUDY . . . . . . . . . . . . . . 165 . C r y s t a l l i z a t i o n of Zoned G r a n i t i c Complexes . . . . . 165 O r i g i n o f T i n-Beari ng Gran i tes 167 . . . . . . . . . . . . The Concept o f Metal l o g e n e t i c Prov inces . . . . . . . 169 T i m i ng o f fracturi ng . F i n a l Crystal 1 i z a t i on.

and M i n e r a l i za ti on . . . . . . . . . . . . . . . . . . 172 Importance o f Depth i n Environments of T i n . . . . . . . . . . . . . . . . . . . M i n e r a l i z a t i o n 176

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

LIST OF FIGURES

F i gure

1.

Page

Loca t i on of t h e Serpen t ine Hot Spr ings area, Seward Pen insu la , Alaska . . . . . . . . . . . . . . . 2

V i e w o f r o l l i n g tundra-mant led h i 11 s west o f Se rpen t i ne Hot Spr ings . . . . . . . . . . . . . . . . 5

View l o o k i n g e a s t across t h e sou thern p a r t of t h e g r a n i t e complex . . . . . . . . . . . . . . . . . . . . . 7

Hand specimens o f b i o t i t e -p l ag ioc lase -qua r t z paragnei ss and g r a n i t e o r t hogne i ss . . . . . . . . . . 12

Outcrop of s t r o n g l y deformed c h l o r i te-muscovi t e - q u a r t z s c h i s t . . . . . . . . . . . . . . . . . . . . . 2 5

Photomicrograph showing complex de fo rma t i on f e a t u r e s i n chlar i te-muscovi t e - q u a r t z s c h i s t . . . . . 27

D i s t r i b u t i o n o f :.T n -g ran i t e p l u tons , Seward Pen insu la , A laska . . . . . . . . . . . . . . . . . . . . P l o t o f modal P I : Q : O r r a t i o s f o r samples c o l l e c t e d f rom a l l p a r t s o-" t h e g r a n i t e complex . . . . . . . . . General i zed s e c t i o n across eas te rn con t a c t of t h e g r a n i t e compqex i 17 us t r a t i ng the nega t i ve topo- g raph i c express ion o f t h e g r a n i t e - c o u n t r y r o c k . . . . . . . . . . . . . . . . . . . . . . . . c o n t a c t

P l o t o f no rma t i ve Q:Ab:Or:SiOn r a t i o s f o r . . . . . . . . . . . . . . . . . . . . f a c i e s 48 rocks

Represen ta t i ve specimens of t h e t e x t - u r a l f a c i es i n z o n e l . . . . . . . . . . . . . . . . . . . . . . . Represen ta t i ve specimens of zone 2 . . . . . . . . . . Rep resen ta t i ve specimens o f t e x t u r a l f ac i es 3A . . . . Represen ta t i ve specimens o f t e x t u r a l f a c i e s 3B . . . . Exposed c o n t a c t between f ac i es 3 A and f a c i e s 3B . . . . Represen ta t i ve specimens of t e x t u r a l f a c i es 4A

. . . . . . . . . . . . . . . . . . . . . . . . a n d 4 E

Page

7 0

7 3

7 6

. . . . . . . . . Photomicrographs of subsol idus a1 bi te

Photomicrograph of i n t e r s t i t i a l myrmeki t e . . . . . . . Photomicrograph of a fringed b i o t i t e crystal . , . . , Photomicrograph of a g raph i c i ntergrowth of quartz in potassi um-feldspar . . . . . . . . . . . . .

. . . . . . . . Mi neral segregations in faci es 3A rocks

Pi sconti nuous and i nterconnected apl i t e di kes . . . . . . . . . . . . . . . . . . i n f ac i es 3A rocks

. . . . . . Composite, 3 f t wide, aplite-pegmatite dike

Quartz-feldspar pegmati t i c pod in facies 3A rocks . . . Vuggy replacement selvage along walls of 2 f t wide pocket in facies 3A rocks . . . . . . . . . . . . . . .

. . . . Pipelike swarm of inclusions in facies 2 rocks

P l agi ocl ase composi tions in the textural f a c i es . . . . o f the granite complex as determined -optical.ly

Variation o f modal averages through the facies sequence . . . . . . . . . . . . . . . . . . . . . . . Variation o f major oxides with percent S. iOz . . . . . . Variation of major oxides through the facies sequence . . . . . . . . . . . . . . . . . . . . . . . Variation o f selected whole-rock t race e l ecmts through the faci es sequence . . . . . . . . . . . . . . Variation of selected t race elements in b io t i t e through the faci es sequence . . . . . . . . . . . . . . . Schematic i l l u s t r a t i o n showing possible relat ions, within the granite stock soon a f t e r emplacement . . . . Schematic i l l u s t r a t ion showing p o s s i b l e relat ions within the granite stock near the end of stage 2 crystal 7 i zatf an . . . . . . . . . . . . . . . . . . . .

35. Schematic i 1 lustrat ion showing possible relat ions within the granite stock a f t e r stage 3 crystal- l i za t ion . . . . . . . . . . . . . . . . . . . . . . .

F i gure Page

36. Schendtic i l l u s t r a t i o n showing possible re la t ions within the grani te stock a f t e r stage 4 c rys ta l - l i z a t i on . . . . . . . . . . . . . . . . . . . . . . . 148

37. Schematic i l l u s t r a t i o n showing possible r e l a t i ons - . w i t h i n the grani t e stock a f t e r complete crys t a l l i z a t i on . . . . . . . . . . . . . . . . . . . . 152

LIST OF PLATES

Pla te

1. Geolog ic map and g e n e r a l i z e d c ross s e c t i o n of t h e Serpentine Hot Springs area, Seward Peninsula, Alaska . . i n pocke t

Table

L IST OF TABLES

Page

. 1. A n a l y t i c a l data f o r K - A r a g e d e t e r m i n a t i o n s . . . . . . 4 3

2. Whol e-rock major ox ide composit ions o f se lec ted samples o f t he g r a n i t e complex i n we igh t percent . . . . 102

3. Whol e-rock t race-e l ement composi t ion o f se lec ted g r a n i t e samples i n ppm . . . . . . . . . . . . 103

4. Resul ts o f semiquanti t a t i ve spectrographic ana lys i s of b i o t i t e separates from t h e g r a n i t e

. . . . . . . . . . . . . . . . . . . . . . . . complex 104

5. Resul ts of quant i t a t i v e spec t rograph ic ana1,yses f o r se lec ted t r a c e elements i n b i o t i t e separates from the g r a n i t e complex . . . . . . . . . . . . . . . . 105

6. Whol e-rock t race -e l ement composi ti on o f , fi ve se lec ted samples o f t h e g r a n i t e complex . . . . . . . . 106

7. Average and ca l cu la ted bul k ma j o r - o x i d e composi t ion o f t h e g r a n i t e comp1.e~ and o the r g r a n i t e averages i n we igh t percent . . . . . . . . . . . 120

8. Average t race-e lemer t composit ion of t h e g r a n i t e complex and of low Ca g r a n i t e s i n ppm . . . . . 122

9 , Average and range o f t r a c e element abundances i n b i o t i t e s f rom some f e l s i c i n t r u s i v e rocks i n the western Un i ted S t a t e s and f rom t h e Serpent ine Hot Springs area . . . . . . . . . . . . . . 123

10. Dimensions and c a l c u l a t e d volumes o f t he g r a n i t e complex based on a c y l i n d r i ca1 she-11 model f o r z o n e s l t h r o u g h 4 . . . . . . . . . . . . . . . . . . . . 133

11, Resul ts o f semiquanti t a t i v e spectrographic analyses o f composite grab samples of s o i 1 , a1 t e r e d rock, and gossan fragments c o l l e c t e d a long f a u l t zone con t a i n i ng fi ne-gra i ned d i ke . . . . . . . . 160

ABSTRACT

A composite epi zonal stock of bi o t i t e grani t e has intruded a d i verse

assemblage of metamorphic rocks in the Serpentine Hot Springs area of

north-central Seward Peninsula, A1 aska. The metarnorphi c rocks i ncl ude

amphibolite-facies orthogneiss and paragneiss, greenschist-facies

f i ne-grai ned s i 1 i ceous and graphitic metasediments , and a variety of

carbonate rocks. Lithologic units within the metamorphic terrane

trend general ly north-northeast and dip moderately toward the southeast.

Thrust fau l t s locally juxtapose l i tho1 ogi c units in the metamorphic

assemblage, and normal faul t s displace both the metamorphic rocks and

some parts of the granite stock, The gneisses and graphitic metasediments

are believed t o be l a t e Precambrian in age, b u t the carbonate rocks

are in par t Paleozoic . Dating by the potassi um-argon method i ndi cates

tha t the granite stock i s L?te Cretaceous.

The stock has sharp discordant contacts, beyond which i s a well-

developed thermal aureol e with rocks of hornblende hornfels faci es.

The average mode of the granite i s 29 percent plagioclase, 31 percent

quartz, 36 percent K-feldspar, and 4 percent b io t i t e . Accessory minerals

.include apa t i t e , magneti te , sphene, a l l ani t e , and zircon. ~a ' te -s tage

or deuteric minerals include muscavi t e , f l u o r i t e , tourmal ine, quartz ,

and a l b i t e .

The stock i s a zoned complex containing rocks wi th several textural

facies that are present in four par t ly concentric zones. Zone 1 i s a.

discontinuous border uni t , containing f ine- to coarse-grained b io t i t e

granite, that grades inward intq zone 2 . Zone 2 consists of porphyritic

b io t i t e granite with oriented phenocrysts of pi nkish-gray microcl i ne in

a coarse-grai ned equi granul ar groundmass of plagiocl ase, quartz, and

b io t i t e . I t i s i n sharp, concordant t o discordant contact w i t h rocks of

zone 3 . Zone 3 consists of seriate-textured b io t i t e granite tha t has

been intruded by bodies of porphyri t i c bi o t i t e grani t e contai ni ng

phenocrysts of pl agioclase, K-feldspar, quartz, and b io t i t e i n an apli t i c

groundmass. Flow structures , pegmati t e and apli t e segregations, and

miarol i t i c cavi t ies are common in the seriate-textured grani t e . Zone 4,

which forms the central part of the complex, consists o f fine- t o

medi urn-grai ned bi o t i t e granite and 1 ocal l y devel oped 1 eucograni t e .

Small miarol i t ic cavi t ies a re common hi th in i t ,

Eight textural facies have been defined within the complex, and

mineralogic, petrographic, modal, and chemical variations a re broadly

systematic within the facies sequence. Study o f these variations shows

tha t the gradational facies of zones 1 and 2 systematl'cally s h i f t toriard

.more mafic compositions inward within t h e complex. Seriate-textured

rocks o f zone 3 a re simi 1ar in composition to those of zone 2 , b u t

porphyritic rocks of zone 3 and rocks of zone 4 mark s h i f t s t o more

f e l s i c composi tions . These late-crystal 1 i zi ng f e l s i c rocks are products

of an in t e r io r resi.dua1 magma system.. This system was enriched in water

and cer tain t race elements including t i n , lithium, niobium, lead, and

zinc. The complex a s a whole has higher concentrations o f these elements

than many other granites . The nature of th i s geochemi cal speci a1 i zation

i s par t icular ly well demonstrated by the trace-element composition of

b io t i te .

a The c ry s t a l l i z a t i on h i s to ry o f t he pluton was complex. The

ava i lab le d a t a suggest t h a t this his tory could have included: ( 1 ) chi 11 i n g

and metasomatic a1 t e r a t i on adjacent t o the contact , ( 2 ) i n - s i b

crys ta l 1 i zation i n several marginal f a c i es accompanied by some t r a n s f e r

of residual const i tuents toward i n t e r i o r parts of the pluton, ( 3 ) s l i g h t

upward displacement of magma t h a t was subjacent t o the c ry s t a l l i z ed

wal ls , accompanied by disequi l i bt.ium c rys t a l l i z a t i on and local vapor

s a tu r a t i on , (4) upward displacement of par t of the residual water-rich

i n t e r i o r magma, accompanied by rapid loss of a separated vapor phase,

and ( 5 ) displacement of t h e margins o f t h e pluton by normal f a u l t s ,

accompanied by loss of an exsolved vapor phase from t h e res idual core

of the pluton.

The vapor phase t h a t escaped along crosscut t ing f a u l t s during the

@ end s tages o f crys ta l 1 i zation produced t i n-mineral i zed zones i n country

rocks adjacent t o the grani te complex. The general geol ogi c envi ronmen t

of the area i s s imi la r t o t h a t of other tin-minerali zed areas, and t he

r e s u l t s o f the study a r e appl icable t o an understanding o f tin-deposi t

genesis as we41 as t o problems of gran i t e o r ig in and crys ta l 1 i z a t i on.

ACKNOWLEDGMENTS

This work was suggested and supported by C, L, Sainsbury, who

prov ided h e l p both as t h e p r o j e c t c h i e f o f t h e U. S. Geological Survey 's

Seward Peninsula P r o j e c t and as a f r i e n d d u r i n g four summers o f geo log ic

i n v e s t i g a t i o n s on the Seward Peninsula, Colleagues o f t h e U. S.

Geol ogi cal Survey who have prov ided mater i a1 and persona1 support a r e

Henry Berg, George Gryc, George P la fker , and Bruce Reed. Lowel l

Kohni tz photographed t h e hand specimens and Norman Prime ass i s ted i n

t he t a k i n g of photomicrographs.

Richard H. Jahns , Konrad B . Krauskopf , Robert C. Compton, and

W i l l i a m C. Lu th o f S tan fo rd U n i v e r s i t y gave h e l p f u l sxggest ions and

:d iscussions du r ing var ious p a r t s o f t h i s s tudy . Jahns v i s i t e d the

Serpent ine Hot Springs area i n 1970, and he and Krauskopf have been

major sources o F guidance and d i r e c t i o n throughout t he i n v e s t i g a t i o n ,

Eugene Foord, John Lufk in, David Mustar t , Jef f rey S t e i n e r , and James

Whi tney p a r t i c i p a t e d i n many he1 pfu l d iscussions concerning prpblems

of o r e genesis and magmatic processes.

Dur ing the summer of 1968, C. L . Sai nsbury, Reuben Kachadoorian,

and Thomas Richards p a r t i c i p a t e d in' t h e f i e l d work as p a r t of t h e

reg iona l i nves t i ga t i ons o f the Seward Peni nsula P r o j e c t . Cl i f f o r d

Weyiounna was an e x c e l l e n t f i e l d a s s i s t a n t du r ing t h e 1969 season and

d i d much o f t h e systemat ic sampling o f t h e g r a n i t e complex. I n 1970,

f i e l d ass is tance was prov ided by Tom Hayward who d i d most o f t h e

ou tc rop p o i n t count ing. The 1969 f i e l d work was p a r t i a l l y funded by

a a Penrose Bequest Research Grant from the Geological Society of

America.

I am thankful to many friends on the Seward Peninsula for the i r

help during various stages of the f i e ld work. Special thanks go t o

the Tweet fami l i e s of Tel ler who, while operating t h e i r mine near

Taylor on the Kougarok River, were my closest neighbors. They made

substantial e f fo r t s t o help re t r ieve and repair disabled track vehicles,

and thei r kindnesses and friendly hospital i t y w i 11 always be appreciated.

Finally, I am deeply g ra te fu l for t h e personal support qiven b v .

Christine Allen through several years of t h i s work and especially during

the sumer of 1969. Her contributions and sacr i f ices t o the study

f a r exceed what I can acknowledge here.

CHAPTER I

INTRODUCTION

The purpose of t h i s study was t o map and describe the geology o f

the Serpentine Hot Springs area of the Seward Peninsula, A1 aska, t o

out l ine the crystal l i r a t ion history of the granite complex within i t , and

t o discuss possible relationships between magmatic processes and the

origin of t i n mineralization in the area. Mapped 1 i thologic and s t ruc-

tural re1 a t i ons and the resu l t s of extensive laboratory investigations

have been integrated in the study. The findings and conclusions

probably are applicable t o several areas o f similar geology on the

Seward Peninsula, and a lso t o other areas characterized by the associa- • tion. of t i n mineralization with b i o t i t e grani te ,

Location

The Serpentine Hot Springs area, in the north-central part of , the

Seward Peninsula, Alaska ( f i g . I ) , i s a 70 sq mi area in the Bendeleben

0-6 and 0-5 quadrangles and 1 ies 120 m i nor th-nor the~s t of Nome, the

1 argest community on the peninsula. A maintained gravel road extends

from Nome t o points about 40 mi south of Serpentine Hot S p r i n g s , and

from th i s road the area can be reached by track vehicle o r l i gh t

a i r c r a f t T h e Arctic Circle i s 45 mi to the north and Bering S t r a i t ,

separating North America f r o m Siber ia , i s 98 mi to the west ( f i g . 1 ) .

Methods of Study

Much o f t h e f i e l d work was done from a base .camp e s t a b l i s h e d in

a l a r g e wood frame cabin a t Se rpen t ine Hot Sp r ings . A t r a c k v e h i c l e

served as a w b i l e camp f o r work i n o u t l y i n g p a r t s of t h e a r e a . The

geology was mapped by f o o t t r a v e r s e on topographic maps a t a s c a l e o f

1 :63,360 and l a r g e r , and r e p r e s e n t a t i v e samples o f rocks were col l e c t e d

throughout t h e a r e a . Many ou tc rop r e l a t i o n s h i ps were documented by

photographs. The f i e l d i n v e s t i g a t i o n s were nade du r ing t h e per iods

June 25 t o , J u l y 2 2 , 1968, June 16 t o August 2 3 , 1969, and August 10

t o August 2 7 , 1970. The 1968 work was p a r t of a reg iona l mapping p r o j e c t

wi th p r i n c i p a l emphases upon s t u d i e s and sampling o f minera l ized a reas

(Sainsbury and o t h e r s , 1970) , b u t dur ing t h i s t ime t h e margin o f the

g r a n i t e complex was a l s o o u t l i n e d . The major l i t ho log i , : u n i t s were

def ined , mapped, and sampled i n 1969, when sys t ema t i c sarnpl i ng of t he

g r a n i t e compl ex , on a 3/8-mi spac ing , a l s o has completed. I n 1970,

impor tan t s t r u c t u r a l f e a t u r e s wi th ~n t h e complex were s tud ied , and

a t r a n s p a r e n t p l a s t i c ove r l ay with g r i d p o i r t s spaced 1 114 i n . a p a r t

was used t o determine modal phcnocryst popula t ions i n severa l of t h e

rock types . The t r a n s p a r e n t g r id was taped t o f l a t ou tcrop su r f aces

fo r t h e counting o f more than 1000 po in t s a t each of 23 l o c a l i t i e s .

Thi n s e c t i o n s and hand specimens were sys t ema t i ca l l y exami ned and

descr ibed i n the 1 abora tory . Deta i led exami na t ion of grani t e specimens

was f a c i l i t a t e d by t h e s t a i n i n g of K-feldspar on sawed s l a b s (Norman,

1974) . The s t a i n e d s u r f a c e s were p o i n t counted wi th t r a n s p a r e n t p l a s t i c

g r i d ove r l ays ; maximum spac ing of po in t s was 2 . 5 mm, a n d 800 t o more

than 1000 po in t s were counted on each s l a b , P l a g i o c l a s e compositions

were determined op t i ca l l y by t h e a-normal method w i t h reference t o curves a published by Deer, Howie, and Zussman (1966, p. 3 3 3 ) . Some mineral

i den t i f i c a t i ons were made by means of X-ray d i f f rac t ion powder pa t te rns ,

and compositions and s t ruc tura l s t a t e s were determined for a few

K-feldspars according t o the method of Wright (1968). Mineral separates

were made through use of magnetic separa tors , heavy l i qu id s , and polished

metal v ibrat ing tab les .

Chemical data were obtained from the Branch o f Analytical Labora-

t o r i e s , U. S . Geological Survey. The major oxide compositions of whole-

rock samples were determined by procedures described by Shapiro and

Brannock (1 962), and semiquanti t a t i ve t r a c e element analyses of whole-

rock samples and mica separates were made chief ly by procedures des-

c r i bed by . Meyers + and others (1 961 ) . Normati ve mi neral composi ti ons

were calculated by computer according t o the C.I.P.W. method, Potassium-

argon dating of two b i o t i t e separates was done by techniques described

i n Dal rympl e and Lanphere (1 969).

Physical Se t t ing

The Serpentine H o t Springs area i s located near the head of the

Serpentine River, a stream with a t i g h t l y meandering, serpent-1 i ke

channel. This par t of the Seward Peninsula i s i n the Intermontane

Plateau of A1 aska (Wahrhaftig, 1965) and i s characterized by a land-

scape with elongate, rounded ridges and hi 11s t h a t a re i n l a rge par t

mantled by tundra ( f i g . 2 ) . Where surface materials a r e exposed,

they a r e dominantly f r o s t r i ven rubble. Bedrock outcrops a re scarce .

The study area s t radd les a north-trending ridge t h a t reaches an

elevation of 2592 f t w i t h i n the area and a maximum of 2720 ft a t

Figure 2. View o f rolling tundra-mantled h i l l s west of Serpent ine Hot Springs.

Midnight Mountain 3 mi to the south. These a re the highest points

,for many mi l e s , and they contribute t o an increased variabi l i ty of

local r e l i e f and an unusual degree of exposure of both surface materials

and bedrock ( f i g . 3 ) .

In areas underlain by grani te , the ridges display spectacular

pinnacle outcrop f o r m . I n other areas , the summits and higher slopes

have we1 1-developed a1 t iplanation terraces. Nonterraced slopes are

convex except where ta lus rubble i s abundant. The uplands are cut

by ravines and sharp V-shaped avenues of drainage on steeper slopes.

These merge a t lower elevations into small valleys occupied by streams.

The area has an annual rainfal l of 7 t o 15 in. and receives 30 t o

34 in . of snow (Hopki ns and others , 1955). Permafrost i s essenti a1 ly

continuous except 1 ocal ly a1 ong larger streams, in the vi cini ty of hot

springs, and in places where the sur f ic ia l cover i s thin or absent,

lacks vegetation, or is we1 7 drained. The vegetation i s dominated by

herbaceous tundra (Sigafoos , 1958), a1 t h o u g h shrub tundra and thick,

brushy wi 11 ows are locally abundant along streams.

Weatheri ng , together w i t h di scon t i nui t i e s i n bedrock types, has

produced some land forms specially charac ter i s t ic of the area and

environment. These include the negati ve topographic expression of the

granite margi n, pi nnacl e l i ke outcrops of granite , and ta l us and sol i -

fluction features. The t race of the contact between grani te and country

rocks l i e s on slopes tha t face inward toward the central parts of the

grani te complex. The re la t ive ly r a p i d r a t e of granite weathering and

the more r e s i s t an t nature of the surrounding zone of hornfels evidently

have combined t o produce t h i s relationship.

Figure 3 . V i e w looking east across the southern part o f t h e granite complex, showing a1 t i p 1 anation terraces developed :n metamorphic rocks beyond. Distance t o the fa r ri dgeli ne i s 6 mi .

The prominent outcrops of granite r e f l ec t f ros t action and

preferential weathering along jo in t s e t s . Some of the outcrops a re

tens of f ee t h i g h and are v is ib le for many miles as highly anomalous

i rregul a r i t i e s in the 1 andscape. These outcrops and. thei r exfo1.i ation

features were f i r s t described and photographed by Coll i e r (1 902).

Talus slopes a re common along margins of t h e stepper sided ridges

and a l t i pl anation terraces. In areas under1 ai n by metamorphic rocks,

these slopes a r e the places where outcrops a re most l ikely to be found;

elsewhere the bedrock i s effectively mantled by rubble. In places

where f ine r grai ned material occurs on the steeper s l opes, well -developed

so l i f l u c t i o n lobes are present.

The s t r e a m , the larger of which are braided, are frozen during

the winter except a t places where they are warmed by hot springs . Thermal springs occur in al luvial materials of lower Hot Springs Creek

a t two local i t i e s (pl . 1 ) . The spri n y waters a re moderately mineral i z e d

and reach temperatures of 171'~. A small undrai ned thaw lake, located

0.5 mi northeast of Serpentine Hot Springs, i s the only standing body

of water in the area.

A1 1 uvial deposits a r e f a i r l y exten.si ve along lower Hot Springs

Creek, especi a1 ly a t i t s juncture with the major t r ibutary entering

from the eas t where the flood plain i s bounded by banks 10 t o 20 f t

high. Terrace gravels are exposed along these banks, which gradually

become lower in an upstream direct ion.

Previous Work

Earlier f i e l d s tudies on the Seward Peninsula were general ly of a

reconnai ssance nature or were d i rected t o speci fi c mineral deposi t s .

Those reports pertinent t o the Serpentine Hot Springs area include

overviews of the regional geology by Col 1 i e r (Col 1 i e r and others ,

1908) and Sai nsbury (1975) , discussions of t i n occurrencqs throughout

the peninsula by Knopf (1908) and Steidtman and Cathcart (1922), and a

detailed description o f the geology of the central York Mountains by

Sai nsbury (1969). Results of an aeromagnetic survey have been reported

by Johnson and Sainsbury (1974), and a regional Bouguer anomaly map has

been published by Barnese(l971) . Regional investigations 0.: grani t ic

rocks in western Alaska have been made by Mi l l e r (1970) and by Csej tey

and others (1971 ) .

A br ief v i s i t to the Serpentine Hot Springs area was described

by Coll ier (1902), who 1 a ter drew attention to the potential there

for t i n mineralization (1904a) and mentioned an unconfirmed report of

lode t i n in the area (1904b). Knopf (1908) confirmed the presence o f

placer c a s s i t e r i t e on Hurnbolt Creek, Moxham and West (1953) vis i ted

the area in search of deposits of radioactive minerals, Sainsbury and

others (1968) described placer concentrates containing c a s s i t e r i t e

,'ram Humbol t Creek, and Sai nsbury and others (1970) provided a pre-

liminary description of the geo l~gy and the resu l t s of a more detailed

investigation of mineral ized. areas. Some .data on the .thermal springs

in the area were included in a report by Mi 11 e r a n d others (1 975 ) .

CHAPTER I1

GEOLOGY

The Serpentine Hot Springs area contains a diverse assemblage

of metamorphic rocks tha t a re in part intruded by an epizonal and

composi t e stock of b i o t i t e grani te. The metamorphic rocks comprise

three d i f fe rent general assemblages, each of which can be divided into

d i s t inc t 1 i thologic uni ts . The stock a1 so comprises d i f fe r ing uni t s ,

the mapped dis t r ibut ion of which reveal s considerable complexity within

the igneous complex and provides important data concern ing structural

relationships i n the surrounding terrane.

Despite the poor exposures, i t has been reasonably well established

that thrust faul t ing, which antedated intrusion of the granite s tock,

was responsible for the juxtaposition of cer ta i n l i thologic units ,

and tha t normal fau l t ing , i n part a t l e a s t , followed the major part of

c rys ta l l iza t ion of the stock. Tin and related mineral ization was

s t ruc tura l ly controlled by normal f au l t s southeast of the grani te

complex. Cass i te r i te occurs with gold in the placer deposits of Hum-

bol t creek, which has i ts headwaters in the mineralized area.

Metamorphic Rocks

The bedrock surrounding the grani te stock consists enti rely o f

metamorphic rocks. These rocks a re l a t e Precambrian and Paleozoic

in age, of greenschist and amphiboli t e facies regional metamorphic

grade, and together comprise three general l i tho logic assemblages:

(1 ) gnei ssi 'c rocks, ( 2 ) graphi t i c metasediments , and ( 3 ) carbonate

rocks. Gneissic rocks extend northward from a segment o f the granite

contact, graphi t i c metasediments a re widespread and a1 most completely

surrpund the pluton, and carbonate rocks occur local ly within the

graphi t ic metasediments and as d iscre te larger masses in the eastern

map area.

Each of the general 1 i thol ogic assemblages can be subdivided in to

two o r more d i s t i n c t l i tho logic uni ts . These units have been mapped

( pl . 1 ) , b u t poor bedrock exposure comnonly prevents a cl ear under-

standing of the nature of the contacts between them. The d is t r ibut ion

of various 1 i tho1 ogi es probably ref1 ec ts original sedimentary 1 ayeri ng ,

b u t the s t ructural and tectonic history of the area i s so complex t h a t

unrecognized s t ructural complications a re probably present. Adjacent

t o the grani t e stock , the regional ly metamorphosed rocks have been

recrystal 1 i zed i n a we1 1 -developed thermal aureol e .

Gneissic Rocks

The gneissic rocks, which extend northward from the north-central

grani t e contact, include both paragnei ss and orthognei 5 s . The two

1 i thologies can be readi ly distinguished ( f i g . 4 ) , and t h e i r separate

d i s t r i butions have been mapped. These amphi bol i te-faci es rocks a re

textural ly and mineralogically the highest grade regionally meta-

morphosed rocks of the area. Simi l a r rocks, bioti te-rich paragneiss

and g ran i t i c orthogneiss, have been found elsewhere on the Seward

Peninsula only in the heart of the Kigluai k Mountains, 76 m i southeast

of the Serpentine Hot Spri ngs area (Sai ns bury, 1972) . The gnei sses

in the Kigluaik Mountains have been dated radiometrically as l a t e

F i g u r e 4 . H?nd specimens o f b i o t i t e - p l a g i o c l a s e - q u a r t z paragnei ss ( r i g h t ) and g r a n i t e o r t h o g n e i ss ( 1 e f t ) .

Precambrian (730 m.y.) ; a s imilar age i s inferred for the gneisses

of the Serpentine Hot Springs area.

The gneisses do not crop out , b u t the two kinds a re not extensively

intermixed in the surface rubble and the location of the contact Getween

them i s reasonably d i s t inc t . The nature of t h i s contact has not been

determined, b u t in the Kigluai k Mountains orthogneiss i s intrusive into

paragneiss (Sainsbury , 1 9 7 2 ) . The sharp break in metamorphic grade

between the gnei sses (amphi bol i t e faci es ) and the immediately adjacent

graphi t i c metasedi ments (greenschi s t faci e s ) , combined with the apparent

absence of conglomerates and other expressions of a deposi tional con t a c t ,

suggests tha t the graphitic metasediments have been s t ruc tura l ly juxta-

posed against the gneisses.

- Paragneiss

Paragneiss i s found adjacent and t o the nor th of the stock.

I t i s mottled l i g h t brownish gray in color and f ine t o medium grained.

Thin concentrations o f f i ne-grai ned biot i t e are discontinuous a1 ong

the fo l ia t ion and o u t l i n ~ 1 ight-colored 1 enses and bands rich in quartz

o r plagioclase. The 1 enses and bands are discontinuous and ordinari ly

a re 1 ess than 0 . 7 5 cm in thickness. The essential minerals in order of

decreasing abundance are quartz, plagioclase, and b io t i t e . Minerals

present in mi nor amounts include ch lo r i t e , cal ci te , muscovi t e , green

amphibole, opaques , apa t i t e , and zircon-(?) . The thin sections tha t were examined a re from samples coll ected

near the granite margin and show e f f e c t s of thermal metamorphism.

Quartz i s unstrained and forms mosaics of anhedral , subequant grains

with simple boundaries. P l agioclase '(An,15) i s consistently f ine -

grained and anhedral . I t commonly i s twinned and forms complexly

interlocking aggregates w i t h simple t o sutured grain boundaries.

Comnly i t i s p o i k i l i t i c , and ovoid quartz i s the most abundant

inclusion. Biot i te i s pleochroic from straw yellow to dark brown,

vi rtual ly i ncl us ion f r ee , undeformed , and wi t h cleavages oriented

both parallel and oblique t o the t race of the f o l i a t i o r . The less

abundant minerals include the accessories and such secondary minerals

as ch lo r i t e , cal c i t e , muscovite, and some iron hydroxides. Chlorite

i s interleaved w i t h b i o t i t e , as i s muscovite to a l e s se r extent.

Muscovite more commonly occurs with c a l c i t e in fine-grained patchy

aggregates of general ly diversely oriented grains tha t replace plagio-

clase. Filmy, reddish iron hydroxides occupy fractures and grain boun-

dari es adjacent t o other secondary mi neral s . Commonly associated with' the paragneiss a re f i ne-grai ned, 1 i neated,

well-fol ia ted, dark greenish-gray amphi bol i tes that may represent

or iginal ly intercalated mafic vol canic rocks or dikes . Samples coll ected

from near the northern granite contact contain granular clouded sphene,

fine-grained granoblastic plagioclase, and green hornblenid.

Orthogneiss

Orthogneiss i s present in the northernmost part of the map area.

I t i s a muscov i t e -p lag ioc lase -qua r t z -m ic roc l i ne gneiss equivalent t o

granite i n modal composition. The plagiocl ase:quartz :K-feldspar r a t io

i s approximately 20:35:45. The color i s a mottled l i g h t orange to gray,

with the orange shades prima.ri ly due to iron hydroxide staining on

mineral surfaces. I t i s dominantly f ine grained except for some micro-

c l ine grains tha t exceed 1 mm i n maximum dimension. Muscovite and

l e s s e r b i o t i t e a r e s t r o n g l y o r i e n t e d w i t h i n segregat ions a long the

f o l i a t i o n . I n c o n t r a s t t o t h e paragneiss, t he f e l s i c minera ls do n o t

occur i n we l l -de f ined, composi t i o n a l l y segregated lenses and bands b u t

a r e more homogeneously d i s t r i buted.

I n t h i n s e c t i o n the q u a r t z g ra ins a re seen t o be anhedral and t h e

l a r g e r ones a r e e longate and o r i e n t e d p a r a l l e l t o t h e f o l i a t i o n .

Undul ose e x t i n c t i o n i s moderately we1 1-developed , and g r a i n boundaries

a r e simp1 e t o moderately complex. The p l ag ioc l ase (An,1 5 ) i s anhedral - and angular," w i t h p o o r l y developed t w i n n i n g t h a t i s hazy and d i s t o r t e d .

The e x t i n c t i o n i s i r r e g u l a r , and some l a r g e r g ra ins have ovo id qua r t z

i n c l u s i o n s . M i c r o c l i n e i s f i n e t o medi um gra ined, anhedral w i t h i n t e r -

s t i t i a l boundaries, and i n p a r t semi po i k i 1 i t i c , i n c l ud i ng bo th qua r t z

and p l a g i o c l ase. I\tuscovi t e forms f i n e , i n t e r g r a n u l a r g ra ins o r aggre-

gates and i s s t r o n g l y o r i e n t e d para1 l e l Lo t h e t r a c e o f t h e f o l i a t i o n .

. B i o t i t e i s p leochrb ic f rom l i g h t t a n t o dark green brown, and i s f i n e r

g ra ined and much l e s s abundant than t h e muscovite. Minera ls p resent

i n m i no r amounts i n c l u d e very fi ne-gra i ned g ranu la r opaque m a t e r i a l s

and subequant b u t anhedral z i r c o n . I r o n hydroxide i s common as hazy

f i l m s a long n e a r l y a l l f r a c t u r e s and g r a i n boundaries.

G r a p h i t i c Metasedi ments

Rocks o f t he g r a p h i t i c metasediment assemblage are t h e most abundant

o f t h e metamorphic rocks i n t h e map area. Many d i f f e r e n t l i t h o l o g i e s

a r e present b u t i n a general way the sequence i s cha rac te r i zed by i t s

f i n e g ra in , i t s dark c o l o r , i t s content o f g raph i te , and i t s metamorphic

s t r u c t u r e s t h a t i n d i c a t e a h i s t o r y of m u l t i p l e deformat ion. These rocks

a r e products o f g reensch is t - fac ies reg iona l metamorphism o f f ine-gra ined,

variably sorted, thin-bedded, and general l y quartz-rich sediments.

They include many layers of cal careous schis t s and some th in , dark-gray

marble interbeds and light-gray marble lenses. Adjacent to the granite

stock thermal recrystal 1 izat ion has produced rocks of hornblende horn-

f e l s facies .

The graphi t ic metasediments a re par t of a thick sequence which

has been ident i f ied throughout the Seward Peni nsul a. The sequence was

original ly called the Kuzi tr i n Series i n the central Seward Peninsula ,

and was correlated with s imilar b u t lower grade metasediments in the

York Mountains by Brooks (Brooks and others , 1901 ) . Most aspects of

t h i s correl a t ian have been accepted by l a t e r workers, b l t tk4e formal

name, Kuzitrin Series, has been dropped and the sequence as a whole

has become infurrnally referred t o as "s la tes of the York region"

(Sainsbury, 1972) o r simply "York Sla te" (Sainsbury, 1975). In the York

Mountai ns , Sai nsbury (1 969) described a conformable t rans i t ion from

1 ower Ordovician limestone downward through a thick unfossi 1 i ferous

section of argil laceous calcareous rocks into these s l a t e s . Based on

t h i s relat ionship, he concluded tha t the graphi t ic sediments of the

York Mountains a r e l a t e Precambrian in age and, because these can be

traced eastward to the Serpentine Hot Springs a rea , t ha t the s imilar

b u t higher grade metasediments in t h i s area a re also l a t e Precambrian

(Sai nsbury and others, 1970, p . H 6 ) .

Variations i n composition, texture, and response to thermal meta-

mrphi sm, combined with margi nal ly adequate exposures, a1 l owed mapping

of 1 i thologi cal ly d i s t inc t u n i t s wi thin the graphi t i c metasediments

eas t of the grani te complex. Their mapped dis t r ibut ion shows tha t they

form a se r i e s o f roughly north-south-trendi'ng bands paral le l t o the

regional s t ructure. The separately ident i f ied 1 i thologic units a re :

( 1 ) banded hornfel s , ( 2 ) dark-gray hornfels , ( 3 ) 1 ineated phyll i t i c

s c h i s t , ( 4 ) dark-gray metasf 1 t i t e , ( 5 ) mi c a ~ q u a r t z schis t s , and

( 6 ) strongly deformed chlori te-muscovi te-quartz schis t s .

Banded Hornfels

Banded hornfels has been mapped in a zone a long part of the eastern

margin o f the granite complex and in a parallel zone approximately

1400 f t fa r ther e s t . The f i r s t zone contains hornfels throughout

b u t the second grades into schistose rocks to the south. Overall the

hornfels i s medium 1 ight gray, b u t commonly i t i s somewhat mottled

l i g h t to medium dark gray with variably distributed brown or green t i n t s ,

Banding i s conspicuous in most hand specimens and i s defined by color ,

composi tional , and textural variations. Indi vi dual bands - a r e commonly

less than 0 .2 in . in width. T h e darker ones have a schistose fo l i a t ion ,

whereas those tha t a re l igh ter colored have a more sugary texture.

In i r regular ly banded rocks the general fabric i s more evenly schis tose,

with disccntinuous stringy aggregates of mafic minerals concentrated

along the fo l i a t ion , Also present a re some light-gray marble >sds and

somewhat coarser cal c-si 1 i c a t e schis t s containing variable proportions

of di opsi de, i docrase, cal ci t e , and pl agi ocl ase.

T h e thermally recrystal 1 ized schis t s and hornfels a r e diopside-

quar tz -amph ibo le -p lag ioc lase rocks tha t a lso contain red-brown b i o t i t e ,

ch lo r i t e , sphene, c a l c i t e , zircon, granular opaque m t e r i a l s and

cordi eri t e . Quartz forms aggregxces of anhedral grains wi t h i r regular

b u t simple interlocking boundaries, The la rges t grains a re p o i k i l i t i c

plagioclase porphyroblasts tha t a r e as much as 0.6 mn across. The por-

phyrobl asts are subequant, have i r regular boundaries, and 1 ack twinni ng

whereas small e r nonpoi ki 1 i t i c pl agi ocl ase d i spl ays s i mpl e twi nni ng . Both diopsi de and amphibole occur i ntergranularly between the generally

larger quartz and plagioclase grains. O f the other minerals, b i o t i t e

i s the most c o m n . I t i s diversely oriented and res t r ic ted t o individual

bands, as i s ch lor i te , b u t the ch lor i te has a preferred orientation

para1 1 el t o the fol ia t ion and i s generally intergrown with amphibole

and b io t i t e . Dark pleochroic halos a re present in some of the mafic

mi neral s. The composi tional variation between bands i s domi nantly the

r e su l t o f the preferred dis t r ibut ion of diopside, amphibole, and b i o t i t e .

Dark-Gray Hornfels - Dark-gray hornfels occurs between the two zones of banded hornfels.

I t i s typical l y a schistose bioti te-cordieri' te-quartz horafels charac-

terized by i t s uniformly f ine grain s i z e and medium dark-gray color.

Most o f these rocks a re hard and break i r regular ly across schistosi t y .

Those south of Hot Springs Creek weather to flaggy fragments t h a t a re

as much as 2 or 3 f t a2ross and tabular parallel t o the fo l ia t ion .

L i t ho1 ogi es present in lesser amounts are dark-gray , f j ne-grai ned marble

with i soc l i nally folded quartz s t r ingers and green, fi ne-grained,

schistose mafic dikes tha t a re a l so thermally recrystal l ized.

Quartz occurs in aggregates of anhedral, angular, simply bounded

grai ns tha t general ly lack undul ose ext inct ion. , The aggregates are

elongate lenses t h a t are oriented parallel to the fo l ia t ion b u t a lso

f i l l in "pressure shadows" adjacent t o cord ier i te porphyroblasts.

Cordi e r i t e occurs as subequant t o elo?gate, very i rregularly shaped

grains oriented along the fol ia t ion and t iddleu with inclusions o f

b i o t i t e , dusty t o granular opaque materials , and quartz. Subequant

cross-sections reach 0.5 m across and in part def lect the adjacent

fo l i a t ion . Bio t i te , the only mineral readily ident i f ied in hand specimen,

i s dark red-brown in color. I t has a bimodal preferred orientation

paral 1 el and approximately perpendicular t o the fol i ation. Other

minerals tha t a re present include muscovite, associated with b i o t i t e ,

and disseminated opaque material t ha t i s dominantly pyrrhoti t e . The

pyrrhotite has a tendency towards idiomorphic forms and i s both

subequant and elongate. In the l a t t e r case, the length i s oriented

paral le l t o the fo l ia t ion .

Phyl l i t i c Schist

Phyl l i t i c s ch i s t occurs about ha1 fway between the granite contact

and Hill 2592. I t i s typical ly somewhat lustrous, medium- to medium

dark-gray , ca1 ci te-a1 b i te-quartz-muscovi t e rock tha t weathers t o pl aty

and tabular fragments as much as 2 o r more ft across. Megascopicalfy

i t l's f ine grained , graphi t ic , locally porphyroblas t i c , and strongly

l ineated. The even and pervasive schis tos i ty i s very f inely crenulated,

producing a s t rorg l ineation paral l e l t o the rnicrofol d axes ,. Oblique

t o t h i s i s a second 1 ineation produced by broader spaced folds with wave

1 engths between 114 and 112 in . An incipient second fol iation has

developed along the axial planes of these broader folds .

Muscovi t e occurs as fe l ted , dense aggregates forrni ng bands paral 1 el

t o the fo l i a t ion , and elsewhere as intergranular, oriented grains or

small er aggregates. The denser aggregates d i splay t igh t crenul a t i ons

with the strong microfold l ineation evident in hand specimens. Quartz

i s subequant and elongate, less than 0.35 mm across, and forms some

inclusions i n a1 b i t e porphyroblasts. The inclusions are'ovoid in cross-

section and intergranul ar grains have simple b u t i r regular ly angul ar

boundaries. A1 b i t e , some w i t h simple twinning, occurs intergranul ar ly

w.1 t h quartz o r as subequant porphyroblasts t o 1 mm across. The por-

phyroblasts a re generally poi k i l i t i c w i t h i ncl uded grains, dominantly

quartz, t ha t form s t r ings , aggregates, or elongate individuals oriented

para1 1 el t o the fol ia t ion . The porphyroblasts a re rounded in out1 ine,

have boundaries tha t are smooth and continuous, and are commonly mantled

by muscovi t e aggregates. Local l y , i ntergranul a r , subequant cal c i t e

grains to 0.8 mm across make up 10 to 15 percent of the rock. Pale

chlori t e w i t h greenish-gray i nterference colors forms aggregates o r

individual grains interleaved w i t h muscovite, and may comprise 5 percent o f

a specimen. Other minerals that a re present include dull-brown b i o t i t e ,

dusty graphite, granular diss'eminated opaque material t ha t i s dominantly

hematite, and a minor amount of disseminated, euhedral tourrnal ine.

In summary, th i s s ch i s t was or iginal ly a thinly bedded drgillaceous

and quartz-ri ch rock. I t i s now characterized by the presence of graphite

and abundant phyll osi l i ca t e s , microscopic compositional banding, and

well -developed metamorphic s t ructures of two d i s t inc t generations.

Metasi 1 ti t e

The most d is t inc t ive (but not dominant) lithology within the

graphitic metasediment assemblage throughout the Seward Peninsul a i s

a dark-gray , very fi ne-grai ned metas i 1 ti t e that weathers to b r i t t l e ,

rectangular fragments less than a foot across. I t consists almost

wholly o f quartz. Throughout the Serpentine Hot Springs area th i s

lithology has been mapped separately and, a1 though the nature of a1 1

contacts has not been determined, i t i s possible tha t there i s more

than one dark-gray metasi 1 t i t e uni t within the graphi t i c metasediment

sequence.

Rocks located on the west s ide of Hi 11 2592 are typical of t h i s

lithology. They are medium to dark gray, weather to platy and tabular

fragments, and contain 80 to 95 percent quartz. The even grain s i ze

i s less than 0.3 mn and commonly around 0.1 mm. Metamorphic s t ruc-

tu res , not everywhere obvious , include very thin bandi ng , a strong

1 ineation due to intense crenulation, and a thinly spaced regular

schi s tosi ty . Where the metamorphic s t ructures a re well -developed,

these rocks a re graphi te-quartz schis t s . The schi sts commonly have

combinations of s t ructures indicating a polydeformational his tory,

Quartz typical ly forms equant, angular grains i n i nterlocki ng

aggregates. Muscovite and lesser chlori t e occur along quartz grain

boundaries, and graphite forms disseminated dusty grains. Other minerals

incl ude tourmaline, plagiocl ase (some as porphyroblasti c a1 bi t e ) , epidote, 4

and apa t i te .

Thermal metamorphism and physical deformation near thrus t faul t s

have produced two important textural variations . W i thin the thermal

aureole of the granite complex, the rretasi l t i t e coarsens in grain s ize

t o a maximum of about 0 . 3 rrun, banding becomes better developed because

opaque material and phyllosi 1 icates a re concentrated a1 ong the fo l i a t ion ,

and the quartz grains become semirectangul a r with very simp1 e grain

boundaries and elongation para1 1 el t o the fol i a t ion , Mi neral s tha t

a re locaTly developed wi thin the hornfel s zone i ncl ude cordieri t e ,

brown b i o t i t e , and porphyroblasti c anhedral andal usi t e , If the meta-

s i l t i t e ha5 been deformed, usually i n the vicini ty o f t h rus t f a u l t s ,

the quartz has strong undulose extinction and complexly sutured grain

boundaries. Crenulation of the principal fo l ia t ion i s comon, and a

strong 1 i neation para1 1 el t o mi crofol d axes i s developed,

Among, other variants known in the map area i s a light-gray weathering

metasi 1 t i t e found near the headwaters of Humbolt Creek. I n thin sect ion,

the l igh te r colored parts of these rocks d i f f e r from the normal dark-gray

parts only in that they lack di sseminated dusty opaque material ,

Apparently, the opaque material has been removed in some manner.

White quartz segregations and large veins are comnon in the meta-

s i l t i t e , especially in deformed zones or near the contact wi t h the

granite. Brecciated metasi l t i t e , encrusted with rust-colored iron

hydroxides, i s locally common in the surface rubble of the metasi l t i t e

u n i t mapped a t the head of Humbolt Creek.

Mi ca-Quartz Schis t - Mica-quartz schis t s occur in a relat ively wide be l t t ha t extends

along s t r i k e from the granite complex across the headwater o f Ferndale

Creek and past Hill 2592. This uni t , which displays more l i thologic

variati'ons than others i n the sequence, i ncl udes chlori te-muscovi te -

quartz sch i s t s , mica-tremol i te-plagiocl ase-quartz sch i s t s , and, in zones

of thermal metamorphism, bioti te-rich . schi s t s . These schis ts a r e

typical ly f a in t ly brownish medium 1 ight gray, and a re l ineated, f ine

grained, and equigranular. A strong 1 ineation i s paral le l t o the axes

of mi crofol ds , and. thin compositional layering can be seen in hand

specimen b u t i s usually more obvious in thin section. Locally these

schis t s are calcareous and contain disseminated pyri te . Oxidation o f

pyrite 1 eaves l i m n i t e spots and patches a1 ong the fol ia t ion. Thin

marble i nterbeds and 1 enses are re1 a t i vely common, and near the grani te

cornpl ex greenish cal c-s i 1 i ca te rocks, formed from i mpure cal careous

i nterbeds , contain diopside with 1 ess abundant sphene and 1 abradori t e .

In thin sect ion, these rocks a r e aggregates of many different

minerals of variable s i z e and dis t r ibut ion. As a r e s u l t , they have a

d i r ty or c lut tered appearance. Quartz i s equant, anhedral , and has

simple t o sutured grain boundaries. Tremol i t e , where present, i s the

next most abundant mineral and forms prismatic, fa in t ly greenish,

i rregularly terminated, and variably oriented grains less than 0.2 mrn

1 ong. Phyll osi 1 i cates commonly compose 40 percent of i ndi vi dual

specimens and i ncl ude chlor i te with gray to deep-bl ue interference

colors, muscovite, and b io t i t e . Chlorite and muscovi t e generally occur

in s t r ingers and aggregates parallel t o the fo l ia t ion b u t b i o t i t e ,

l ess than 0.1 m long and amber, 1 ight tan , or moderate red brown in

color , i s variably oriented, inclusion f r e e , and in places inter1 eaved

with ch lor i te or muscovite. Both b i o t i t e and tremoli t e appear t o be

products of thermally induced recrys ta l l iza t ion . Plagioclase forms

anhedral , equant, semi porphyroblastic grains with inclusions o f mica

and quartz. Twinning i s obscure t o absent. Granular pyri te and other

opaque materials a re common, and individual grains, some to 0.1 mrn

across, tend to be angular and subidiomorphic. Other mi neral s incl ude

small euhedral tourmaline crystals , very f i ne-grained epidote, in te r -

granular c a l c i t e , and t race amounts of apa t i te and subhedral zircon.

Hornfels textures are well developed in some o f these rocks adjacent

t o the granite complex. In these, porphyroblastic cordieri t e ( ? ) and

andalusi te(?) accompany b io t i t e , plagioclase, and quartz. The quartz

i s coarser than elsewhere in th i s unit and forms angular, equant grains

with simple boundaries and even extinction.

Chlori te-Muscovi te-Quartz Schists

The chlorite-muscovite-quartz schis t s have been severely deformed

during thrust faul t ing and a re textural ly and s t ructural ly d i s t inc t

from other parts of the graphitic metasediment sequence. They occur

in two kl i ppen, one on the summi t and eastern s ide of Hi 11 2592 and the #

other on a small spur on the east s ide of Hi 11 2592. In general , they

are greenish gray and weather t o very i r regular fragments. Megascopic

s t ructures are complex, characterized by 1 ensoid white quartz segregations

that are severely contorted and i soc l ina l ly folded ( f i g . 5 ) . They cre

composed of quartz and muscovite i n subequal amounts and less abundant

. chlori te . Minerals present in minor amounts a re dusty to granular

opaque material , epidote, a1 b i t e , and tourmaline.

Some quartz i s f ine grained and i r regular ly angular to r e c t ~ n g u l a r

with simple grain boundaries, b u t more generally i t i s strongly strained

wi t h complex, granulated to sutured grain boundaries . Muscovite i s

f ine t o medium grained and occurs in deformed aggregates which out l ine

d is tor ted compositional banding, I t i s complexly microfol ded and has

elongate s t r ingers of opaque material along cleavages and grain boundaries,

Chlorite i s a moderate grass-green color and i s concentrated in layers

or in dense aggregates which form sporadically dis t r ibuted patches.

Individual grains w i thin these patches a re bent and s t rained. Smal I

opaque grains wi t h square and rectangul a r out1 ines occur as i ncl usions

in ch lor i te .

Figure 5. Outcrop of st rongi iy deformed chlori te-muscovi te-quartz s c h i s t w i t h complexly fo lded wh i te qua r t z segregations.

Of the other minerals present, fine-grained opaque material ,

probably graphite, i s the most abundant. This material commonly i s

associated with muscovite in thin s t r ingers tha t sharply out l ine the

complex crenulations resul t ing from deformation. A1 bi t e forms small

subequant, anhedral grains with i r regular boundaries b u t i t a lso commonly

forms large grains tha t vcrge on being porphyroblastic. Tourmal ine

occurs as small rods along the axes of microfolds.

As a whole, t h i s uni t i s compositionally similar to other parts

of the graphi t ic metasediment sequence, b u t textural ly and s t ruc tura l ly

i t i s much more complexly deformed. Megascopic and mi~;~oscopic s t ruc-

tures vary in complexity b u t they c lear ly r e f l e c t a t l e a s t two episodes

of deformation. A strong fo l ia t ion i s parallel t o compositional banding

defined by variations in ch lo r i t e , quartz, and muscovite concentrations.

I t has been intensely crenulated, variously folded, and displaced along

axial -plane shears ( f i g . 6 ) .

Carbonate Rocks

Carbonate rocks occur as thin interbeds and small lenses as much

as a few hundred feet long within the graphitic metasediment sequence

and as la rger , isolated marble masses with complex s t ructural- re lat ions

to surrounding metamorphic rocks. The carbonate rocks intercalated

in the graphi t ic metasediments occur throughout the sequence-in minor

amounts b u t a r e most common within the mica-quartz schis t s and similar

rocks found in the eastern part of the map area. The la rger , isolated

marble masses occur along the ridge east of Humbol t Creek. They do not

support vegetation we1 1 , and stand out as 1 ight-colored knobs, mounds,

and h i l l s .

Figure 6. Photomicrograph .showing complex deformation features i n c h l o r i te-muscovi t e -qua r t z s c h i s t . Plane p o l a r i z e d 1 i g h t ; dark dusty and g ranu la r opaque m a t e r i a l s i n p a r t o u t l i n e crenul a t i o n s .

Carbonate Rocks Within Graphitic Metasediments

The carbonate rocks i ntercal ated i n the graphi t i c metasediment

sequence include cal careous schis ts and i nterbeds and 1 enses of marble.

They have been mapped separately only in the area north of lower Fern-

dale Creek.

Marble interbeds a re commonly less than 2 f t th ick , dark gray, f ine

to medium grained, graphi t ic , and equigranular. The coloration of

weathered surfaces i s ,simi l a r t o t h a t o f fresh surfaces. Weathering

does accentuate thinly spaced color banding and some specimens have

th in , t igh t ly folded, quartz bands that s t a n d out conspicuously on

weathered surfaces. More impure interbeds a re medium dark gray t o

medium gray and rnicaceous.

The. largest of the marble i nterbeds occurs in a klippe of chlor i te-

muscovi te-quartz schis t 3/8 mi south of Tin Mountain. Here a medium-gray

marble band i s approximately 10 f t thick, dips in general 5 t o 10 degrees

eastward, and crops out for more than 1/4 mi i n an east-west direct ion.

Carbonate lenses, comonly l e s s than 100 f t in length, are generally

a t h i n l y cleaved, medium-gray, thinly banded marble t h a t weathkrs t o

platy fragments. Near the granite complex the lenses- are recrystal l i zed

t o more massive, splotchy medium-gray, variably grained marbles with

coarse white cal ci te segregations. Impure layers in these thermal ly

metamorphosed rotks have been converted t o rus t - s t a i ned , brown-green

cal c-si 1 i c a t e bands t h a t are 'a combi nation of di opside , i docrase,

pl agi ocl ase, cal ci t e , and garnet.

Calcareous schis t s and associated marble a r e mapped separately

north of 1 ower Ferndal e Creek. Here buff-orange weathering gr-aphi t i c

muscovi te-cal ci t e schis t s grade upward into medi urn-gray, semi platy

marble. The r e l a t i on o f these rocks t o underlying chlori te-muscovi te-

quartz schis t s i s not c l ea r , bur fo l ia t ion a t t i tudes in closely spaced

outcrops of the two schist: are nearly the same and the sequence may

be ent i rely transi tional .

Marble in Isolated Masses

East of Humbolt Creek are the largest carbonate bodies in t h e map

area. They form nonvegetated h i l l s , knobs, and mounds on which outcrops

are limited and the surface i s usually an accumulation of frost-riven

rock fragments, The la rges t mass i s exposed over aoout 1 s a wi and forms

the northern end of the ridge east of Humbolt Creek. Other s imilar b u t

smaller bodies occur fa r ther south along th i s ridge and on i t s eas t

flank.

The marble in these masses weathers to chunky, i r regular fragments

that are a f a i r l y uniform medium gray in color, I t i s variably colored

1 ight and medium gray on fresh surfaces, f ine t o medium grained, and

massive to thinly banded. The banding i s defined by changes in the

darkening of gray coloring and i s caused 'primari ly by variation in

graphite content. Impurities other than graphite a re uncommon.

Small-scale l i thologic, textural , and structural variations are

common. Calcite may be f ine and medium grained i n a s ingle specimen,

whi t e cal ci te-cemented marble breccias can be found in some places ,

and fractures containi ng i ron hydroxides are locally abundant. Indi vi-

dual cal ci t e c rys ta l s Inore than 1 f t across and 1 arge boulders of

milky-white quartz were found in the surface rubble of the northernmost

body. Fragments of dark-gray, very f i ne-grai ned sch i s t have

been found in surface rubble of the body south of elevation 1808.

In the mass nearest elevation 1320, strongly deformed, dark-colored,

graphitic cal ci t e schis ts with small quartz segregations and vein1 e t s

are i nterbedded and i nterfol ded with massive, buff-orange weathering,

medium-gray, fine-grained dolomite.

One mile eas t of elevation 1320, adjacent t o the eastern margin

of the map area, abundant remnants of Paleozoic spaghetti corals

(probably Si lur i an or Devonian, according to Char1 es Merriam, 1973,

verbal commun.) were found in one of these bodies. Here the marble i s

massive and variable gray in color. The coral remnants are white

c a l c i t e spots and short rods in a much f ine r grained, ddrk-gray,

graphitic ca l c i t e matrix. These are the only fossi 1s t h a t have been

found in the map area and serve to help ident i fy th i s and s imilar

isolated carbonate masses as s i gni f icant ly d i f fe rent i n age from

surrounding graphi t i c metasediments.

The contact relations of these marble masses t o surrounding meta-

sediments are complex and generally s t ructural in nature. Imbricate

thrust faul t ing i s a dominant a ~ p e c t of the regional s t ructure of the

Seward Peninsula (Sainsbury, 1969 ) . The res t r ic t ion of the marble masses

t o the higher elevations of the north-south-trending ridge eas t of

Humbolt Creek, combined with the i r isolated nature as indicated by

the i r mapped dis t r ibut ion and form, suggests that they are the remnants

of a once larger thrus t sheet tha t juxtaposed them against the graphitic

metasediments. This possibi l i ty i s a1 so supported by the age discrepancy

that i s indicated by the foss i l corals in some of the marbles, the

var iab i l i ty of deformation and recrystall i zation within the marbles,

the complex folding that i s evident in the outcrops j u s t e a s t of -levation

1320 (Sai nsbury , 1969). Normal faul t i ng apparently has downdropped

the northernmost thrust remnant and i n places i s responsible f o r

the development of local brecciated zones.

In summary, the la rger , isolated marble masses a re Paleozoic

in age and they a re probably the remnants of a once larger th rus t sheet.

The Paleozoic marble masses d i f f e r from carbonate bodies intercal ated

in the graphi t ic metasediments in tha t they have more uniform weathering

chdrac ter i s t ics , a more conspicuous var iab i l i ty of small -scale l i tho logic ,

textural , and s tructural features , are less schistose or thinly cleaved,

contain graphi t e as the principal l'mpuri ty , and tend t o be larger i n

s ize .

Thermal Metamorphism

The granite complex i s sharply discordant and i s surrounded by

a we1 1 -developed thermal aureole. Thi s aureole vari es i n hori zon tal

width and i s widest in the southeast part of the map area ( p l . 1 ) .

Rocks wi thin the aureole have been metamorphosed t o hornblende hornfels

and a1 bi te-epidote hornfels f a c i es (Turner, 1968). Rocks of a1 b i te-

epi dote hornfels . . faci es have complex textures and mineral assemblages

gradational between those of the regionally metamorphosed greenschist +

facies rocks and the more extensively recrystal 1 ized rocks of hornblende

hornfels fac ies , In general , i nci pient thermal metamorphism i s indicated

by the development of b io t i t e a n d textural adjustments of quartz grain

boundaries . Bioti t e i s present local ly and replaces previous phyllosi l i c a t e s .

I t i s l i g h t colored and pl eochroic i n various shades of t an and ye1 low.

I n some samples i t has a preferred dis t r ibut ion and i s e i the r spa t i a l ly

a s s o c i a t e d w i t h ' o t h e r mineral g r a i n s , such as opaque m a t e r i a l , o r i s

concen t r a t ed i n d e f i n i t e composi t ional bands. I nc reas ing tempera ture

c o n d i t i o n s a r e i n d i c a t e d by a darkening of t h e c o l o r t o red-browns and/

I o r i n c r e a s e d g r a i n s i z e . In the h ighe r g rades , b i o t i t e tends t o

develop a bimodal o r i e n t a t i o n of t h e c l eavage t r a c e s both p a r a l l e l and

pe rpend icu l a r t o t h e t r a c e o f t h e f o l i a t i o n .

Q u a r t z g r a i n boundaries a r e ano the r s e n s i t i ve i n d i c a t o r o f thermal

g r a d i e n t s i n t h e lower grade rocks . Q u a r t z boundaries i n rocks t h a t

have n o t undergone the rma l ly induced r e c r y s t a l l i z a t i o n a r e c h a r a c t e r i z e d

, by complex t e x t u r a l r e l a t i o n s t h a t i n genera l i n c l u d e s u t u r e d g r a i n

boundar ies , v a r i a b l e g r a i n s i z e , and undulose e x t i n c t i o n . Upon r e c r y s t a l -

l i z a t i o n , t h e r e i s a g r ada t iona l change from t h e s e re1 a t i o n s t o one

where t h e q u a r t z boundaries have developed granobl a s t i c t e x t u r e s .

1nc ip i ent r e c r y s t a l 1 i z a t i o n i s evidenced by boundary ad jus tments t h ~ t

tend t o occu r i n i nd iv idua l zones o r domains where q u a r t z is t h e doriinant

mineral ( e s s e n t i a l l y monomineral i c domains). Here the i n i t i a l changes

a r e i n c r e a s e s i n t h e l eng th o f i nd iv idua l g r a i n boundary segments , a

h o t ~ g e n i z a t i o n of g r a i n s i z e , and a dec rease i n the deg ree of i n t e r n a l

s t r a i n i n g . These i n i t i a l t e x t u r a l ad jus tments may be p r e s e n t i n rocks

having only greenschi s t f a c i e s mineral assemblages. As metamorphism

proceeds , q u a r t z g r a i n s throughout t h e specimen become r e c r y s t a l l i zed

s o t h a t g r a n o b l a s t i c t e x t u r e s a r e we1 1 developed. These granoblas t i c

t e x t u r e s a r e u n l i k e t h o s e desc r ibed by Spry (1 969), i n t h a t t h e t r i p l e

p o i n t s t end t o be or thogonal r a t h e r than equ iangu la r . Th i s seems t o

be due t o t h e i n f l u e n c e o f o r i g i n a l banding and 1 aye r ing on the s c a l e

o f one g r a i n s i z e i n wid th . The advanced s t a g e o f t e x t u r a l ad jus tments

i n t h e s i l i c e o u s inetasediments i s reached when ind iv idua l q u a r t z g r a i n s

become rectangular i n out1 ine and form 1 ayers one t o a few grains in

width tha t are separated by th in , discontinuous, oriented mica-rich

segregations . Gradational assemblages between those of greenschist and hornblende

hornfels facies a r e most obvious in rocks that were or iginal ly argi l l a -

ceous and compositionally inhomogeneous. In these, thermal recrys ta l l iza-

t i on tends to be i ncomplete and di sequi 1 i bri um assemblages and compl ex

( d i r t y ) textures develop. Disequil i bri um assemblages a re characterized

by mineral compositions t h a t vary and the coexistence of greenschist

and hornblende hornfels facies mineral s . Both p l agi ocl ase and amphi bol e

(tremol i te-actinol i te-hornblende) can have a range of composi t i ons

within an i,ndi vidual specimen.

The charac ter i s t ic mi neral s of the more extensively recrys ta l l i zed

rocks of the thermal aureol e are andal usi t e and cordieri t e . Staurol i t e

was observed in one thin section of hornfels. Andalusi t e tends to

occur as subhedral porphyroblasts oriented parallel to the fo l ia t ion ,

where cordi e r i t e occurs domi nantly as very i rregul a r , elongate, poi ki 1 i t i c

porphyroblasts tha t create webli ke textures in which o;ented i ncl usions

of mica are very abundant.

I n summary, the grani te complex i s surrounded by- a d i s t inc t thermal

aureole in which rocks of hornblende hornfels facies have been formed.

The contact. re1 a t i ons and thermal ly induced assembl ages are charac-

t e r i s t i c of an epi zonal plutonic environment , and the temperature gradient

between the pluton and the surrounding rocks evidently was s teep . The

greater horizontal width of the thermal aureole along the southeast

margin of the pluton i s probably a ref lect ion of the shallow dip of the

grani te body in t h a t direction.

Structure

The s t ructural relations and features discussed in t h i s section

are principally developed in the metamorphic rocks of the area. The

graphi t ic rnetasediments s t r i k e north-south to s l igh t ly northeast and

have a low to moderate dip t o the east . Isoclinal overturned folding

may have affected parts of the metasediment sequence, high-angle

f au l t s a r e abundarrt in the southeast map area , and thrus t f au l t s a re

present in three areas. The thrust f au l t s a re part of the Coll ier . th rus t be l t (Sainsbury, 1969), a t h rus t terrane that includes the

ent i r e Seward Peninsula and i s characterized by imbri ca te , compl i cated

faul t ing.

Fol ds

I t was hoped tha t mapping of d i f fe rent l i thologic units within the

metasediments would help c l a r i fy s t ructural re la t ions , and i t has

done so with respect to some high-angle f au l t s . B u t the question of

the i mportance of 1 arge-scal e i socl i nal fol di ng has not been resol ved . c>

Such folding i s suspected because of the presence o f a re la t ive ly small

overturned ant ic l ine near the south-central boundary of the map a rea ,

tha t plunges to the south and has an amplitude of a few hundred f ee t .

This an t ic l ine i s i n fine-grained, l ineated and banded, quartz-calci te-

al b i te-muscovi t e schis t s containing some b i o t i t e . These schis t s a re

mapped as part of the banded hornfels as they apparently grade in to

such rocks along s t r i k e t o the north. Repetition of apparently similar

units of banded hornfels, or related rocks was noted in the south-central

map area between the small an t ic l ine and the grani te contact and may

be an indication o f larger isocl inal folding. The stratigraphy i s

so poorly known tha t the presence of such folds can only be suspected.

Faults

The presence o f thrust faul t ing i n three separate areas has been

mentioned earl i e r in the discussion of the metasedimentary uni t s . These areas include: (1 ) the gneiss-metasilti t e contact in the north-

central map area, ( 2 ) the klippen of highly deformed chlori te-muscovi te-

quurtz sch i s t and associated marble overlying the graphi t ic metasediments

in the southern map area, and ( 3 ) the discontinuous marble masses l o c a t ~ d

along the ridge eas t of Humbol t Creek.

In the area north o f the granite the principal evidence for thrust

f au l t s i s the marked discrepancy in metamorphic grade between the meta- " . .

s i l t i te and gneisses, the lack of features tha t would characterize a

depositional contact between. them, and the low clipping a t t i t ude of the

mapped gneiss-metasi 1 t i t e contact. Two such contacts have been recognized,

and the mapped relat ions indicate tha t they dip in opposite direct ions.

This suggests tha t the original contact may have been broadly war ped.

Thrust faul t ing in the southern map area i s evidenced by the severe

deformation o f the chlori te-quartz-muscovi t e schis t s ( f i g s . 5 , 6 ) and

by t h e i r mapped d is t r ibut ion , which defines re la t ive ly t h i n and shal low

dipping plates that were probably once connected. The a1 mos t f l a t - ly i ng

a t t i t u d e o f these plates i s in sharp contrast to the strong north-south

continuity of the other 1 i thologic units tha t have been mapped i n the

graphi t i c metasedi ments . On the ridge eas t of Humbol t Creek, isolated masses of Paleozoic

marble overl ie the graphi t ic metasediments and a thrus t re lat ion i s

suspected because: (1) the mapped dis t r ibut ion of isolated masses

indicates that they could have once been part of a continuous sheet ,

( 2 ) s i l i c i f i c a t i o n i s localized a t the base of the marble mass that i s

immediately eas t : ~ f the placer workings on Humbol t Creek, ( 3 ) 'dolomi t i -

zation and comp1e:c folding are present in the second mass eas t of the

placer workings on Humbol t Creek, and ( 4 ) the apparent age discrepancy

between the underlying rocks, believed t o be l a t e Precambrian, and

the Paleozoic (probably Devonian) overlying marbles. Si l i c i f i ca t ion and

dolomitization a t and near the base of such marble masses i s considered

by Sainsbury (1969) t o be indicative of thrust zones in the Coll ier

thrus t be1 t.

High-angle f au l t s a re the most widespread and eas i ly recognized

of the s t ructures found i n the area. Individual f a u l t s a re evidenced

by topographic discont inui t ies , 1 inear zones of a1 te ra t ion o r deforma-

t ion , and o f f s e t of 1 i thologic uni ts . Glhere of fse ts have been recognized,

the sense o f displacement i s normal. They are best developed i n the

metasediment sequence southeast of the graqi te complex and near o r

within the eastern margin of the granite.

Several apparently preferred or ientat ions fo r the h i gh-angl e f au l t s

were recognized by Sainsbury and others (1970, p. H7-H8). The most

conspicuous f au l t s are those that trend northeast and northwest.

Cormonly, f a u l t intersect ions a re a t high angles and the f au l t s define

rectangular to i r regular f a u l t bounded blocks. The granite m r g i n

i s o f f se t i n many loca l i t i e s by steep f au l t s , and one re la t ive ly large

s t ructure extends into the i n t e r i o r of the complex where i t terminates

as a s e r i e s of hydrothermal ly a1 tered splays i n Zone 4. The somewhat

regularly oriented faul ts concentrated w i t h i n the grani te complex near

i t s eastern margin are large-scale primary jo in ts along which differen-

t i a l ver t ical displacements and local o f f se t of the granite contact

have occurred. The dis t r ibut ion of high-angl e fau l t s in the metamorphic.

rocks south and eas t of the grani te i s important because these struc-

tures , par t icular ly those tha t trend northwest, have local i zed cassi t e r i t e

and su l f ide mineralization.

Age of Structural Features

The re la t ive age of various s t ructures can be determined i n par t ,

b u t the only absolute age reference i s the 68 to 69 may. K-Ar age date

f o r the grani te complex. The deformation and metamorphism of the meta-

sediments took place before emplacement of the granite complex, and

fol di ng wi thi n the graphi t ic metasediments may have preceded or accom-

panied thrust faul t ing. Microfolding in the graphi t i c metasediments is

common, and as the axial planes a re obl i lue to the primary schis tosi t y ,

the micrufolds may indicate tha t two periods of deformation affected

these rocks. However, much microfolding could be related t o local

tectonic deformation associated with thrust faul t i n g , The thrust

f au l t s themselves may have been warped, as suggested by the opposite

d i p s of the gneiss-metasil t i te contact north of the grani te . The

thrust ing and possible warping of the thrust sheets predates granite

emplacement. The high-angle fau l t s may be of a t l eas t three ages.

From 01 der to younger these are: (1 ) northeasterly trending s t ructures

apparently conforming t o regional s t ruc tura l trends, ( 2 ) northwesterly

t o westerly trending s t ructures tha t displace the grani te margin and

some thrust sheets , and along which mineralization comnonly i s localized,

and ( 3 ) somewhat regularly oriented primary jo in ts near the eas:ern

margin of the grani te , along which d i f fe rent ia l verti-cal movement has

occurred b u t mineralization i s not known t o be localized.

Igneous Rocks

The igneous rocks of the area a r e a l l rel&ted to a Late Cretaceous

epizonal stock of b i o t i t e grani te . This st6ck i s one of several very

similar grani te plutons t h a t occur in a northeasterly trending bel t

across northern Seward Peninsula ( f i g . 7 ) . All these plutons a re

related t o t i n mineralization and a re informally referred t o as " t in -

granites." The grani te stock in the Serpentine Hot Springs area i s

exposed over a larger area and to deeper levels than any of the other

t in-grani tes in the 'be l t and i t i s therefore best suited for t h e study

of the magmatic processes tha t were operative during the i r c rys t a l l i za -

t ion.

The Granite Complex

The grani te complex of the Serpentine Hot S p r i n ~ s area i s a tex-

tu ra l ly and compositionally zoned intrusion exposed over an area of

about 26 sq mi, roughly oval in plan except for a bul.ge in the .

northeast direct ion. The complex i s composed of var ie t ies of

b i o t i t e grani te and minor leucogranite. The overall f e l s i c nature of

the pluton i s i l l u s t r a t ed by the modal ra t ios plotted in f igure 8.

The average mode for the essential minerals, determined from

samples collected f r o m a l l parts of the complex, i s 29 percent plagio-

c lase, 31 percent quartz, 36 percent K-feldspar, and 4 percent b io t i t e .

Accessory minerals, amounting t o less than 1 percent, include apa t i t e ,

0 50 103 M I L E S I - - : I

F igu re 7. D i s t r i b u t i o n of t i n - g r a n i t e p lu tons , ~ e w a r d Peninsula, Alaska (e, exposed; l, i n f e r r e d a t depth;*, g r a n i t e complex o f t h e Serpent ine Hot Spr ings a rea) .

Q u a r t z

F igu re 8. P l o t of modal P I :Q:Or ratios for samples collected from a1 1 parts of the granite complex.

magnetite, zircon , sphene , a1 1 ani t e , and monazite(?) . Late-stage

or deuteric mi neral s i ncl ude muscovi t e , f l uori te', schorl , quartz,

and a l b i t e ,

The complex i s composed of several d i f fe rent textural facies tha t

a re par t of four principal zones (p l . 1 ) . Some of the facies are tex-

tural ly and compositional ly gradational w i t h one another, b u t some form

dis t i ,nct ly crosscutting intrusions, A sequence of the textural facies

i s defined by t h e i r mutual f i e l d relationships and by important s h i f t s

in major and trace-element composi tions , modal r a t io s , mineral composi - t ions, and petrographic re1 at ions. Study of these variations shows that

the i n i t i a l l y emplaced magma probably underwent f ract ional c rys ta l l iza-

tion and evolved a residual magma system tha t was saturated with

respect t o an aqueous phase. A coexisting aqueous phase probably was

also present a t the contact upon i n i t i a l crystal 1 ization and locally

when some in t e r io r parts of the complex were almost to t a l ly crystal1 i red . I

The evolved aqueous phase was enriched in t i n and cer tain related elements.

Final crystal 1 izat ion of the complex accompanied the local i zation o f

t h i s t i n-beari ng f lu id along late-formed f a u l t s t ructures that transect

the pl uton, These fau l t s presumably represent the dynamic adjustments

t h a t t ~ o k place within the complex as i t neared complete so l id i f i ca t ion .

In .the following sect ions, the f i e ld and petrologic character is t ics

of each zone and textural facies are desiri bed, with emphasis on those

parameters tha t show var iab i l i ty from one facies t o the next . The tex-

tural , structural , compositional , and spat ia l re lat ions that a re best

studied i.n the context of the pluton as a whole a re treated separately

in l a t t e r sections. In t h e l a s t sec t ion , the avai lable data a re inte-

grated into an evolutionary model of the crystal l i zi ng pluton.

A Late Cretaceous age i s assigned t o the granite complex on the

basis of two K-Ar radiometri c 'age determi nations, provided by Marvin

A. Lanphere o f the U. S . Geological Survey, on b io t i t e separates from

samples representative of facies 1C (69 AH 283) and facies 38 (69 AH 228).

The analytical data are shown in table 1 . These dates help substantiate

correlations with other bodies of b i o t i t e grani te to the west (Coll ier

and others, 1908; Sai nsbury and others, 1968) , one of which also has

been dated by the K-Ar method and found to be 75,1 + 3.0 m.y. old

(Sainsbury, 1969, p . 61).

Contact Relations

The granite complex i s in sharp and discordant contact with

adjacent country rocks. In general, the t race of the principal contact

is easi ly ident i f ied on a i r photos o r on the g r o u n d , and i s continuous

and regular. The c o n t a c t i s in many places top.ographi cal ly expressed

by changes i n surface slopes between borderi ng country-rock areas

and marginal areas within the pluton. These slopes face towards the a

i n t e r i o r areas o f the pluton and are generally steeper i n the country

rocks than in the imnediately adjacent parts of the pluton ( f i g . 9 ) .

Rocks of hornblende hornfels facies are present in the country rocks

adjacent t o the contact with grani te , and t h e i r resistance t o weathering

compared t o tha t of the granite i s in large part responsible for the '

topographic re1 a t iors between them. On a local sca le , the granite-

country rock contact can be very i r regular , and diking into the country

rocks i s common. The dikes, discussed in more detai 1 in a l a t e r sect ion,

can. be several hundred fee t 1 ong and in many places are clear ly di rect

Sr Q 0 ar a .. 'A a, X U 0 rl C U m

m

A h A rn 2 h 4 m c rd

E 3 -d m m (d U

8, .- m m W

c 0 >

U

'7

h A m C 0 .rl 4J a d 3 U rl m 0

(I)

2' F a cn a, cn h d a C m c 0 m LI 4 V

7.

k Q (I)

X .

ZONE 2 Z 0 N E . I :o 1345 Feet -

Figure 9. General ized s e c t i o n across eas tern contac t o f the g r a n i t e compl ex i 11 us t r a t i ng the negat i ve topographi c expression o f t he gran i te-coun t r y rock con t a c t .

offshoots of the granite complex. T h e principal contact gains addi-

tional i r regular i ty where i t i s o f f se t by high-angle f au l t s .

Depth - of Emplacement

A shallow environment fo r emplacement of the pluton i s indicated

by the nature of the contact re la t ions , the presence of many open voids

within i t , and a t en ta t i ve correlation with experimentally determined

compositional re lat ions for vapor-saturated parts of the haplogranite

system. The development of rocks of hornblende hornfels facies within

the thermal aureole indicates maximum 1 i thos ta t ic pressures on the

order of 2 o r 3 kbar' (Hietanen, 1967). T h e many miarol i t i c cavi t i e s

in facies lA, 38, 4A, and 48 rocks, including an open pocket 1 .5 f t

across, suggest that to ta l confining pressures may well have been less

than the maximum pressures indicated by the contact metamorphic mineral

assemblages. For parts of the pluton tha t crystal1 ized under vapor-

saturated conditions and have bulk compositions t h a t can be nearly

completely defined within t h e haplograni t e system (NaA1 S i 308-KA1Si308-

Si02-H20), a possible indicator of depth i s the relat ion of the observed

b u l k composition to the trend of successive isobaric minima for the

haplogranite system (Duffield and Jahns, 1975). The only rocks that

s a t i s fy the requirements of being water saturated and of simple b u l k

composition a re those of zone 4 , The plot of normative Q:Ab:Or ra t ios

f o r the analyzed rocks of facies 48 f a1 1 close t o the isobaric minimum

f o r a confining pressure of 1 kbar (Tut t le 2nd Bowen, 1958, p. 7 5 ) ,

as shown in figure 10. This suggests an emplacement depth o f about

4 ' k m i f the confining pressure was to t a l ly l i thos ta t ic but, as i s shown

1 a t e r , facies 4B rocks probably formed under condi t ions in which the

Figure 10. Plot of normative Q:Ab:Or:Si02 r a t i o s f o r fac ies 4B rocks (m ) i n r e l a t i o n t o the t rend o f success ive i s o b a r i c minima determined by Tutt le and Bowen (1958, p. 75) f o r t h e system NaAlSi ,O,-KAISi ,0,-SiO, - H20 .

pressure regime was complex and not necessari l y to ta l l y l i thos t a t i c.

For t h i s reason, the 1 kbar confining pressure probably should be

considered a minimum !pressure, and the depth of emplacement i s there- " .

fore thought t o have been between 4 and 8 kw.

Peri pheral D i kes

Granitic dikes in the country rocks surrounding the granite

complex i ncl ude d i rec t offshoots of the complex, is01 ated quartz

porphyry dikes, and fol ia ted dikes whose relationship to the granite

complex i s not well defined.

The dikes that a re clear ly d i r ec t offst,oots of the grani te complex

occur peripheral ly to i t throughout the area. These dikes a re as much

as a few hundred f e e t long, of differ ing width b u t commonly less than

5 f t across, and display no systematic or ientat ion. They a re very l igh t

gray in color , f ine grained, and wholly or partly allotriomorphic

equi granul ar i n texture. Seri a t e hypi di omorphi c textures and re1 a t i vely

abundant graphic and myrmeki t i c intergrowths are charac ter i s t ic o f

some. The essenti a1 minerals include quartz and unzoned, variably ST

twinned p l agiocl ase in about equal proportions, somewhat 1 ess abdndant

pe r th i t i c microcline, and less than 5 percent mica which i s dominantly

b i o t i t e or muscovite. Accessory minerals are not abundant b u t locally

include a few grains of a l l a n i t e and more abundant schorl. The schorl

i s f ine to medium grained and occurs as suthedral intergranular grains ,

i r regular i n t e r s t i t i a l grains, and in small vugs. Vugs are relat ively

common, especially so in the m r e al tered specimens. Alteration i s

the r e su l t o f replacements, local ly extensive, by muscovite. Oxidation

has caused discoloration and f i lmi ng of grain boundaries by iron hydroxides

together w i t h chlori t izat ion of b io t t t e .

Quartz porphyry dikes have been found in the peripheral zones of

the thermal aureole southeast of the granite complex. These dikes

intrude rocks of the graphi t i c metasediment assembl age, and a t the

present surface a re not connected di rect ly wi t h the granite pl uton.

They are very f ine grained, very 1 i ght gray to whi t e i n color, and . contain rounded-appearing megacrysts of gray quartz t o 1 mrn across,

di ssemi nated and commonly oxidized euhedral pyri te , a:~d pervasive fine-

grained muscovi t e . Some specimens contai n numerous small vugs that in

part display mica concentrations and secondary iron s taining. Some

of these dikes a re apparently localized along high-angl e f a u l t s t ruc-

tures , and a t l e a s t one of them has been affected by postintrusion

faul t ing ( loca l i ty C , pl . 1 ) . Soil samples collected along t h i s dike

contain anomalous metal values (see table 11 ) . An area with abundant diking borders part of the northern granite

complex and has been ident i f ied separately on the geologic map. The

dikes here include typical offshoots of the pluton, b u t more commonly

they d i f f e r in that they are fol iated and oriented in a general north-

south direction. This orientation i s para1 le l t o the general s t r i k e

o f the metamorphic host rocks, and the dikes may actually be s i l l s .

They are found in association with b o t h paragneiss and graphitic meta-

sediments.

In general, the foliated dikes a re l i gh t t o medium gray and f ine

graincd. In hand specimen, the fo l ia t ion i s defined by discontinuous

segregations o f oriented b i o t i t e ( l e s s c o m n l y muscovite) and strung-out

aggregates of quartz. Sugary textures are common and some gradations

from coarser, m r e obviously fo l ia ted , border zones into f ine r grained,

sugary in ter iors have been observed. Some specimens a r e porphyrit ic,

with individual plagiocl ase and perthi t i c K-feldspar phenocrysts tha t

reach 5 mm in length. In thin sect ion, the fo l ia ted dike rocks a re

dominantly a1 lotriomorphic and contain, in decreasing order of abundance,

quartz, plagioclase, perthi t i c K-feldspar, b i o t i t e , and muscovite.

Secondary mineral development i s common, consisting of rrinor t o exten-

sive chlori t i za t ion of b i o t i t e and replacements by muscovite and ser i ci t e ,

Oxidation produces discoloration due t o iron s taining.

Rocks of Zone 1 - - - - Zone 1 i s a discontinuous border zone tha t was mapped in the f i e l d

on the general basis of i t s charac ter i s t ic nearly hypidiomorphic equi-

granular texture! . . and K-feldspar c rys ta l s t h a t a r e cl early 1 ight gray

t o white in color. These character is t ics distinguish Zone 1 rocks from

those tha t are t ransi t ional inward from them, i . e . , the rocks of lone 2

t ha t a re porphyritic and have large pinkish K-feldspar c rys ta l s . Within

Zone 1 the rocks systematically increase in grain s i ze and show important

textural s h i f t s inward from the contact. These inward changes permit

convenient subdivision i n t o three d i f f e ren t textural facies . From the

contact inward, these facies a re referred t o as facies 1 A , 1B, and IC,

and representative examples of each are shown in figure 11 . I n addition

t o the regular increase in grain s i z e , both the K-feldspar and quartz

populations show important t ransi t ional textural changes inward.

K-feldspar changes from small anhedral grains that are n o t generally

connected t o one another ( facies I n ) t o those that a re s l ight ly above

average in grain s i ze , d i s t inc t ly less interconnected in nature, and

with be t te r def ined crystal morphology (facies 1 C ) . In th i s same

sequence q u a r t z changes from equant and g e n e r a l l y unconnected g r a i n s

t o p a r t i a l l y aggregated c l u s t e r s o f anhedral g r a i n s , and thence t o

t o t a l l y aggregated e l u s t e r s of anhedral g r a i ns.

Fac ies 1A--Medi um-grai ned Equi granul a r G r a n i t e

Rocks o f f a c i e s 1A a r e medi um-grai ned, hyp id iomorphi c, and equ i -

g r a n u l a r h i o t i t e g r a n i t e . They a r e found ad jacen t t o t h e c o n t a c t and

f o r a s h o r t b u t unknown d i s t a n c e inwards . They a r e s i m i l a r t o t he

p e r i p h e r a l d i kes t h a t a r e d i r e c t o f fshoots o f t h e p l u t o n . The e s s e n t i a l

m i ne ra l s occur as i n d i v i dual equant, subhedral t o anhedral g r a i n s t h a t

a re dominant ly 0.3 mm i n s i z e . Smal l , g e n e r a l l y open m i a r o l i t i c

c a v i t i e s a r e common b u t i n some p laces they a r e l a r g e r and c o n t a i n

t ou rma l i ne , muscov i te , and f l u o r i t e . Local inhomogeneity i s caused

by g raph i c i n t e r g r o w t h s , domains a few cen t ime te rs a c r o s s o f q u a r t z

o r f e l d s p a r concen t ra t i on , and a few f i n e - g r a i n e d i n te rg row ths of

p l a g i o c l a s e and b i o t i t e w i t h some hornblende. I n a few p laces muscovi te

e x t e n s i v e l y rep laces some o f these r ocks .

P l a g i o c l ase d i s p l a y s a wide range o f composi t i o n (Ang-An2,) and

3 a c h a r a c t e r i s t i c a l l y even e x t i n c t i o n . H few s c a t t e r e d , appa ren t l y

unzoned phenocrysts o f p l a g i o c l a s e a re p resen t i n t he f i n e s t g ra i ned

p a r t s o f t h i s f a c i e s . zoning' i s uncommon and, where p resen t , i s n o t

w e l l developed. The zoned c r y s t a l s have cores w i t h even e x t i n c t i o n

and narrow n o r m a l l y zoned r ims . I n a few l a r g e r g r a i n s , f a - i n t o s c i l l a -

t i o n s a r e p resen t i n t h e r ims . The compos i t iona l he te rogene i t y

o f t h e p l a g i o c l ase e x i s t s between c r y s t a l s i n one specimen as we1 1 as

between specimens. The zoned c r y s t a l s and phenocrysts commonly have

h i g h e r a n o r t h i t e con ten t s , b u t t h e compos i t ion o f more t y p i c a l c r y s t a l s

cannot be p r e d i c t e d w i t h i n t h e o v e r a l l rznge o f composi t ion.

Some K-feldspar i s euhedral b u t most i s anhedral , has undulose

extinctdon and obscure grid twinning, and i s s t r ing or s t r i n g l e t micro-

perthi t e . I t i s character is t ical ly very clouded by minute inclusions

of c lay(?) minerals. Some quartz grains contain small inclusions of

plagiocl ase, muscovite, and b i o t i t e whereas elsewhere in the complex

small muscovite grains are usually the only inclusion. Biot i te i s f ine

grained, variable in pleochroism (moderate brown, root-beer brown,

dark red brown, 01 i ve, and dark green brown), and contains abundant

pl eochroi c ha1 0s. The accessory mi neral s i ncl ude zi rcon , magneti t e ,

and monazi t e ( ? ) , They occur characteri s t i cal ly as small i ncl usi ons

in b io t i t e , generally too small fo r optical ident i f ica t ion . A tabular ,

hi gh-re1 ief mineral w i t h high birefringence and i ncl ined exti ncti on

may be monazite; i t has not been observed elsewhere in the comp1.e~.

Facies 1B--Coarse-grained , Approximately Equi granul ar Granite

The b i o t i t e granite of facies 1 B i s transit ional inward from tha t

o f facies 1 A and occurs within a few hundred f ee t of the contact and

into the medial parts of zone 1. The rocks are characterized by a

texture in which 1 i nearly ser i a t e , anhedral , and i nterconnected grains

o f K-feldspar occur with dominantly equi granul a r plagi oclase and quartz.

As a whole the rocks are coarse grained b u t the se r i a t e K-feldspar,

which i s u p t o 2 cm across, has a median grain s ize generally greater

t h a n the average for quartz and plagioclase. Quartz i s l i gh t t o medium

l igh t gray and displays a variable degree of aggregation; c lusters of

two o r more grains are common, b u t isolated subequant grains are also

present. Xenoliths(?) of fine-grained plagiocl ase a n d b i o t i t e , u p t o

3 cm b u t commonly less ' than 1 cm across, are present in more than

ha1 f of the specimens studied. Other i nhomogenei t i e s incl ude small

concentrations of quartz and K-feldspar,

Plagiocl ase (AnEZ0) grain in te r iors display patchy extinctions,

some localized or oriented se r i a t e replacements, and narrc.1 rims

normally zoned t o a1 bi t e . Some larger grains have we1 l-developed f ine

osci 11 atory zoning, b u t with 1 i t t l e overall composition change outward

t o the rim. Twinning follows combinations of Carlsbad and a1 bi t e twin

laws. The K-fel dspar i s vein microperthi t e , commonly grid twinned,

and local l y has small plagiocl ase inclusions tha t o u t 1 ine growth bolln-

daries. Biot i te i s dark reddish brown and i s coarser and contains much

fewer pleochroic halos ( 1 per every 4 or 5 grains) than tha t in facies

1 A . The accessory minerals a r e intergranular a1 lani t e , sphene, magnetite,

and zircon. Apatite, character is t ical 1y occurring as oriented small ,

euhedral inclusions in b io t i t e , f i r s t appears in th i s fac ies .

Faci es 1 C--Semi porphyri t i c Granite

Biot i te granite of facies 1 C i s transit ional inward from tha t o f

faci es 10 and makes u p about the inner ha1 f of zone 1 . T h e charac ter i s t ic

texture i s one in which subhedral K-feldspar, coarser on the average

than other minerals, occurs wi t h coarse-grai ned plagiocl ase and quartz;

the quartz i s anhedral and almost to t a l ly aggregated into clusters

containing two or more grains. The K-feldspar i s s e r i a t e b u t has a

skewed grain s ize dis t r ibut ion such that more than ha1 f of the grains

are larger than the middle value in the K-feldspar grain s i ze range.

The interconnected nature of K-feldspar i s s ignif icant ly diminished

compared t o that in facies 1B. Quartz, darker colored than in facies

1 A and 10, commonly i s medium l i g h t gray. As a whole, plagioclase and

quartz form an equi granul a r hypidiornorphi c population with an average

grain s i ze of 0 .4 t o 0 .5 cm. As the median K-feldspar grain s i ze i s

commonly 1 .5 times greater t h a n th i s , faci es 1 C i s considered t o be

semi porphyri t i c .

Plagi ocl ase ( A n l g M z 4 ) grains have d i s t inc t central cores with

patchy zoning, and marginal normal zoning w i t h thin and somewhat

f a i n t osci 1 la t ions. The K-feldspar i s vein and'patch perthi t e , has

we1 1-developed grid twinning, and i s marked by growth boundaries

defined by both plagioclase inclusions and thin a1 bi t e lamellae.

Biot i te and the accessory minerals are similar to those in facies 1B

except tha t i diomorphi c development and grain s i ze increase inwards ,

Rocks of Zone 2 ---- Rocks of zone 2 are textural ly t ransi t ional inwards from those of

zone 1 . The contact between these zones was ident i f ied in the f i e ld

on the basis of K-feldspar. The rozks of zone 2 contain pinkish-gray

K-feldspar that i s conspicuously coarser grained than other minerals.

The t ransi t ion from typical facies 1 C rocks t o those typical of zone 2 .

takes place over a distance of a few hundred f ee t - Rocks of zone 2

are texturally the m s t homogeneous in the en t i re complex a.nd are con-

sidered t o represent one textural facies , facies 2 .

Facies 2--Porphyri t i c Granite

Representative specimens of facies 2 a re shown in figure 12.

Typically, the rocks are porphyritic w i t h large (commonly t o 3 or 4 cm

long) , 1 ight pink-gray K-feldspar phenocrysts in an approximately

equi granular and coarse-grained groundmass of whi t e t o very 1 i ght-gray

pl agioclase and medi urn-1 i ght-gray t o medi urn-gray , anhedral quartz

F i g u r e 12 . Representa t i ve specimens of Zone 2 ( t e x t u r a l facies 2 ) .

t h a t i s t o t a l ly aggregated into clusters of two or more grains. The

K-feldspar i s dominantly euhedral and a t l e a s t two times the average

grain s i z e of quartz and plagioclase. Ccnsiderable range in grain s i ze

ex i s t s , however, and some specimers contain medi um- t o f7 ne-grained,

subhedral t o anhedral grains tha t are i n part i n t e r s t i t i a l . The boun-

daries of large K-feldspar grains are i r regular on a small sca le , and

the grains have i n te rs t i t i a1 offshoots and partly include adjacent grains.

Preferred orientation o f the phenocrysts i s common and, combined with the

variable aggregation o f quartz and plagiocl ase, produces a small degree

of 1 ocal i nhomogenei ty . The plagioclase ) i s d i s t inc t ly zoned, Grain in t e r io r s

a re generally homogeneous b u t some di splay patchy exti ncti on or marked

optical discontinuity with surrounding ma,erial . Marginal bands with

strong osci l la tory normal zoning in which the osc i l la t ions are rela-

t ive ly sharp a r e wider than in rocks of the marginal facies . K, feldspar

i s we1 1 twinned , vein and patch perthi t e wi t h a high proporti on of

inclusions, mostly plagioclase b u t a lso q u a r t z and b i o t i t e . Included

pl agioclase general l y out1 i nes growth boundari es . Bi ot i te i s euhedral ,

commonly greater than 2 mm across, pleochroic t o dark brown, and con-

ta ins relat ively few pleochroic halos. In some specimens b io t i t e shows

a preferred d is t r ibut ion , with concentration in a zone adjacent t o large

K-feldspar phenocrysts . Sphene, a1 1 ani t e , zi rcon, and magneti t e are

commonly euhedral , intergranular, and coarser grained than in more

marginal facies . Allani te , for example, forms crys ta l s as much as

1.5 mm long. Magnetite and b i o t i t e a re more abundant in t h i s f ac ies

than in any other.

Rocks of Zone 3 - - -- - Zone 3 was or iginal ly mapped as a separate uni t because i t i s much

less consistent in 1 i thology and s t ruc ture than other parts of the

complex. I n general i t i s concentric with respect t o zones 1 and 2

b u t the northeastward bulge of the pluton i s due primarily to the

intrusive emplacement of parts of t h i s zone along the northeast contact

of the original , pre-zone 3 pluton. The contact of zone 3 with e a r l i e r

crystal l ized tones i s re la t ive ly sharp, especially i f compared t o the

highly t ransi t ional zone 1 -zone 2 contact. The intrusive character of

the zone 3 contact i s c leares t along the east-central and northeastern

margins of the pluton, where zoqe 3 rocxs have been emplaced into

adjacent country rocks and now separate what probably orce were con-

t i nuous parts of zones 1 and 2 . Along the western boundary of zone 3

the contact i s concentric, with e a r l i e r zones indicating tha t displace-

ment o f the central zone 3 magma was relat ively minor. This part of

the contact i s b e s t seen about 1000 f t north of the m o u t h of McAble

Creek. I t i s n o t well exposed, b u t zone 2 a n d zone 3 rocks occur a

short distance apart in adjacent outcrops. A t t h i s l oca l i ty , flow

structures in zone 3 trend parallel to the contact and d i p 80 deg to

the north.

Characteris t i c s of zone 3 tha t serve t o distinguish i t in the

f i e l d a re variations i n texture and s t ruc ture . As a whole the zone i s

inhomogeneous. I t contains large crystals of white K-feldspar ( u p to

22 i n , long) tha t a re about half as a b u n d a n t as the K-feldspar pheno-

crysts in zone 2 . Structures tha t are common include b i o t i t e sch,lieren,

coarse feldspar clots and segregations, discontinuous a p l i t e s t r ingers

and di kes , pegmati t i c pods and dikes , miarol i t i c cavi t i e s with euhedral

crystals of quartz and schorl , and some larger openings associated

with pegmati t e s . Most of these a re discussed i n l a t e r sections ,

The ,zone contains two textural facies: 3A i s s e r i a t e and 3B

i s porphyritic with an apli t i c groundmass. Rocks o f facies 3A a re

i r regular ly intruded by those o f 3B, and a large dike of faci es 38

granite extends several hundred f e e t in to zone 2 rocks a t one loca l i ty .

Most of the zone 3 discordance i s due t o emplacement of rocks typical

of facies 3B. Outcrops, and the s t ructures l i s t ed above, a re more

charac ter i s t ic of facies 3A rocks t h a n those of facies 3B. .

Faci es 3A--Seri a t e Granite

Rocks typical o f facies 3k have an excellent seriaT,e texture, shown

in a representative example in figure 13. All the e s s e ~ ~ t i a l minerals

, are se r i a t e , and the idiomorphic character of these beccmes bet ter

developed wi t h increasing grai n s ize . Larger quartz crystal s , for

example, are doubly terminated, noncggregated, and have a good basal

parti ng. K-feldspar varies from small anhedral i ntergranular grains

t o large euhedral crystals wi t h boundaries that generally are ragged

on a small scale . Plagioclase appears t o be subhedral i n even the

smallest grain s izes .

Some important textural and composi t i onal charac ter i s t ics of the

feldspars correspond t o the variations in crystal grain s i ze and mor-

phology. The larger plagiocl ase crystals di splay zoning t h a t

i s grossly s imilar to tha t in rocks of facies 2 , b u t i t i s better

defined, and both marginal bands with osci l la tory zoning and more evenly

normal-toned rims are wider than in facies 2 ?lagioclase c rys ta l s .

The f ine r grained plagioclase c rys ta l s lack such well-developed zoning

:>

F igure 13. Representat ive specimens o f textural f ac i es 3A.

and c o n t a i n d i s t i n c t l y l e s s a n o r t h i t e (Anlldl5 ) The p e r t h i t i c charac ter

and g r i d tw inn ing o f the K- fe ldspar increase w i t h g r a i n s i ze . Larger

c r y s t a l s a re good ve in p e r t h i tes s i m i l a r t o t h e phenocrysts o f zone 2

except t h a t some a r e a l i t t l e l a r g e r and they a r e n o t p i n k i s h i n c o l o r .

The sma l l e r K- fe ldspar c r y s t a l s a re n o t s t r o n g l y p e r t h i t i c and have

obscure g r i d tw inn ing . X-ray s tud ies of b o t h the l a r g e p e r t h i tes and

the smal l e r K- fe ldspar c r y s t a l s t h a t do n o t c o n t a i n microscopic a1 b i t e

segregat ions show t h a t they are bo th maximum microc l i nps b u t t h a t t he re

i s a s l i g h t d i f ference i n t h e i r present a l b i t e contents. Two o f t h e

k e r t h i t e s have composit ions o f O r g 5 and O r g 2 , and two of the sma l l e r

c r y s t a l s have composit ions of O r g 7 and Orlo0 As the composit ions o f

p e r t h i t e s must have o r i g i n a l l y been much more a1 b i t i c , these data show

t h a t the f a c i e s 3A rocks a c t u a l l y c o n t a i n four fe ldspars : (1 ) l a r g e r

we1 1 -zoned c a l c i c. 01 i goclase, (2 ) smal l e r unzoned sodi c 01 i goclase,

( 3 ) l a r g e r sod ic K- fe ldspar , and ( 4 ) smal l e r nonsodic K- fe ldspar .

B i o t i t e i s anhedral t o euhedral and p leoch ro i c i n shades o f amber

brown, roo t -beer brown, and redd ish brown i n c o n t r a s t t o the general i3r

dark brown c o l o r i n g o f fac ies 2 b i o t i t e . The f a c i e s 3A b i o t i t e has

v a r i a b l y abundant p leoch ro i c halos and commonly i n c l udes a p a t i t e .

A1 1 an i t e , sphene , z i r c o n , and magnet i te a r e a1 1 present , dominant ly

as i n t e r g r a n u l a r g ra ins , b u t t h e i r abundance i s v a r i a b l e .

Facies 30--Cornposi t e - tex tu red Gran i te

Rocks o f t h i s f ac ies are found a long the nor theas tern contac t

o f t h e p l u t o n and i n smal l i n t r u s i v e bodies w i t h fac ies 3A rocks. A

d i k e of t h i s fac ies extends several hundred f e e t i n t o the rocks o f

fac ies 2 f rom a p o i n t on t h e no r th -cen t ra l contac t o f zone 3. The

facies 3A rocks generally do not form outcrops, and t h e i r dis t r ibut ion

has mostly' been determined mainly from t h e i r occurrence in frost-riven

surface rubble. Even so, t h e i r contacts with adjacent rocks a l l appear

to be sharp and discordant ( f i g . 1 5 ) . The bulge along the northeastern

s ide of the pluton i s composed chiefly of rocks of t h i s fac ies .

Representative examples of facies 38 are shown in figure 14.

This facies i s characterized by the presence of K-feldspar, plagioclase,

quartz, and bi o t i t e phenocrysts i n a very f i ne-grai ned, apl i t i c ground-

mass of the same minerals. Because of the marked bimodal nature of

th i s texture, i t i s referred t o as composite. The feldspar phenocrysts

a re subhedral t o euhedral. Quartz phenocrysts are equant and in part

id iomrphic , b u t generally appear rounded in cross section. They a re

fractured, and some of the fractures appear to be f i l l e d by groundmass

K-feldspar fingers. The groundmass varies from as much as 80 percent

to about 10 percent. Flow structures and feldspar segregations within

the groundmass are locally present. There seems t o be a complete

textural gradation i n hand specimens between compos i t e and ser i a t e ,

although the two are clear ly the dominant textures within zone 3.

The t ransi t ional textures are those in which most of the minerals a re

s e r i a t e t o coarser grain size 's , b u t with a small proportion as. dis-

t i nctly f iner and even-grai ned aggregates i nters t i t i a1 t o 1 arger crystal s .

The phenocryst and groundmass mineral populations have d i s t inc t ly

d i f fe rent petrographic charac ter i s t ics . The plagi ocl ase phenocrys t s

display variable zoning relat ions and have maximum anorthi t e content

of about Anl8 . Where the zoning relat ions a re most obviour , they closely

- resemble facies 3A plagioclase, b u t the most d is t inc t ive charac ter i s t ic

of plagioclase crystals in th is facies i s t ha t many phenocrysts do not

F i g u r e 14. kep resen ta t i ve specimens o f t e x t u r a l f ac i es 3B.

F igu re 15. Exposed con tac t between f a c i e s 3A (above) and fac ies 38 (below) rocks l oca ted 1.5 m i nor theas t o f Serpent ine Hot Springs. Fac ies 38 rocks a t t he con tac t a r e composed o f d iscont inuous very f i ne -g ra ined l i g h t and dark bands. Dark bands con ta in more b i o t i t e , m ~ g n e t i t e , and fl u o r i t e .

Feldspar-quar tz pegmati t e i s l o c a l i z e d a t t h i s con tac t immediately t o 1 e f t o f photo. Pocket con ta in ing .replacement selvage i n f ac i es 3A rocks ( f i g . 24) i s exposed a long a separated j o i n t p lane about 30 f t t o t he l e f t of t h e photo.

show mar~ed internal compositional variations. This i s t rue fo r large

crystals tha t cl early have we1 1 -developed osci 11 atory zoning as well

as more typical phenocrystc tha t only appear t o be markedly zoned

because of twinning discont inui t ies or se lec t ive secondary mineral

replacements such as ser ic i t i c bands or cores replaced by se r i c i t e ,

clay minerals, or iron hydroxides. The textural contrasts within the

phenocrys ts , i ncl udi ng the apparent or real zoning b u t a1 so we1 1 -developed

a1 bi t i c overgFowths , suggest a degree of compositional vari abi 1 i ty

that actual ly i s not. present . The textural i nhomogenei t i e s appear t o

be ghost features inherited from original crystals that l a t e r were

compbsition~lly homogenized.

The K-feldspar phenocrysts can be as large as any in the compl ex

b u t they a re re1 a t i vely less perthi t i c and contain fewer i ncl usions . Individual crystals a r e s t r ing microperthites and vein per th i tes ,

have grid twinning, and contain small plagioclase and ovoid quartz inclu-

sions in sutequal amounts. Overgrowths of nonperthi t i c K-feldspar a re

c o m n . Biot i te phenocrysts a re subhedral t o euhedral, d a r k brown t o

greenish brown, contain a moderate abundance of pleochroic halos, and in

addi t i on t o apati t e , commonly i ncl ude magneti t e and some re1 a t i vel y 1 arge

euhedral zircon. -Overall , .accessory minerals a re n o t abundant; magnetite

i s most prevalent, being b o t h intergranular and included in b i o t i t e , and

some well -formed sphene, a1 lani t e , and zircon crys ta l s a re present.

The groundmass i s a1 lotriomorphic t o hypidiomorphi c and equigranul a r ,

containing poorly twinned homogeneous plagioclase (Anl0), nontwinned

i n t e r s t i t i a l and par t ia l ly connected K-fel dspar, subequant quartz,

and generally shreddy b i o t i t e crystals that a re commonly replaced by

chlori t e and 1 esser muscovi t e .

Rocks of Zone 4

Rocks of zone 4 are exposed over an area of l e s s than 0 .5 5q mi

in the centra1,part of the complex. 0utc.rops are n o t present in th i s

a rea , and a1 1 rock exposures are 1 imi ted to surface accumulations. ,of

frost-riven rubble. The zone i s roughly t ea r shaped in plan and i s

topographically expressed along i t s southzrn end as a small domal hi 11 . I t was or iginal ly recognized as a 5eparat.e unit because of the presence

of fine-grained equigranular and lqucocratic granite which i s not

found elsewhere in t he complex. Other distinguishing aspects a re the

dominance o f fine-grained a1 lotriomorphi c t o hypidiomorphic equigranul a r

rocks, and the occurrence of abundant small cpen miarol i t ic cavi t ies

about 1 mm acrors , quartz s t r ingers and 'veins, and some disseminated

su l f ide mineral ( s ) . The rocks a re transected by several f au l t s along

which zones of a r g i l l i c and related a l te ra t ion are present,

As outcrops a r e lacking, t h i s zone was studied by examination of

thin sections and hand specimens o f rocks coll rc ted, fo r the most pa r t ,

from f l o a t along a t raverse across i t s maximum dimension. I n t h i s

t raverse , a1 1 1 i thologi c variations were sampled. The speci men5 show

that two textural facies predominate and tha t rocks textural ly and - . cornposi tional l y simil a r to those typical of facies 3A- also are present.

The l a t t e r are s e r i a t e and subordinate in abundance to those of facies

4A and 4B, which a re both f ine to medium grained, a1 lotriomorphic to

hypidiomorphic, and equi granul a r , b u t facies 4A i s b io t i t e granite and

- facies 48 i s leucograni t e ( f i g . 16) . Rocks o f facies 4A are widespread

through the zone, b u t facies 4B rocks a re res t r ic ted t o arzas near

hydrothermally a1 tered f au l t zones. The presence of facies 3A type

rocks, largely surrounding t h i s zone a s well as scat tered through i t ,

Figure 16. Represen ta t i ve specimens of t e x t u r a l f a c i e s 4A ( t o p ) and 4B (bo t tom) .

suggests tha t zone 4 may be intrusive into facies 3A, b u t the f i e l d

relat ions a re ,inadequate t o clear ly reveal the nature of the contacts

of th i s zone.

Facies - - A 4A--Fi ne- to rnedi urn-grained equigranul a r yrani t e . Rocks

of th i s facies a re f i n e t o medium grained, allotriomorphic t o hypidio-

morphic, and equi granular with a few scat tered megacrysts of feldspar

and quartz. The quartz i s l i g h t gray t o medium 1 ight gray, and together

with b i o t i t e ( 2 t o 3 percent) imparts a l i g h t gray color to the rocks.

Spotty oxidation of b i o t i t e , se r i ci t i za t ion of pl agiocl ase, and oxidation

of disseminated su l f ide (? ) resu l t s in minor buff or cream di scolorati ons,

Miarolit ic cavi t ies a r e open, about 1 t o 2 mm across, and appear t o

make u p 1 to 2 percent of individual specimens.

Most plagioclase c rys ta l s in facies 4A are not obviously zoned,

b u t a few are , The zoning charac ter i s t ica l ly consists o f d i s t inc t

cores, defi ned by sel ec t i ve secondary mineral rep1 acements and di f fe ren-

t i a l ext inct ion, t ha t a re surrounded by a s l igh t ly normal zoned rim.

Oscillations a re local ly present, b u t generally a r e not common. The

optical and compositional character is t ics o f the facies 4A plagioclase

are s imilar t o those of facies 4B except for the presence of the scattered

zoned crystals in facies 4A.

The K-fel dspar i s dominantly anhedral , has undulose e x t i nction ,

i s microperthi t i c in par t , and has variably developed grid twinning.

In some specimens i t i s clouded by development o f very fine-grained

c l ay (? ) minerals. Quartz i s anhedral and i n t e r s t i t i a l t o subequant.

Graphic intergrowths a re locally well developed. Biot i te i s reddish

brown, subhedral t o euhedral , has moderately abundant pl eochroi c ha1 os ,

and contains small incl usions of sphene, zircon, and apa t i te . General l y

i t i s spo t t i l y interleaved with ch lo r i t e , b u t extensive replacement

occurs in those specimens in which the K-feldspar i s clouded. Accessory

minerals a r e sparse and res t r ic ted t o the sphene, zircon, and apa t i te

included in b io t i t e .

Facies .4B--Leucocratic grani te . The rocks of t h i s facies are the - most conspicuous in zone 4 , and they appear to be spa t ia l ly associated

with the hydrothermally al tered f a u l t zones. They are character is t ical ly

f ine grained, a l l otriomorphic (except fo r plagioclase) , equigranul a r ,

and leucocratic. Fresh surfaces a re very l igh t gray because b io t i t e

i s sparse and the quartz i s l i gh t gray. Buff discolorations are common,

due t o spotty iron hydroxide stainings t h a t are in part associated with

miarol i t ic cavi t ies . T h e cavi t ies a re about 1 mm across, contain small

amounts of earthy rust-stained material , and appear to b e as abundant

as those in- facies 4A. Thin quartz veinlets a re common and are discon-

tinuously sheeted and interfingering in places, A few scattered medium-

grained megacrysts of quartz and bioti t e are locally present.

The feldspars a re pa r t i a l ly clouded by clay minerals and contain

scattered small grains of muscovi t e . . Plagioclase ( A n g y l 5 ) forms short

subhedral grains tha t characteri s t i cal ly have even ext inct i on and are

unzoned. K-feldspar i s fa in t ly microperthi t i c , has undulose extinction,

and the grain boundaries range from orthogonal t o very i r regular and

i n t e r s t i t i a l . Graphic intergrowths are locally present. Quartz forms

intergranul a r anhedral grains and small aggregates. Biot i te occurs

as remnant grains that have been par t ia l ly replaced by muscovite.

Muscovite a1 so occurs as we1 1 -formed i ntergranul a r c rys ta l s of average

grai n s i ze. Accessories a re conspicuously absent except for very

small unidentifiable remnant inclusions in a l te red b io t i t e and relat ively

coarse f luo r i t e . Opaque material i s sparse, and most i s pa r t i a l ly

oxi di zed t o bri g h t orange-red i ron hydroxi des .

Microscopic Textures

' Microscopic textures of speci a1 i nteres t i ncl ude t h ~ s e re1 ated

to subsol idus a1 bi t e , myrmeki t e , graphic i ntergrowths , and fringed

b i o t i t e . Subsol i dus a1 b i t e , associated myrmeki t e , and fri nged b i o t i t e

a re present in most parts of the complex, b u t graphic intergrowths Lre

fotlnd only in those rocks tha t can be inferred t o have crystal l ized

rapidly and in the presence of a vapor phase.

Subsolidus A1 b i t e

A1 bi t e (An,5.) i s charac ter i s t ica l ly associated with coarser grains - of K-feldspar throughout the complex. In addition to 1 amel lae , patches,

and veins in perthi t e , i t comnonly forms mantles on plagioclase inclu-

sions, marginal rims on intergranular plagioclase, and small , discre te ,

i n t e r s t i t i a l grains that a re generally associated with K-feldspar

boundaries. Some examples are i 11 ustrated i n f igure 1 7 .

Rims on plagioclase c rys ta l s are by f a r the most common occurrenc.

and are present a t most K-feldspar-plagioclase contacts. The a1 b i t e

i s o ~ t i c a l l y continuous with substrate plagioclase b u t the contact

between them i s sharp and i s usually defined by differences i n exti nc-

tion and in many places by variations in twinning. Where the contacts

w i t h K-feldspar a re semi paral le l t o (001 ) of the substrate plagioclase,

the a l b i t e forms wide rims t h a t a re i r regular ly convex into the K-feld-

, spar c rys ta l s . Where the a1 bi t e has formed a t boundaries semi paral l el

t o (010) of the substrate plagioclase, the rims a re thinner and s t r a igh t

along both contacts. A1 b i te local ized along other K-feldspar boundari es

F igu re 17. Photorni crographs (UXN) of subsol i dus a1 b i t e ; overgrowth on i n c l uded p l a g i o c l a s e crysta l i n p e r t h i t e ( t o p ) and sha rp l y de f ined r i m on p l a g i o c l a s e a t c o n t a c t w i t h p e r t h i t e (bo t tom) .

i s much less common, and where found i s present as small d iscre te grains.

Inclusions of pl agioclase in K-feldspar are mantled with a1 b i t e ,

The pl agiocl ase nuclei range from homogeneous subhedral cores with

sharp differences i n extinction and/or twinning from the a l b i t e t o

i rregul a r , hazy, and anhedral centers wi th i n strongly normal -zoned

grains tha t a re generally small e r than the fir s t type. Cornpos1'tions

of incl usions were determined in many specimens (mostly from zones 1

and 2 ) , and grains with sharply defined homogeneous cores a re charac-

terized by a bimodal anorthi te content in t h a t the a l b i t e rims surround

cores that a re Anz0 regardless of t h e composition of intergranular

plagioclase elsewhere in t he specimen. Some larger inclusions do not

show th i s relat ionship, b u t i t i s nonetheless con,mon.

The a1 b i t e i s s imilar in many ways t o t ha t described by Ramberg . .

.

(1962) in granulite-facies metamorphic rocks. T h a t i t i s due t o

subsolidus crystal1 ization i s evidenced by the pronounced association

with perthi t i c K-feldspar crystals and especial ly by the local ization

of the a l b i t e a t K-feldspar-plagioclase boundaries. The rims on in ter -

granular plagioclase appear to have grown in part by the replacement of

K-feldspar, and essent ia l ly they form overgrowths on the substrate

plagiocl ase (Ramberg, 1962, p . 21 ) . However, tha t the crystal 1 ization

of a l b i t e adjacent to plagioclase i s not s t r i c t l y an exchange process

within the K-feldspar s t ruc ture i s evidenced by the composition of the

plagioclase inclusions in the K-feldspar, as well as by the fac t tha t

the contact with substrate plagioclase i s not s t r i c t l y conformable

with the primary crystal out l ine. The bimodal compositions of many

i ncl uded crystals indicate that sodi um t r ans fe r , in or o u t deperidi n g

on the composition of the or iginal ly included pl agiocl ase, has horno-

genized the nucleus plagioclase to a composition of about Anpg This

intimate coexistence of two plagioclases i s probably another example

of the lack of sol id sol ution in the low-temperature plagiocl ase se r i e s

(Laves, 1954; Barth, 1969 , p. 38-39). Inclusions tha t d o not show the

bimodal compositional relationship are probably of two kinds: (1 ) those

tha t are the resu l t of a1 b i t ? precipi ta t ion on an or iginal ly small

nucleus in an environment within the host K-fel dspar crystal where i t

cou1 d be a1 bi t ized almost completely, 1 eading to products that range

from homogeneous grains of a1 b i t e t o the small , strongly normal -zoned

grains with inhomogeneous i r regular cores, and ( 2 ) those that a re the

r e su l t of a1 b i t e precipitation on plagioclase incl usions too 1 arge t o

have been extensively homogeni zed by the subsol idus processzs , yi eldi ng

grains with unchanged original cores. The ava i lab i l i ty of a l b i t e i n

the vicini ty of included plagioclase, the s i ze of the incl u ~ z d plagio-

c lase grain, and the character of the K-feldspar s t ruc ture i n the vicini ty *

o f the inclusion a1 1 probably influenced the subsol idus development of

the products now seen in the specimens. Detailed study of the included

plagioclase crystals would y ie ld much information about the low-tempera-

ture relat ions in the low-An plagioclase s-cries and the mechanism of

exchange responsible fo r subsolidus adjustments i n these minerals.

Myrmeki t e

Myrmeki t e ( f i g . 18) i s charac ter i s t ica l ly associated w i t h subsolidus

a1 b i t e throughout the grani te complex. I t consists of very f i ne-grained

intergrowths of quartz and recrystal l ized pl agiocl ase on margins o f

primary plagioclase c rys ta l s , in mantles surrounding plagioclase

Figure 18. ~ho tom ic rb3 raph ( U X N ) o f intersti t ial myrmeki te.

inclusions in K-feldspar c rys ta l s , and i n discrete i n t e r s t i t i a l grains

of a1 b i te .

In rims on primary plagioclase the quartz in the intergrowths

occurs as thin plates t h a t generally radiate outward from very near the

substrate plagioclase boundary almost t o the edge of the a1 b i t e rim,

b u t in many places the q u a r t z becomes less abundant and terminates a t

some intermediate distance inward from the rim margin. The outer contact

of myrmekite conforms to the original p l a g i o ~ l a s e boundary. or i t i s

smoothly convex and bulbous into adjacent K-feldspar c rys ta l s , thus

producing the typical wartl i ke forms (Barker, 1970, p. 3 3 4 3 ) . As

described by Phi 11 ips (1964, p. 53) the marginal band containing myr-

meki t e can be subtly zoned outward t o . a lb i t e compositions, and th i s

i s the case in some of the rims that were examined. In some others ,

the zoning i s not so subt le and the quartz occurs in a d i s t inc t band

tha t has a composition intermediate between a l b i t e and the substrate

plagiocl ase.

I n t e r s t i t i a l myrmeki t i c a l b i t e contains quartz blebr , plates, and

rods tha t are variously oriented and general ly evenly di s t r i buted

throughout individual grains. These occur as discrete grains local i zed

a t the contacts between K-feldspar and the other framework s i 1 icates ,

as we1 1 as i n fine-grai ned aggregates. Grains within aggregates a re

generally anhedral and somewhat rounded 1 ooking , b u t d i scre te grains

along boundaries a re generally subhedral.

Myrmeki t e mantles on p l agiocl ase inclusions in K-fel dspar are

commonly simi l a r to the myrmeki t i c rirrts on i ntergranul a r pl agiocl ase.

However, the smaller and strongly normal-zoned inclusions tha t have

been more extensi vely a1 b i t i zed have i rregul ar ly oriented quartz bl ebs

only in central areas where the anorthi te content i s higher.

Fringed Biot i te

Throughout the compl ex, bi o t i t e c rys ta l s commonly have f r i nged

borders ( f i g . 19). These fringes a re very f ine , interfingering exten-

sions of the b i o t i t e tha t form a patchy interconnected network. The

spaces within the fringe network appear t o be occupied by material of

the crystals immediately adjacent to the fr inge or by very fine-grained

aggregates of feldspar and quartz. Some skeletal-1 i ke b i o t i t e c rys ta l s

occur as small discrete grains not associated with larger b i o t i t e plates .

The fringes a re most obvious a t the borders of b i o t i t e sections semi.-

para1 1 el t o (001 ) . They are developed along segments of the host b i o t i t e

border and do n,ot completely surround i t . The spotty occurrence and

local izat ion a t borders of larger c rys ta l s suggest t ha t t he fringes

a re t h e product of 1 a t e crystal 1 i za t i on processes.

Graphic Intergrowths

Graphic intergrowths ( f i g , 20) occur in plagioclase b u t a re be t t e r

developed and more c o m n in K-feldspar. They are found only in rocks

which can be inferred t o have crys ta l l ized re la t ive ly rapidly and in

the presence of a vapor phase, facies 1A (contact-zone rocks) , facies 38

(composite-textured rocks with apli t i c groundmass), and the fine- t o

medium-grained equigranular rocks of Zone 4 ( facies 4A and 4 B ) . In

addition, large s ingle host c rys ta l s with intergrowths (graphic grani tes)

a r e u p t o 8 i n , long i n the in t e r io r parts of pegmati t i c dikes. These • di kes , and the graphic grani t e within them a re discussed separately in

a l a t e r sect ion.

F igu re 19. Photomicrograph o f a f r i n g e d b i o t i t e c rys ta l (p lane p o l a r i zed 1 i gh t) . -.

F igu re 20. Photomicrograph (UXN) o f a graphic i n t e r g r o w t h of l u a r t z i n po tass i um-feldspar t h a t defines a d i s t i n c t marginal band i n a 1 arge p e r t h i t e c r y s t a l .

The graphic i ntergrowths tha t form complete grains in parts of the

granite a re anhedral and have i r regular boundaries. The quartz in these

grains forms equant t o elongate blebs tha t show no preferred orientation

b u t ' are optical ly continuous . The bl ebs in K-feldspar hosts are small e r

and more abundant than those in plagioclase.

Margi nal bands w i t h graphic i ntergrowths mantle many feldspar

phenocrysts in rocks of facies 1A and 38. K-feldspar phenocrysts have

marginal bands wi t h a few scat tered, angular quartz inclusions , b u t the

bands are characterized by thin zones with abundant small quartz blebs

tha t sharply define what may be the original phenocryst outline. Where

the quartz inclusions are elongate, they are generally se'miperpendi cul ar

to the interface between the marginal band and the in t e r io r K-feldspar.

As many as two such paral le l zones of small inclusions have been observed

in a s ingle margin, and t h e principal difference between the K-feldspar

of the in t e r io r and tha t of the margin i s tha t the margin lacks micro-

scopic a1 b i t e lamellae and has a m r e uniform extinction. The quartz

blebs in the thin zones consis t of several different opt ical ly continuous

subsets, and the inclusions belonging to a par t icular subset do not

a1 1 occur together. In pl agioclase phenocrys ts , rnargi nal bands with

graphic intergrowths are not conyon. Where present, they consist o f

sca t te red , equant, and relat ively large quartz inclusions in a band

characterized by having homogenpous plagioclase extinction. This band

i s external t o a d i s t inc t plagioclase core that d i f fe rs from the margin

in both extinction and zoning charac ter i s t ics . Individual quartz blebs

in the plagioclase are opt ical ly continuous with only a few others

scattered through i ntergrowths .

Meqascopi c Structures

Megascopi c s t ructures a re abundant, varied i n nature, a n d i nhomo-

geneously distributed in the complex. They include those related t o

fo l i a t ions , diking, incl usjons, joint ing, and cav i t i e s , These s t ruc-

tures , especially the planar fabrics , are locally very well developed,

b u t e f fo r t s toward defining three-dimensional aspects of the complex

are handicapped by the lack of outcrops and the i r var iab i l i ty of dis-

t r i bution. Many of the s t ructural features are in te r re la ted , b u t for

convenience they are discussed separately be1 ow.

Fol i at ions

Foliations a re defined primari ly by mi neral segregations, pheno-

c rys t or inclusion or ientat ion, and tabular swarms of inclusions.

Mineral segregations are by f a r the mas t important; good examples

a re shown in f igure 21. The segregations l'nclude biot i te-r ich bands,

th in , discontinuous fine-grained l eucocrati c zones, and c ? ots and

bands of coarse K-fel dspar crystal s . These commonly occur together,

and the b io t i te - r ich bands generally border the fine-grained zones and

coarse K-feldspar segregations.. - Individual segregations range in thick-

ness from about 1 in . to almost a foot . Large K-feldspar crystals

a re tabular and show a weak t o strongly developed preferred orientation

in several parts of the complex. Thei r orientation generally defines

a steep fo l i a t ion , and in areas where segregations ex i s t they a re

semiparallel t o t h e fo l ia t ion tha t i s defined by the mineral segregations.

Incl usions generally take the form of f la t tened e l l ipso ids , and t he i r

orientation a s well as the orientation of tabular swarms also defines

planar s t ruc ture in several l o c a l i t i e s . T h e d a r k inclusions in figure 21

F igu re 21. M ine ra l segregat ions i n fac ies 3A rocks . Dark b i o t i t e - r i c h bands a r e separated by l i g h t e r c o l o r e d bands of f i ne -g ra ined g r a n i t e and c l u s t e r s o f o r i e n t e d , 1 arge po tass ium- fe ldspar c r y s t a l s . Lensoid, dark xenol i t h s ( r i gh t cen te r , t o p pho to ) a r e o r i e n t e d para1 1 e l t o t h e segrega t ion bands.

i 11 us t r a t e the preferred orientation of i ndi vi dual s and, as shown ,

such orientations a re commonly parallel t o other planar fabr ic i n the

immediate v ic in i ty .

The planar fabrics are developed in zones 2 and 3 of the complex.

In zone 2 , K-feldspar phenocrysts a re locally aligned and the best

developed inclusion swarms a re present. In zone 3 , the seriate-tex-

tured faci es ( 3 A ) contains the schlieretil i k e mineral segregations, and

in the composi te-textured faci es (3B) a1 i gnment of K-feldspar pheno-

crysts i s obvious in many hand specimens as a r e some flow structures

within the a p l i t i c groundmass. Unfortunately, rocks of faci es 3B

crop out a t very few l o c a l i t i e s and fur ther information about the planar

fabrics i s unavailable.

Dikes

Porphyri t i c , apl i t i c , and apl i te-pegmati t e dikes a re present in

the complex. They a r e a l l of grani te composition and represent segre-

gations of residual materials w i thin mostly crystal 1 i zed par ts of the

pluton or d i s t i n c t intrusions of such materials into previously

c rys ta l l ized zones. Most a re l e s s than 2 f t in width.

The porphyri t i c di kes contain phenocrysts of bioti t e , quartz ,

K-feldspar, and plagioclase in an a p l i t i c groundmass. Only two such

dikes have been located. One i s a foliated dike 2 t o 5 in . thick,

a t an elevation of 800 f t in zone 2 , 1 m i south of Serpentine H o t Springs.

The other i s a d i r ec t offshoot, about 1000 f t long, of the main intrusive

body of facies 38 into zone 2 rocks a t a point on i t s northern contact

with zone 3 (shown on pl . 1 ) . These represent d i s t inc t intrusions

of par t ia l ly c rys ta l l ized materials into previously crysta-11 ized parts

of zone 2 .

Aplite dikes, commonly less than 3 in . t h i c k , are by f a r the most

widespread and abundant and are present in b o t h zones 2 and 3. In

zone 2 , they are the. most cormnon dikes and occur in groups of 2 o r 3

separate di kes tlrat are or i ented approximately paral 1 el t o one another.

They are more abundant in zone 3 b u t in t h i s zone they are discontinuous

and i nterconnected ( f i g . 2 2 ) .

Composite apl i te-pegmati t e di kes a re res t r ic ted to faci es 3A rocks

except fo r a few in zgne 2 . They a re textural ly and compositionally

zoczd. Apli t e typical ly forms the major portion of each dike and i t

borders the more central ly located pegmatite. The pegmatite contains

coarse K-feldspar, plagiocl ase, quartz, and in places tourmaline and

muscovi te . Composi tional i nhomogenei t y i s defi ned by variations i n

feldspar dis t r ibut ion and, t o a cer tain extent, i4i quartz d is t r ibut ion ,

Coarse I(-fpldspar has preferent ial ly crystal 1 i zed a t the s t rat igraphic

roof of the pegmati t e zone, and in some thicker dikes i t actually

forms graphic crysta's , several inches long, t h a t extend downward into

central parts o f the zone. The K-feldspar concentration i s accom-

panied by quartz, and plagioclase in turq has crystal1 ized a t the L>

bottom of the zone and i s comnonly f iner grained than the K-feldspar.

Some o f the thicker dikes, for the most part apli te-pegmatite

dikes , have many a1 te rna t i ng mineral segregation bands paral 1 el t o ,

b u t not symmetrically distributed about them, Figure 23 i 11 ustrates

th i s relationship and shows how f i ne-grained and apli t i c granite within

a zone of mineral segregation becomes more clearly defined up dip

where i t eventually has a pegmatite pod segregated in the central part

of an a p l i t e dike. Development of the aplite-pegmatite dikes in facies

F i gure 2 2 . Di scon ti nuous and interconnected ap l i t e di kes i n faci es 3A rocks. Thicker parts of the dikes form ridges on the weathered and p a r t i a l l y lichen-covered outcrop surface.

Figure 23. Composite, 3 ft wide, a p l i te-pegmati te d i k e ( t o p ) t h a t grades down d i p i n t o banded, f i ne-grai ned g r a n i t e tone (bot tom).

3A rocks i s believed to be the resu l t of segregation processes in which

resi dual mel t s and vol a t i 1 es were concentrated i nto di sconti nuous di kel i ke

bodies within the almost completely crystal l ized parent rocks. ' I t i s

probable tha t a l l variations from the hazy and thin mineral segregations,

such as i 11 ustrated in figure 21 , t o the well-defined, three-foot-thick

aplite-pegmatite dike of figure 23 a re present in facies 3A rocks.

There i s one known example of a pegrna t i t e pod wi thi n se r i a te-

textured, facies 3A rocks. Figure 24 shows th i s body, which i s zoned

into a lower apl i t e and an upper coarse quartz-feldspar intergrowth

with some vugs containing euhedral tourmaline and quartz. The apll ' te

i r in sharp cont?ct with the host rocks, b u t the coarse i ntergrowth

i s separated from the se r i a t e granite by a th in zone o f equigranular

ma t e r i a1 .

Cavities

Three principal types of cavi t ies occur within the granite complex:

( I - ) miaroli t i c cavi t ies tha t contain l a t e crystal 1 i zing minerals,

( 2 ) larger openings (pockets) that occur i n or near pegmatitic bodies,

and ( 3 ) small miarol i t ic cavi t ies that a re open voids without l a t e

crystal 1 izing minerals.

The miarol i t i c cavi t ies that contain l a t e c rys ta l l iz ing minerals

are found in the seriate-textured rocks (faci es 3 A ) of zone 3 . The

cavi t ies are i r regular openings u p to 2 cm across and contain euhedral

c rys ta l s o f dark-colored quartz, clevelandi t e , muscovite, tourmal ine,

and f luo r i t e ( i n order of decreasing abundance). The cavi t ies are

associated with coarser K-feldspar crystals and c lo t s ~f such crystals.

Characteristically small e r cavi t ies a re located immediately adjacent

f igure 24. Quartz-feldspar pegmatitic pod i n facies 3A rocks. Apli te segregation forms lower part o f the pod.

t o t h e b i g c r y s t a l s . They a r e r u s t s ta ined and an ea r thy buf f -brown

,mater ia l commonly coats t h e euhedral minera ls t h a t p r o j e c t i n t o the

voi ds . The pockets a re a l s o assoc ia ted w i t h s e r i a t e - t e x t u r e d rocks

o f f a c i e s 3A and most ly form openings as much as several inches across

i n pegmat i t i c d ikes and pods, b u t one i s an open v o i d 2 f t across who1 l y

w i t h i n s e r i a t e - t e x t u r e d rocks. ,The pockets i n pegrnati ti c rocks c o n t a i n

euhedral minera ls as i n t he case of t h e m ia ro l i t i c c a v i t i e s descr ibed

above, t h e p r i n c i p a l d i f f e rence between them being p r i m a r i l y one o f

scale. The w a l l s of t h e 2 ft wide pocket (noted on p l . 1 ) con ta in a

vuggy equi granul a r e n c r u s t a t i o n of euhedral c l eve1 andi t e , muscovi t e ,

quar tz , tourmal ine , f 1 u o r i t e , and a t r a c e of i lmeni t e t h a t rep1 aces

the s e r i a t e - t e x t u r e d g r a n i t e , i n a se lvage t h a t i s about 3 i n . wide.

Pseudomorphs o f muscovite ' and a1 b i t e a f t e r K- fe l dspar phenocrysts can

be d iscerned w i t h i n i t ( f i g . 2 5 ) . This pocket i s found near a massive

qua r t z - fe ldspa r pegmati te t h a t i s l o c a l i z e d a t t h e con tac t between

f a c i e s 3B rocks and t he fac ies 3A rocks they i n t r u d e ( f i g . 14) .

C o n t i n u i t y between the pegmati te and t h e 20 ft d i s t a n t ( a s now exposed)

pocket i s suspected b u t cannot be demonstrated because o f t h e l ack of

exposure.

The t h i r d t ype o f c a v i t y i s smal l m i a r o l i t i c c a v i t i t e s , t o 2 mrn

across, t h a t a re abundan t - i n t h e equ ig ranu la r rocks of zone 4 . They

a r e c h a r a c t e r i s t i ca l l y open and do n o t con ta in 1 a t e c r y s t a l 1 i z i ng minera l s . However, some a r e r u s t s ta ined and as i n t e r s t i t i a l f l u o r i t e and muscovite

a r e observed i n t h i n sec t i ons of these rocks , these minera ls may i n • p a r t occur i n t h e smal l c a v i t i e s . The c a v i t i e s a r e est imated t o

compose 1 t o 2 percent by volume o f the hos t rocks. Th is abundance

Figure 25. Vuggy replacement selvage along walls o f 2 f t wide pocket in facies 3A rocks. Selvage i s composed of subequal amounts of euhedral rnuscovi t e and a l b i t e w i t h lesser amounts of f luo r i t e , tourmal i ne , and ruti 1 e.

indicates tha t more i n t e r s t i t i a l aqueous f lu id was present in zone 4

than anywhere a l s e in the pluton.

As discussed above, openings a re most closely associated with

rocks of zone 3 and zone 4. However, some miarol i t i c cavi t ies containing

euhedral minerals a re found in dikes in country rocks and in rocks of

zone 1 ( fac ies 1A) very near the granite-country rock contact. Therd

i s a conspicuous absence of voids within the composite-textured rocks

o f facies 38.

Incl usi ons

Two principal tyyes o f i ncl usions a re present in the granite complex,

One type i s f ine t o medium grained, dark gray colored, and contains a

few megacrysts o f quartz, plagioclase, and pink K-feldspar. The other

i s brownish gray and has a "porphyritic" texture with coarse megacrysts

of quartz, plagioclase, and pink K-feldspar in a fine-grained aggregate

o f the same minerals together with b io t i t e . Those of the f i r s t type

may be t rue xenoliths b u t the others are auto1 i ths .

Common to both types. of i ncl usi ons a re f iner grai ned anhedral

microcl i ne and equant quartz , plain brown b io t i t e , and fine-grained

plagioclase with charac ter i s t ica l ly smoothed crystal edges. The darker,

finer grained inclusions contain more abundant b i o t i t e , d i sseminated

acicular apa t i t e , and green hornblende (<5 - percent). I n these, the

microscopic textures include inter1 ocking aggregates of small anhedral

grains , networks of skel eta1 , i ncl usi on-ri ddl ed quartz grai ns , and

radiating c lus te rs o f plagioclase la ths . In the inclusions with

"porphyri t i c " texture the f ine r grai ned m a t e r i a1 forms an inter1 ocking

aggregate of semi-anhedral grains similar to tha t in many darker,

fine-grained inclusions, b u t i t contains relat ively more abundant

microcline, and sphene and al lani t e as accessory mineral s . The mega-

crysts a re generally well formed b u t somewhat rounded. Reaction rims

are present on plagioclase c rys ta l s , and some K-feldspar i s mantled

by thin bands of plagiocl ase. Large plagioclase crystals have f a i n t

osci 11 atory zoning, 1 arge d i s t inc t in te r iors with homogeneous cxti nction

and well developed a1 bi t e twinning, and +hin reaction rims that local ly

contain minute inclusions and d i f f e r s l ight ly in extinction and twinning

from in te r io r areas. K-fel dspar megacrysts a re perthi t i c , with i rsegul a r

margins and par t ia l ly a1 bi t ized pl agiocl ase inclusions . Coarser b io t i t e

grains form some megacrysts and contain magnetite and zircon incl usions.

The f iner grained b io t i t e i s generally inclusion f ree .

I n form the inclusions are commonly i r regular flattened el l ipsoids

1 f t or l e s s in maximum dimension, a very f e w are angular, and some

a re tabular and u p t o 3 f t long. Where planar s t ructures such as

schlieren are present t h e inclusions a re aligned with them ( f i g . 2 1 ) .

They can be is01 ated or occur in groups tha t range from a few irregu larly

scattered in a small area to dense pipelike or tabular swarms that can

be several feet wide and several tens of f ee t long ( f i g . 2 6 ) . I n the

groups, ,both principal types a re found together though one can predominate

over the other.

The inclusions are exposed in outcrops of both zones 2 and 3 b u t

they are most abundant in zone 2 , Zone 2 contains the dense swarms

as well as the numerous small c lo t s , 1 ess than 2 cm i n t he i r maximum

dimension, of fine-grained plagioclase, b io t i t e , and 1 esser green horn-

bl ende tha t are present in more than ha1 f of the hand specimens examined.

F i g u r e 26. P i p e l i ke swarm o f i n c l u s i o n s i n f ac ies 2 r ocks on r i dge - 1 i n e west o f Serpent ine Hot Spr ings. I n c l usions are as much as 1 f t across i n t h e i r maximum dimension. Photograph by R, E w i ng , 1970.

Joints

Joints a re present in a1 1 large outcrops, and the outcrop forms

are determined by the orientation and spacing of the jo in ts because of

preferential weathering along them. They a r e best developed in zone 2

b u t also a re present in parts of z3ne 1 and zane 3. They are systemati-

cal ly oriented b u t are variably spaced, and three se t s usually can be

recogni zed i n i ndi vi dual outcrops.

Two part icular ly prominent se t s in te rsec t a t a high angle. One

trends about N . 65' E. and i s steeply dipping, and the other trends

N. 37' W . and dips moderately to the northeast in central areas b u t

i s 1 ess systematically oriented and steeper in marginal areas. The

third s e t s t r ikes obliquely to one of the other sets a n d generally dips

in the opposite direct ion.

Joints crosscut l a t e intrusive units such as a p l i t e and pegmatite

dikes and small porphyritic plugs. They also have been observed t o

transect dark inclusions. In many places, dikes s t r i k e parallel t o one

of the jo in t s e t s b u t dip in an opposite direction. Locally (especially

in zone 2 ) , some fracture systems containing l a t e muscovite, quartz,

tourmaline, and f luo r i t e a re parallel t o one of the clear ly recognized

jo in t se ts , and the l a t e minerals appear t o have healed them. In

eastern parts of the pluton where rocks of zone 1 a re present , much

d i f fe rent ia l movement has occurred along para1 1 el s e t s of throughgoi ng

j o i n t s ( ? ) that i n part appear t o extend in to country rocks. They are

evidenced by l inear topographic expressions, the presence of sl icken-

sided and pol i shed granite fragments, and zones of increased hydroxide

staining due to oxidation. A1 though oxidation appears t o have been

1 ocal i zed a1 ong these fractu-res , they have not been mi neral i zed and are

mapped as f a u l t tones on plate 1 ,

Deuteric Features

Products o f deuteric crystal1 i zation a re present i n minor amounts

in a l l the textural facies of the complex. They include materials

related to the a l te ra t ion of b i o t i t e and feldspar and t o precipitation

in i n t e r s t i t i a l voids. The character of the deuteric crystal 1 i zation

changes through the facies sequence.

In the facies of zone 1 , the principal deuteric product i s muscovite.

I t forms i n t e r s t i t i a l grains, scat tered and relat ively abundant inclu-

sions i n plagioclase, and small grains tha t are adjacent to o r in te r -

leaved w i t h b io t i t e . The bioti ee i s commonly preferential ly replaced

along the cleavages by a pale green ch lo r i t e w i t h gray interference

colors. Deuteric crystal1 ization may merge w i t h hydrothermal a1 terat ion

in facies 1A where se r i c i t e i s common in plagiocl ase, K-feldspar

i s very clouded by minute inclusions tha t are probably clay minerals,

f l u o r i t e i s associated with late-formi ng muscovi t e , and schorl i s present

i n many cavi t i e s . I n zone 2 , deuteric c rys ta l l iza t ion i s characterized by the develop-

ment of, g r a ~ s - ~ r e e n ch lo r i t e ( w i t h b l ue-gray i nterference colors) t h a t

replaces b io t i t e along cleavages and in patches. Some of the b i o t i t e

i s a bright red brown near chlor i t ized areas , b u t other la te-s tage

products a re res t r ic ted to scat tered muscovite c rys ta l s i n parts of some

plagiocl ase grains.

In zone 3 , the seriate-textured rocks ( fac ies 3A) characteri s t i ca l ly

contain much deuteric rnuscovi t e tha t forms in t e r s t i t i a l grains, as we1 1

as patchy crystals in a1 tered plagioclase, particul ar ly i n the in t e r io r s

of zoned. crystals . I t i s general'ly accompanied by small , anhedral ,

disseminated and i n t e r s t i t i a l grains of f l uori t e . Grass-green chlori t e

wi t h bl ue-gray interference colors patchily replaces some bioti t e

that i s in part s imilar t o a l tered b i o t i t e in zone 2 . In composite-

textured rocks (facies 3 B ) , the groundmass crystals a re character is t ical ly

more al tered than the phenocrysts. Small shreddy grains of b io t i t e

a re par t ia l ly t o completely replaced by pale to deep-green chlori t e ,

and muscovite i s re lat ively common as intergranul a r t o i n t e r s t i t i a l

grains and as crystals attached to the margins of larger b io t i t e grains.

I n t e r s t i t i a l and anhedral f luo r i t e i s re lat ively common, and feldspars

a re variously clouded by minute f ncl usions,

In zone 4 , the effects of deuteric c rys ta l l iza t ion merge with

those o f hydrothermal a1 te ra t ion . In facies 4 A , equi granul ar rocks

w i t h b i o t i t e , the b i o t i t e i s par t ia l ly oxidized, i s bright red brown,

and i s interleaved with muscovite. Muscovite a lso occurs i ntergranul ar ly

and as replacements in plagioclase. I n t e r s t i t i a l f luo r i t e i s common

and some i n te rs t i t i a l tourmal i ne i s present. Chlori t e i s lacking..

In faci es 48, equi granular and 1 eucocratic rocks, muscovi t e occurs as

somewhat shreddy i ntergranul a r grains and i n t e r s t i t i a l f luo r i t e i s

common. The shreddy muscovite has thin inclusions of opaques and sphene

oriented along the cleavages, as well as discontinuous shreds that are

pleochroic in shades of brown, thus indicating that i t largely represents

replaced b io t i t e c rys ta l s . The altered nature of the feldspars in

facies 4B varies considerably. In some specimens both plagioclase and

K-feldspar are extensi-vely clouded by minute inclusions, and in others

plagi ocl ase has been preferenti a1 1y rep1 aced. Yet there a re faci es 4B

specimens i n which n e i t h e r f e l d s p a r shows s igns of l a t e - s t a g e a l t e r a -

t i o n , Probably t h e r e i s complete t r a n s i t i o n f rom rocks o f f a c i e s 4B

t h a t con ta in u n a l t e r e d fe ldspars , q u a r t z ve in ing , bleached qua r t z , and

muscovite a f t e r b i o t i t e t o t h e e x t e n s i v e l y a r g i 11 i z e d and s i 1 i c i f i e d

rocks present i n the hydrothermal l y a1 t e r e d f a u l t zones t h a t c u t Zone 4.

I n summary, muscovite, c h l o r i t e , and c l a y minera ls a r e the dominant

1 a te-s tage products o f d e u t e r i c c r y s t a l l i z a t i o n . These m i nera l s a r e

accompanied by i n t e r s t i ti a1 anhedral f l u o r i t e and l o c a l l y by schor l . Both fac ies 1A and f a c i e s 48 rocks show evidence of ex tens i ve l a t e -

s tage changes t h a t appear t o merge w i t h those o f hydrothermal a1 t e r a t i o n .

These changes i n v o l v e l o c a l l y ex tens ive replacements o f f e ldspa rs by

s e r i c i te-muscovi t e and c l ay mi ne ra l s and muscovi t e rep1 acements of

b i o t i t e . I n t he f a c i e s 1B-1C-2 t r a n s i t i o n , the p r i n c i p a l d e u t e r i c

product i s c h l o r i t e . The c h l o r i t e rep laces b i o t i t e and becomes da rke r

green and more anomalous i n i n te r fe rence c o l o r s inward through t h i s p a r t

o f t he f a c i e s sequence. C h l o r i t e s i m i l a r t o t h a t i n f a c i e s 2 i s p resent

i n f a c i e s 3A b u t t he l a t t e r conta ins more muscovite and f l u o r i t e than

f a c i e s 2. The groundmass of f ac i es 3 B conta ins very few n o n c h l o r i t i z e d

b i o t i t e g ra ins , and muscovite, f l u o r i t e , and c l a y ( ? ) m ine ra l s a re a1 1

r e l a t i v e l y abundant. Deuter ic c r y s t a l 1 i z a t i o n i n zone 4 i s l o c a l l y

complex b u t o v e r a l l i t i s charac ter ized by the presence of muscovite

and t h e absence o f c h l o r i t e .

P l ag i o c l ase Compos i ti on

P l ag ioc l ase composition. va r i es through the fac ies sequence, sugges-

t i n g t h a t t he re are severa l d i f f e ren t p l a g i o c l ase popu la t ions w i t h i n

t h e complex.

Composition of the plagioclase was determined opt ical ly using the

a-normal wthod, and the resul ts a re shown diagrammati cal ly in f igure 2 7 .

Only one composition. was measured f o r any part icular grain, even though

zoning commonly was present. This i s because compositions within

individual c rys ta l s commonly bracket Anpl , the composition about which

extinction angles vary only s l igh t ly from oO, and because, even in

c rys ta l s w i t h conspicuous zoning features , the compositional va;,i a t i on

commonly i s s l i g h t . The compositional variation within zoned crystals

i s comparable t o the overall variation of plagioclase composition

in facies l B , l C , 2 , and 3A, but in facies 1A and 3B, the overall

variation within the facies is greater than tha t observed in any single

zoned crystal . Composi t i ons determined on subsol i dus a1 b i t e crystals

are shown in f igure 27 for facies 3A rocks only. Similar subsolidus

a l b i t e i s 'present in the other faciks. The data fo r facies 4A and 48

are not s igni f icant ly d i f fe rent and have been plotted together in

figure 27 .

As shown in figure 2 7 , plagioclase composition in facie: 1 A varies

over a wide range. I t i s relat ively uniform within facies 18, lC,

and 2 , and +he anorthi t e con ten t increases s l igh t ly i nward through

these fac ies , The range in composition i s greatest i n facies 3A, and

the determinations f a l l i n to a group a t about An25-27 and a group between

A n l 2 and AnZ0. The composition i s somewhat evenly dis t r ibuted a b o u t

average values i n facies 3B and Zone 4 , and shows a s l i g h t s h i f t toward

a l b i t e in the l a s t c rys ta l l iz ing fac ies , The general trend in the

variation of plagioclase composition i s one in which the average

anorthi te content increases from f a d e s 1 A through facies 2 ( A n l 7 to

A n 2 5 ) , goes through some intermediate val ue i n facies 3 A , then drops

l b a * *. e e l m * ma me. a . ma. 6 0

. * * m e .

4.. m * l me.. .** .

toward a1 bi t i c compositions (<Anll) i n facies 38 through 48. The

detai 1 s of the compositional var iat ions, corrbined with petrographic

variations, a re important and are discussed more completely below.

The compositional data suggest t h a t there a re several different

pl agiocl ase popul ations within the complex: (1 ) heterogeneous crystals

o f facies 1A, ( 2 ) a t ransi t ional population that includes crystals of

facies 1B, 1 C , 2 , and the ca lc ic oligoclase of facies 3A, ( 3 ) a l b i t i c

o l i gocl ase in faci es 3 A , ( 4 ) the facies 38 c rys t a l s , and ( 5 ) the Zone 4

crys ta l s . The facies 3B plagioclase compositional data apply to both

phenocrysts and groundmass crystal s and, a1 though there i s no dis t i nct

break in composition between them, the sharp break in grain s i ze clearly

defines two populations in th i s facies . The playioclase compositional

and petrographic data actually earmark several composite crystal popula-

t ions in the complex. These crystal populations a re ident i f ied and .'

discussed more completely in a l a t e r section.

Modal Composition

Modal determinations were nude on selected mineral populations

in facies 3B, 4A, and 48 rocks. Modes on these facies were res t r ic ted

t o the f iner grained m i neral populations that surround a we1 1 -dew1 oped

phenocryst population in the case of facies 3B rocks and a few

scattered metacrysts in the case of facies 4A and 4B rocks. The modal

composition of the groundtiass in facies 38 rocks was determined by

point counting thin sections stained for K-feldspar. The modal composi - tions of a1 1 other specimens were deterrni ned by point counting ,.I stained

rock s labs.

Mod21 compositions were obtained fo r samples coll ected from a1 1

parts of the complex, and a l l the modal P1:Q:Or ra t ios a re plotted

t o g e t h e r i n f i g u r e 8. The r a t i o s a r e even ly s c a t t e r e d and do n o t de f ine

any obv ious v a r i a t i o n t r ends . Because o f t h e o v e r a l l g r a n i t e com-

p o s i t i o n of t h e complex, t h e modal v a r i a t i o n s a r e n e c e s s a r i l y sma l l ,

b u t s i g n i f i c a n t d i f f e r e n c e s i n modal compos i t ion do e x i s t among t h e

p r i n c i pa l t e x t u r a l f a c i es . Only t h e e s s e n t i a l m ine ra l s and b i o t i t e

were i n c l uded i n t h e modal determi n a t i o n s ; accessory minera l s a r e

p resen t i n amounts l e s s than 1 percen t .

V a r i a t i o n --- o f Modal Averages

The modal averages f o r each f a c i e ~ a r e p l o t t e d i n f i g u r e 28.

A11 i n v o l v e whole- rock modes except t h a t f o r f ac i es 3B, which i s t h e

average mode of t h e a p l i t i c groundmass. The p l o t t e d averages de f i ne

seve ra l i mpor tan t v a r i a t i o n s : ( 1 ) b i o t i t e v a r i e s independent l y of t h e

o t h e r e s s e n t i a l m l n e r a l s ' a n d shows a gradual i nc rease t o a maximum o f

6 pe rcen t i n f a c i e s 2, t hen decreases i n more i n t e r i o r f a c i e s u n t i l i t

i s o n l y a m inor c o n s t i t u e n t o f t h e l e u c o c r a t i c f a c i e s 40 rocks, ( 2 ) t h e

p l a g i o c l ase and K - f e l dspar p r o p o r t i o n s d i v e r g e s i g n i f i c a n t l y f r om t h e i r

more sys tema t i c t r ends i n 1By 1C, and 2 , ( 3 ) t h e sys tema t i c v a r i a t i o n s

among f a c i e s 1B, l C , and 2 a r p d e f i n e d by an i n c r e a s e i n p l a g i o c l a s e ,

a s 1 i gh t l y i ncreas i ng o r approx imate ly cons tan t K- fe l dspar percentage,

and a cor respond ing d e p l e t i o n i n q u a r t z , ( 4 ) t he groundmass o f f a c i e s 30

i s s h a r p l y d e p l e t e d i n p l a g i o c l ase and cor respond i ng ly con ta ins i nc reased

q u a r t z and K - fe l dspa r percentages, ( 5 ) f a c i e s 4A c-ont inues t h e p l a g i o -

c l a s e enr ichment t r e n d and i s dep le ted i n K- fe ldspar , and ( 6 ) f ac i es 40

-con ta ins more p l a g i o c l a s e t han any o t h e r f a c i e s , an i nc reased K- fe ldspar

c o n t e n t ove r chat of f ac i es 4A, and a l ow percen tage o f qua r t z . Q u a r t z

i s b e l i e v e d t o be t h e dependent modal v a r i a b l e i n t h e f l u c t u a t i o n s o f

F igu re 28. V a r i a t i o n o f modal averages through the f a c i e s sequence. All data determined from whole-rock p o i n t counts except t h a t f o r f a c i e s 3B which i s f o r t h i n s e c t i o n p o i n t counts o f t h e a p l i t e groundmass o n l y .

the framework s i l i c a t e percentages in a l l the facies except 4B. Many

of these variations a r e important t o an understanding of the nature

of the separate crystal populations in the complex tha t a re ident i f ied

and discussed far ther o n ,

Chemi cal Composition

The major- and trace-element composi tion o f 16 who1 e-rock samples

and the trace-element composition of 18 mica separates from the complex

have been determined by personnel of the Branch of Analytical Laboratories

of the U . S. Geological Survey, and individual analysts are credited

in the tables . The samples were collected from points throughout the

complex and are typical of the facies they represent. The resu l t s fo r

major oxides a re l i s t e d in tab le 2 , for whole-rock t race elements in

tab1 es 3 and 6 , and for b i o t i t e t race el ements in tables 4 and 5 .

The purpose of t h i s section i s to c l a r i fy the overall composition of

the complex by comparison of i t s average whole-rock nlajor- and t race-

element and biot i t e trace-element compositions wi t h some average grani t e

and b i o t i t e compositions and to determine the n ~ t u r e o f the principal

chemical variations. Much of the major- and trace-el ement variation

i s systematic through the facies sequence and helps to c l a r i fy important

aspects o f the crystal 1 ization history.

Variation o f Major Oxides

Most o f the major-oxide compositions l i s t ed in table 2 are shown

diagrammatically in figure 29. The variation diagrams of figure 29

bring out several important relationships: ( 1 ) there i s a marked

l inear dependence of Ti02, CaO, MgO, Fe203, and A1 203 values with

Si02 content, ( 2 ) K20 values decrease s l igh t ly with increasing Si02,

m o m

~ ~ ~ ~ Z q ~ ~ o o a U . . U E r ~ o ~ o o o o d d d d d o o o o o '3 2 PI m 3 3 m m o o ~ ~ ~ ~ ~ r - m m ~ 0 3 0 m

0 0 0 3

c N . . . . , . . ? ? ' 1 1 9 9 9 ? 9 a '1 a ~ o ~ o o o o d o o o o o o o o o a : *

Table 3.--Whole-rock trace element composition of selected granite samples i n p p ; C h r i s Heropoulos, a n a l y s t * .

F i e l d No. Facies Mn B Ba Be C e C o C r Cu La Nb Nd Pb Sc Sn Sr V Y Zr Ga (1) (7) ( 2 ) (1) (70) (2) (1 ) (1) (301 (7) 170) (7) (2) (7) ( 5 ) ( 3 ) (10) ( 5 ) (2)

Average 2 9 7 13 486 7 131 2 2 4 - 9 8 44 63 66 4 15 i73 1 0 6 0 148 21 <ZOO 6

' S e m i q u a n t i t a t i v e s p e c t r o q r a p h i c a n a l y s e s ; prscedure described i n U . S . G . S . Bulletin 1084-1.

Looked for but n o t d e t e c t e d (N); A s , Au , N i , rrl, P t , Sb, Te, U, W, Zn G e , Hf, I n , Re, Ta, ~ h , TI, Pr, Sm, Eu.

( ) Lower Limit of d e t e c t i o n i n ppm.

Table 4 . - - R e s u l t s of semiouanti tative spectrographic analysis of biotite separates from the qrani te complex, all results in p p ; R . E. M a w , a n a l v s t .

F i e l d N o . Fac ies Ag B Ba B? Co Cr Cu Nb Ni Pb Sc Sn Sr V Y Z n Zr Li (1) t7) ( 2 ) (51 (2) (1) tll (7) 1 ( 7 ) (2) ( 7 ) (5) ( 3 ) (101 t100) (5) 1200)

Average (excluding 6 9 A H 3 7 D and 69AH434) 0 . 5 0.0 285 1 . 8 20 14 36 175 9 52 17 175 10 71 9 920

'Muscovite from an albite-muscovite selvage a long wa l l s of a large pocket i n rocks o f f a c i e s 3A. G Greater than va lue shown. 1 Not detected a t l imi t of detect ion. Looked for but n o t detected; A s , Au, Bi, Cd, La, Mo, P d , P t , Sb. Te, U , W, Ce, Ge, Hf, In, Re, Ta, T h , T1, and Eu ( I Lower l i m i t s of detect ion, i n ppm.

Table 5 . - - R e s u l t s of quantitdtive spectrographic a n a l y s e s ( 1 ) for selected trace elements in biotite s e p a r a t e s from t h e grdnite complex, a l l r e s u l t s in pDm; R. E . Mags, a n a l q s t .

Field No. Facies B ( 7 )

1 A N 2 0

18 N 2 0

1B N 2 0

1C N 2 0

2 N 2 0

2 N2O

2 N 2 0

2 N20

2 N2O

3A N20

3 0 N2O

. 3B H 2 0

3 0 N 2 0

4B N 2 0

4B ' N2O

4B N 2 0

4B , N20

* 250

(1) Accuracy is 215 percent, except near the limits of detection, where only one digit is included.

Muscuvltc from an albite-musccvite selvage along walls of a large pocket in rocks of facies 3 A .

N N o t detec led at value shown.

( ) I f iwt .1 limit of detection,

Table 6. --Whole-rock trace element cornpsi t i o n of five selected s a m p l e s of the g r a n i t e complex analyzed in 1 9 7 5 b y R . E . Mays .* A l l results i n ppm.

FieldNo. Facies Mn B . Ba Be Co C r 31 La Nb Ni Pb Sc Sn Sr V Y Zn Zr Ga Li Yb, 1 . 7 ) ( 2 1 (1) (.71 ( . 7 ) ( . 7 ) 151 ( 7 ) ( - 7 1 ( 3 ) ( . T I { 2 ) ( 1 ) (1) ( 5 1 ( 1 5 ) 13) ( . 7 ) (100) 1.7)

GS52 1A 300 10 70 7 N 5 1 N 30 N 5 0 5 7 15 N 20 N 50 20 500 5

69AH195 1B 500 10 500 7 1.5 10 3 100 10 1 50 5 7 200 7 20 30 SO 20 300 3

69AH118 2 500 10 1.000 7 2 15 2 100 10. 1 50 5 10 300 15 30 50 50 2 0 300 5

69AH452 3 A 700 10 1,000 10 L.5 10 1 100 10 2 7 7 10 300 10 50 50 70 20 500 10

70AH756 48 300 10 100 10 N 10 5 N 30 N 70 5 7 70 N 100 N 50 20 300 20

*Semiquantitative spectrographic analyses ; procedure nodif icd frnw t h a t d c s c r ~ h c d I n USGS Bulletln 1084-1

N-Not detected at limit of detection.

Looked for but not detected; A g , A s , Au , Bi. Cd, Mo, Pd. Pt. Sb. Te, U . W, Ce. Ge. ~ l f , In. Re, Ta, Th. TI. Eu.

( 1 Lower limit of detection In ppm.

Figure 29. Variation o f major oxides w i t h percent S i O n . Data from analyses l i s t ed i n t ab l e 2 normalized t o 100 percent. Dashed 1 ines sketched by eye t o i l l u s t r a t e general t rends .

b u t Na20 values generally a re higher in samples high in Si02, and

(3 ) there i s a break in Si02 content between 74 and 75 percent, and

t h i s corresponds to s h i f t s in the K20 and NapO values. The regular

and marked decrease of mafic oxides with increasing Si02 content suggests

that the c rys ta l l iza t ion history of the complex i s essent ial ly one of

magmatic d i f fe rent ia t ion . As the re la t ive age relations of the different

facies are known, the variation of major-oxide values with the facies

sequence helps to c la r i fy the nature of t h i s d i f fe rent ia t ion .

The major-oxi de val ues are plotted against the facies sequence

in f igure 30. Comparison of the variation in figure 29 with that in

figure 30 quickly reveals major differences and shows tha t the facies

sequence does not represent a simple trend toward more sal i c compositions.

The variation within the facies sequence i s of three types: ( 1 ) a

decreasing Si02 trend tha t s h i f t s to higher values in facies 3B and 48,

( 2 ) increasing trends fo r Ti02, CaO, MgO, FeO, Fep03, and A1 203 wi t h

s h i f t s t o lower values in facies 30 and 48, and (3 ) less regular trends

fo r Na20 and K20 in which i t appears t h a t the Na20/K20 r a t i o does n o t

change s ignif icant ly through facies IB , 2 , and 3A, b u t tha t i t i s

d i s t inc t ly high in racies 1 A , 3B, and 4B.

The trends in ,f igure 30 indicate tha t crystal1 ization resulted

sequentially in : (1) i n i t i a l so l id i f ica t ion of s a l i c melts in the

f iner grained border facies , ( 2 ) .an intermediate crystal l i ta t ion of

relat ively femic compositions in facies 1C through 3A, and (3) . final

c rys ta l l iza t ion of s a l i c melts in facies 3B and 4B. Although they

a re not represented in the analyses, i t i s believed t h a t sampl es of

facies 4A would closely follow the trends in figure 30 and plot with

samples from facies 3B and 48. The negative slope of the Si02 variation

F igu re 30. V a r i a t i o n o f major ox ides through "the f a c i e s sequence. Data from analyses l isted i n t a b l e 2 normal ized t o 100 percent . Dashed l i n e s sketched by eye t o i 11 u s t r a t e general t rends .

indicates tha t the residual me1 t experienced enr'i chment in Si02;

conversely, the posit ive slopes fo r Ti02, CaO, MgO, FeO, Fe203, and

A1203 indicate tha t the residual me1 t was depleted in these components

by facies 1C through 3A crystal1 izat ion. The marked s h i f t in the

direction and continuity of the trends upon crys ta l l iza t ion of rocks

of facies 3B and zone 4 indicate that these facies can represent res i -

dual me1 t s that were developed by crystal 1 i zation and separation from

the melt-crystal system of facies 1C, 2 , and 3A. The f i e l d data i ndicattng

tha t facies 38 and zone 4 were the parts o f the complex to c rys t a l l i ze

1 a s t , the cogenetic character of a l l the facies that i s evidenced by

the trends in f igure 29, a n d the whole-rock and b i o t i t e trace-element

data discussed be1 ow confi rm t h i s relationship.

The gap in SiOp values evident i'n figure 29 can be seen in figure

30 to be essent ial ly a break between the values for the i n i t i a l and

final facies (1A, 3B, and .48) and the values f o r the facies t h a t

crystal l ized a t an intermediate stage ( I C , 2 , and 3 A ) . The final facies

clear ly represent residual melts; the i n i t i a l facies are composed of

relat ively rapidly crystal l ized rocks adjacent to or near the contact

w i t h the hornfels country rocks. Both the major- and trace-element

irregul a r i ty a re more pronounced in facies .lA rocks t h a n in any other

facies of the complex. This i r r sgular i ty probably i s the resu l t of

the complex interaction of several processes as discussed in the section

dealing with the c rys ta l l iza t ion model for the pluton.

Variation o f Trace Elements

The variations of some of the whole-rock trace-element abundances

l i s t e d i n t ab le 3 through the facies sequence are shown diagrammatically

in figure 31. The concentrations in facies 1 A are more i r regular than

in the other fac ies . In general, the concentrations increase from

facies 1B to 3A and s h i f t t o divergent values in facies 3B and 48.

The trends a re of two principal types: (1 ) markedly increasing values

with maxima in rocks of facies 2 and s h i f t s to low concentrations in

facies 30 and 4B (Ba and Sr) , and ( 2 ) gently increasing or nearly

constant values with marked s h i f t s t o higher concentrations in facies

38 and 48 (Nb, P b , and Be). Tin i s an apparent exce'ption t o the general

trends and i s discussed separately below.

The trends of the f i r s t type, those fo r barium and strontium,

a re explained by preferential subst i tut ion of bari um fo r potassium in

K-feldspar and of strontium fo r calcium in plagioclase. The modal

percentage of K-fel dspar decreases then increases s 1 i ghtly from faci es

1B through facies 2 , then becomes s l ight ly less in rocks of facier 3A .

This modal trend closely follows the whole-rock barium t r e n d through

these facies . The modal faci es t r e n d for bioti t e i s simi l a r t o t h a t

f o r K-feldspar, b u t the barium content of b i o t i t e i s t o o low ( t ab le 4 )

t o explain the observed whole-rock barium contents and variations.

Stronti um c lear ly fol 1 ows cal ci urn through the faci es sequence, and as

plagiocl ase ' i s the only calci um-rich mineral o f appreciable abundance,

the dis t r ibut ion of strontium i s mostly a function of the nature and

dis t r ibut ion of t h i s mineral. B o t h the modal-facies t r end and composi-

t ion of plagiocl ase cor re la te we1 1 with t h e observed s t r o n t i urn variation

from facies 1B t o facies 3A. The s h i f t t o low values for barium and

strontium in facies 3B and 4C i s in keeping with the more f e l s i c nature

of these l a t e r fac ies , b u t in addition i t probably indicates that the

barium and strontium concentrations tha t were present in the i n i t i a l

F igu re 31. Variation of selected whole-rock trace eleinents through the fac ies sequence. Data l is ted i n table 3. Dashed lines sketched by eye t o i 11 ustrate general trends.

magma were essent ia l ly used up by incorporation in the K-feldspar

and plagioclase of the more mafic intermediate fac ies , An interest ing

aspect of the barium and strontium dis tr ibut ion i s that t he i r higher

concentrations in the intermediate fac'ies probably were a1 so enhanced

by the slow crystal l i za t ion rates tha t existed during crystal 1 ization

of these facies .

The second type of trend, for niobium, lead, and beryllium,

i s explained by the inab i l i t y of these elements to readily subs t i tu te

f o r any of the major elements. These elements would be expected to

concentrate throughout crystal1 ization in the residual me1 t . Their

concentrqtion i n facies 38 and 48 i s commonly a t l e a s t twice the i r

concentration in e a r l i e r formed fac ies . The sharp s h i f t to these higher

concentrations in the l a s t crystal 1 i zed facies supports the conclusion

that these facies represent mostly sol idl'fied residual me1 t .

The trend fo r t i n through the facies sequence could be expected

to be s imi lar t o that fo r the other t r ace elements tha t do n o t subs t i tu te

readi ly fo r any of the ma~or elements, There i s some indication in 3

the who1 e-rock trace-el ement data tha t t i n fol lows the other fugi t i ve

elements in tha t the highest observed t i n concentration i s in one of

the samples of facies 30 rocks, b u t the dro?-off t o lower values in

facies 4B rocks must be explained, Rocks of facies 4B, unlike a l l

other rocks in the complex, essent ia l ly lack b i o t i t e . They charac-

t e r i s t i c a l l y have many small miarol i t i c cav i t i e s , i n t e r s t i t i a l f l u o r i t e ,

and muscovite t h a t in part has c lear ly replaced b i o t i t e . I f high t i n

concentrations were present in these rocks when they f i r s t crystal 1 i zed,

i t must have been removed by subsequent reactions. In the process,

muscovi t e rep1 aced pre-exi s t i ng bi o t i te .

V a r i a t i o n o f Trace Elements i n B i o t i t e

Seventeen b i o t i t e separates from una l te red who1 e-rock samples and

one muscovite separate f rom a muscov i te -a lb i t e replacement selvage

a long t h e w a l l s o f a l a r g e pocket have been spec t rog raph ica l l y analyzed

f o r se lec ted t r a c e e l ements. These da ta a r e i n two p a r t s : (1) r e s u l t s

o f semiquan t i t a t i ve analyses f o r a l a r g e number of t r a c e elements

( t a b l e 4 ) , and ( 2 ) r e s u l t s o f q u a n t i t a t i v e analyses f o r t h e se lec ted

elements B , Be, Cu, Pb, Sn, and Zn ( t a b l e 5 ) . The semiquan t i t a t i ve

data p rov ide a scan o f t h e trace-element composit ion and i n d i c a t e the

general abundance of those t r a c e elements present i n apprec iab le amounts

i n t h e micas. The q u a n t i t a t i v e data were obta ined t o more c l o s e l y

s tudy t h e v a r i d t i o n i n trace-element ahtindance through the f a c i e s

sequence. The elements determined quant:i t a t i ve l y were chosen i n p a r t

because Sainsbury and Hamil ton (1968, p . F11) repo r ted them t o be

present i n read i l y de tec tab le quan t i t i e ? i n b i o t i t e from some o the r

t i n - g r a n i tes of t h e Seward Peninsula. I n a d d i t i o n , b i o t i t e i s t he

p r e f e r r e d minera l hos t f o r most o f these elements (Nb, Sn, Zn, and L i ) .

The data i n t a b l e 4 show t h a t many elements a re present i n low b u t

de tec tab le amounts, and t h a t barium, niobium, t i n , z i nc , and l i t h i u m

a r e present i n apprec iab le quan t i ti es . The semiquanti t a t i ve data revea l

t h a t some of these elements, such as n iobium and t i n , show s i g n i f i c a n t

v a r i a t i o n through the fac ies sequence, b u t f o r many, such as bar ium and

z inc , t h e na ture of the v a r i a t i o n , if any, i s no t c l e a r .

Most o f t h e q u a n t i t a t i v e data i n t a b l e 5 a r e shown d iagrammat ica l l y

i n f i g u r e 32. The v a r i a t i o n i n copper content o f the b i o t i t e through

t h e f a c i e s sequence i s i r r e g r t l a r , and systemat ic d i f f e r e n c e s f rom one

f a c i e s t o t h e nex t a r e n o t present . On the o the r hand, t h e v a r i a t i o n

Figure 32. Variation of selected trace elements i n b i o t i t e through the f a c i e s sequence. Data l i s t e d in tab le 5 . Dashed l ines sketched by eye t o i 11 us t ra te general trends.

fo r Be, L i , Nb, S b , and Zn through the facies sequence i s systematic,

and the data fo r a l l these elements clear ly show a s l ight ly decreasing

trend o r almost constant value from facies 1A or 1B through facies 3A

and then a s h i f t to higher concentrations in facies 38 and 4A. In

general , the highest val ues fo r these elements are in b i o t i t e from

facies 4A. The highest observed lithium value i s in facies 38 rocks.

The elements that show nearly constant o r s l igh t ly decreasing con-

centrations through the intermediate facies and higher values in the

f inal facies a re those tha t substitut;? i n limited amounts for major

elements in the b i o t i t e s t ructure. These variation trends indicate

tha t , throughout i ntermedi ate-faci es crystal 1 i zation ; these elements

were being select ively parti t ioned to the residual me1 t .

The variation o f t i n concentration i n the b i o t i t e provides some

explanation of the observed whole-rock variation of t h i s element. '

The nearly constant t i n concentration in b i o t i t e through the in te r -

mediate faci es indicates that the whole-rock variations through these

facies shown in figure 31 must be due t o variations in the amount of

b i o t i t e present o r the contributions of other'"mi neral s . Both factors

probably contri bute t o the s 1 i g h t whol e-rock ti n i ncrease; modal bi o t i t e

increases t o a maximum i n facies 2 ( f i g . 28) and sphene, a l ike ly con-

centrator of t i n , i s present in minor b u t above-average amounts in th is

faci es . The whol e-rock t i n content apparently cannot be completely

explained by the dis t r ibut ion and composition of b i o t i t e alone, i n that

the whole-rock t i n concentration i s commonly about twice the amount

tha t could be contributed by b io t i t e . Either the whole-rock concen-

t ra t ion of t i n reported i n table 3 i s too high, o r s ignif icant amounts

o f t i n are present in minerals other than b io t i t e .

Some recent data suggest tha t the semiquanti t a t i ve whole-rock

determinations f o r t i n l i s t e d in table 3 a r e too high. Five additional

whole-rock samples, two from facies 2 and one each from facies 1 A ,

3A , and 4B, were analyzed i n 1975 fo r the en t i r e trace-element s u i t e

by semiquanti t a t i ve techniques . The procedures used i n 1975 a re revised

from e a r l i e r ones and are considered t o be more accurate ( R . E . Mays,

1975, pers. commun.), The 1975 resu l t s a re l i s t e d in table 6 and show

t i n contents that a re less than or equal t o 10 ppm. The conclusion

i s t ha t the whole-rock t i n concentrations i n table 3 are too h i g h ,

and tha t the t i n content and dis t r ibut ion of b io t i t e readily explains

the whole-rock t i n variation shown in figure 31. The newer da ta in

t ab le 3 have not been used t o study the t race element variation between

facies because data from a l l the facies a re not available. The newer

data 'do not reveal any differences in the general trends exhibited by

the older data in tab le 3, and, as the older data were obtained under

the same laboratory and analytical conditions , they a r e enti rely

sa t i s fac tory fo r s tudyi ng the t race element variation through the facies

sequence. The new data d i f f e r fram the old only in tha t : ( 1 ) nickel

and zinc a re present in detectable amounts, ( 2 ) values for chromium

are consistently higher., b u t those for t i n , and probably niobium, a re

consistently lower, and ( 3 ) the values fo r 1 i t h i u r n are more consistently

w i t h i n detectable l imi ts .

The mica trace-element data provide one more important l ink to

the. whole-rock data. The lack of whole-rock analyses f o r facies 4A

i s a serious gap in the analytical data, more so for the t race elements

thzn fo r the major oxides. The mica da ta partly f i l l the t race element

void f o r t h i s fac ies , and show tha t t i n was concentrated to high values

i n the residual fac ies of the complex. As discussed more completely

l a t e r , facies 4B rocks a r e leached facies 4A rocks and the t i n they

once contained has been removed, resul t ing i n the low whole-rock values

as i 11 ustrated in figure 31 . . .

Trace-El ements in Pocket Muscovi t e

Muscovi t e was separated from the rep1 acement selvage ( f i g. 25)

that i s present along the wall s of a 1 arge pocket i n faci es 3A rocks.

Tne semiquanti t a t i ve and quant i ta t ive trace-el ement data for th i s

mineral are included i n tab1 es 4 and 5 along with those f o r the other

micas from the complex. The muscovite has h i g h concentrations of

beryllium, lithium, and t i n , b u t low concentrations of copper, niobium,

and zinc, The high boron content shown in table 5 for th i s sample i s

probably due to contamination by small grains of schorl , which i s a

minor b u t important consti tuent of the selvage material ,

The selvage material probably represents the products of reaction

between a separated and trapped aqueous phase and the enclosing ser ia te -

textured rocks of facies 3A, The most obvious chemical change has

been the addition of Na20, and the selvage i s essent ial ly albi t ized

grani te . Water, boron, and fluorine a1 so have been added. As the . .

muscovite.is part of the reaction products, i t s trace-element-composition

should r e f l ec t , a t l eas t in par t , the trace-element composition of the

coexi s t i ng aqueous phase. The h igh concentrations of beryl 1 i urn, 1 i thi u m ,

and t i n i n the muscovite support the conclusion tha t the coexisting

aqueous phase was enriched i n these elements. As muscovi t e forms about

50 percent of the selvage material , the whole-rock t i n concentration

for t h i s material i s about 300 ppm. This i s 30 times t he average t i n

concent ra t ion o f t he g r a n i t e samples and c l e a r l y shows n o t o n l y t h a t

t i n was concentrated toward t h e r e s i d u a l me1 t throughout magmatic

f r a c t i o n a t i o n , b u t a l s o t h a t once a separate vapor phase was evolved,

t h e t i n was s t r o n g l y p a r t i t i o n e d t o it. Th is r e l a t i o n s h i p i s a l s o

supported f o r b e r y l 1 i urn and 1 i t h i um. The muscovi t e t race -e l ement da ta

do n o t revea l what has happened t o t h e o t h e r elements t h a t were concen-

t r a t e d through magmatic f r a c t i o n a t i o n , b u t i t seems l i k e l y t h a t they

were a l s o p a r t i t i o n e d t o the evolved vapor phase. Once i n t h i s phase,

t h e i r d i s tri b u t i on apparent ly became governed by the complex re1 a t i ons

o f a high-temperature hydrothermal system ( o n l y p a r t o f which i s exposed

i n t h e pocket ) , and they were e f f e c t i v e l y n o t a v a i l a b l e f o r d i r e c t

p r e c i p i t a t i on o r s u b s t i t u t i o n i n t h e muscovi t e s t r u c t u r e under t h e

c o n d i t i o n s t h a t must have preva i l e d i n t h e pocket env i ronment.

Comparison w i t h Average composit ions of Gran i te and B i o t i t e

The o v e r a l l chemist ry of t h e complex i s de f i ned by comparisons

w i t h compiled major-oxide, whole-rock trace-element, and b i o t i t e t race -

element averages. For t h e major ox ides, t h e average and c a l c u l a t e d

b u l k composit ions of t h e complex a re shown i n t a b l e 7 along w i t h the

average f o r most of t h e analyzed t i n - g r a n i t e s o f t h e w o r l d (stemprok

and Skvor , 1974) , some b i o t i t e g r a n i t e s (Nockol ds , 1954), and general

calc-a1 k a l i c g ran i tes (Nockolds, 1954). The c a l c u l a t e d bu l k composit ion

o f the complex i s s l i g h t l y more mafic than t h e a r i t h m e t i c average.

Comparison o f t h e bu l k composit ion w i t h t h e o t h e r g r a n i t e averages

shows t h a t t h e complex i s s i m i l a r t o the average f o r t i n - g r a n i tes

b u t t h a t t he re i s one s i g n i f i c a n t d i f fe rence w i t h respect t o t he o t h e r

g r a n i t e averages. The Serpent ine Hot Springs complex and e v i d e n t l y

o t h e r ti n-grani tes have a d i s t i n c t l y lower K20/Na20 r a t i o .

The average values f o r se lec ted t r a c e elements i n the complex a re

shown i n t a b l e 8 a long w i t h the average f o r t r a c e elements i n low-

ca l c i um gran i tes as compi l e d by Turek i an and Wedepohl ( 1 961 ) , The

complex conta ins d i s t i n c t l y l e s s Mn, C r , Cu, and V and d i s t i n c t l y more

Be, La, Nb, Pb, Sn, and L i t h i n t h e compiled l ow-ca l c i um g r a n i t e average.

O f those elements t h a t a re more abundant i n t he complex, t h e ~l ues

f o r Be, Nb, Pb, and Sn a re two times o r more above t h e average values

f o r 1 ow-cal c i urn gran i tes . B i o t i t e i s t h e p re fe r red minera l h o s t i n the complex f o r many ~f

t h e elements l i s t e d i n t a b l e 8, and i t s t race-e lement composit ion more

c l e a r l y i n d i c a t e s the na tu re o f t he o v e r a l l t race-element composit ion

of t he complex. The average trace-element composi t ion o f b i o t i t e i n

t h e complex ( t a b l e 9 ) has been ca l cu la ted from the r e s u l t s of 16 o f

t h e analyses l i s t e d i n t a b l e 4. Data f o r sample 69AH434 were no t

i nc l uded i n the cal c u l a t i o n because i t s composi t i a n i s somewhat anomalous.

A1 so shown i n t a b l e 9 i s t he average t race -e l ement composi ti on o f 28

b i o t i t e s f rom 7 d i f f e r e n t i n t r u s i v e bodies i n t h e western Un i ted Sta tes .

The average f o r t he western Uni ted s t a t e s b i o t i t e s was ca l cu la ted -from

data repo r ted by Lover i ng ( 1972).

Compared t o t h e data repo r ted by Lover i ng (1972), the b i o t i tes

f rom t h e g r a n i t e complex i n t h e Serpent ine Hot Springs area have a

di s t i n c t t r ace -e l ement composi ti on t h a t i s cha rac te r i zed by two d i f f e r e n t

element s u i t e s . The f i r s t s u i t e i nc ludes those elements w i t h h igh

concent ra t ions (Be, Nb, P b , Sn, Zn, and L i ) and t h e second inc ludes

those w i t h low concent ra t ions (Ba, Co, C r , N i , Sc, V , and Y ) . L i r con i urn

Element

Mn

B

Table 8. --Average trace element composition of the qrani te

complex and of l o w Ca granites (1) in ppm.

Average (from table 3)

Low Ca granite average

(1) Turekian and Wedepohl, 1961

"from table 6

Table 9. --Average and range of t r a c e element abundances i n biotites from some felsic i n t r u s i v e rocks i n

the western United States and from the S e r p e n t i n e Hot Springs area . A l l d a t a i n ppm.

1/ Western United States- Serpentine Hot Springs area Element Average Range Average Range 2/ Concentration factor-

l/calculated from data reported by Luvering (1972) for 28 biotites separated from seven intrusive complexes d

of quartz monzonite and granite composition. w IU

L'Average Serpentine Hot Springs area/average western United Stbtes .

appears t o be par t of the depleted s u i t e , but since zircon inclusions

play a major role in the dis t r ibut ion of t h i s element the data in

t ab le 9 a r e inconcl usive without additional petrographic information.

The copper averages are about the same fo r the two d i f fe rent b i o t i t e

groups, b u t the range of values for the bioti tes from the Serpentine

Hot Springs area i s narrower. The degree of enrichment fo r some of

the elements i s spectacular; the lowest val ues for t i n , zinc, and

lithium in bioti tes from the Serpentine Hot Spr ings area a r e equal

t o , o r greater than, the highest values f o r these elements in b io t i t e s

s tudi ed by Loveri ng . Much work has been done t o c l a r i f y the trace-element geochemistry

of t in-granites and t h e i r constituent minerals, and the data in table 9

confirm many general .relationships ident i f ied by others ( f o r example,

Bradshaw, 1967; Odi kadze, 1967; Sa i nsbury and others , 1968; jtemprok,

1971; Groves, 1972). B u t because the calculated averages in table 9

characteri ze the trace-el ement compos i t i on of the grani te complex as

a whole and the data a re f o r a re lat ively large su i t e of elements,

they a1 so provide t hz basis for making some interpretat ions concerning

the origin of the t in-granite magma, as discussed l a t e r .

The comparison of averages has shown tha t the complex i s s imilar

in major-oxide composition to other t in-granites and tha t i t has a

d i s t inc t ly lower K20/Na20 r a t i o than average bioti t e grani t e as compi led

by Nockol ds ( 1 954). The whole-rock trace-element composition i s a lso

s imilar t o that in other t in-granites (Stemprok, 1970, p. 116; Sains-

bury and Hamilton, 19G8), b u t i t d i f fe rs s ignif icant ly from the averages

compiled fo r low-calcium granites by Turekian and Wedepohl (1961).

Compared t o the averages i n low-calci um granites , the granite compl ex

*

i s more fractionated and contains enriched concentrations of several

fugi t i ve el ements i ncl udi ng t i n . The trace-el ement composi tion of b i o t i t e

c lear ly ident i f ies the overall fractionated nature of the complex;

the biot i tes a r e exceptionally enriched in the fugi t ive elements.

The important differences a l l point t o the f a c t tha t the complex, as

a whole, i s more fractionated than average grani tes .

crystal Populations

The textural , petrographic, and compositional d a t a together help

to ident i fy and characterize eight major crystal populations within

the complex. As referred t o here a crystal population includes those,

crystal s , regard1 ess of mineralogy , t h a t grew together and were simul-

taneously affected by any l a t e r changes in the c rys ta l l iz ing environ-

ment. The d i f fe rent crystal populations include: (1 ) equigranular

c rys ta l s of fac ies 1 A , ( 2 ) the tex tura l ly and compositional ly t ransi-

t ional c rys ta l s of facies lB, 1C, and 2 , ( 3 ) the larger well-formed

crys ta l s of facies 3A, ( 4 ) the f iner grained poorly formed crys ta l s of

facies 3A, ( 5 ) the phenocrysts of facies 38 , ( 6 ) the f i ne-gra'med

crys ta l s in the a p l i t i c groundmass i n facies 3B, ( 7 ) the c rys ta l s of

facies 4A, and (8) the c rys ta l s o f facies 40. These crystal populations

a re discussed separately below. Thei r characteri s ti cs together defi ne

the framework within which a complete c rys ta l l iza t ion model fo r the

complex can be s e t up.

The equigranular c rys ta l s of facies 1 A form a population tha t has

many anomal ous textural and compositional charac ter i s t ics . This

population has h i g h Si02, NaZO, Sn, Pb, Be, and Nb val ues and low KpO,

CaO, MgO, MnO, FeO, Y , Ba , and Sr values . The variation diagrams of

f i g u r e 30 show t h a t the values f o r a1 1 t h e :najor oxides except Na20

and K20. a r e n o t markedly d i ve rgen t from t h e v a r i a t i o n t rends i n the

f a c i e s t r a n s i t i o n a l inward from fac ies 1A; t h e Na20 values a r e en r i ched

and t h e K20 values a r e dep le ted i n t h i s f a c i e s . A lso anomalous a r e t h e

h i gh ly v a r i a b l e composit ions of the dominant ly unzoned p l ag ioc l ase

( f i g . 27), the s h i f t i n modal popu la t i on of f e ldspa rs ( f i g . 2 8 ) , t he

clouded na tu re o f the fe ldspars , and t h e l o c a l b u t ex tens ive rep lace-

ments by muscovite. Together, these data suggest t h a t processes a f f e c t i n g

t h e a l k a l i con tent of these rocks a r e i n l a r g e p a r t respons ib le f o r

t h e i r anomal ous nature.

The t e x t u r a l l y and composi t ional l y t r ans i t i o n a l p l a g i oclase,

K-feldspar, and b i o t i t e c r y s t a l s of fac ies 16, l C , and 2 c o n s t i t u t e

another d i s t i n c t c r y s t a l popu la t i on w i t h i n t h e complex. Nowhere

i n t he f a c i e s 1B t o 2 t r a n s i t i o n does qua r t z d i s p l a y any evidence

o f having coex is ted as a 1 i qu idus phase w i t h p l a g i o c l ase, K-fel dspar,

and b i o t i t e . It i n v a r i a b l y forms anhedral g ra ins o r aggregates o f

g ra ins t h a t a r e i n t e r s t i t i a l t o t h e o the r essen t i a l minera ls , and i t

i s t h e r e f o r e n o t considered t o be p a r t o f the f a c i e s lB, 1C, and 2

c r y s t a l popu la t ion . The t r a n s i t i o n a l composit ions between fac ies 1B

and 2 a re e v i d e n t i n t he chemical data ( f i g s . 30, 31 , 3 2 ) , i n t he

modal data ( f i g . 2 8 ) , and i n the inward increase i n a n o r t h i t e content

o f t h e p l a g i o c l a s e ( f i g . 27 ) . The inward change i n b i o t i t e c o l o r and

i n t h e p e r t h i t i c na tu re of K-feldspar suggests t h a t these minera ls

a l s o g radua l l y s h i f t i n composi t ion inward.

The na tu re of t he t r a n s i t i o n s w i t h the fac ies 1B, 1C, and 2

c r y s t a l popu la t ions i s especi a1 l y w e l l i 1 lus t ra - ted by megascopi c

t e x t u r a l s h i f t s inward, such as t h e i nc reas ing aggregat ion, s i ze ,

and morphological development of K-feldspar, and also by the pro-

gressi ve changes in plagioclase zoning relat ions. In general , the

zoning divides the indi vidual pl agiocl ase crys ta l s into three parts :

(1 ) homogeneous cal ci c cores tha t comonly show even extinction , b u t

can have patchy ext inct ion, twinning di sconti nui t i e s , and se lec t ive

development of secondary mi neral s , ( 2 ) normal-zoned marginal bands t h a t

surround the cores and have thin and somewhat f a i n t o sc i l l a t ions , and

( 3 ) sodic rims t h a t lack osc i l la t ions and have even or normal -zoned

extinction. The compositional variation within the zoned crystal s

i s comnonly not great, b u t i t i s consistently normal in t h a t the cores

a r e cal ci c 01 i gocl ase , the margi nal bands are normal -zoned

toward sodi c 01 i gocl ase, and the rims are sodi c 01 i goci ase (An1 0-18).

The progressive changes in the zoning relat ions from facies 1 B t o

facies 2 incl ude changes i n the composi t i on and physical characteri s t i cs

o f . cores, the degree of osci 11 a t i on' devel oprnent wi t h i n the margi na1

bands, the sharpness of normal zoning wi thi n the rims, and d i s t inc t

changes in the r e l a t ive proportions o f marginal bands and rims. Through

t h i s pa r t of the facies sequence, the cores change inward from being

the dominant part to being one-third of individual c rys ta l s , and from

being poorly defined and having even extinction to being sharply defined

by twi nni ng di scontinui t i es and i ncl usi ons . The rnargi nal bands gradual l y

change from being th in , poorly defined, and containing only a few fa in t

osc i l la t ions in facies 1 B t o having many f a i n t osc i l la t ions and being

half o r more o f individual c rys ta l s in facies 2 . The rims are we1 1

developed in facies 1B, where they are one-fourth to one-thi rd of

indi vi dual crystals and a r e moderately normal toned, b u t they become

narrower and commonly inconspicuous to absent in rocks of facies 2 .

The 1 arger , morphological ly we1 1 -developed cal ci c 01 i gocl ase ,

perthi t i c K-feldspar, quartz , and b i o t i t e crystal s of facies 3A consti-

' tu te a separate crystal population. This i s the f i r s t facies in t e r io r

from the facies 1A rocks in which quartz appears t o have coexisted

as a s t ab le liquidus phase with the other essential minerals. Some

aspects of t h e feldspars in th i s crystal population a re s imilar t o those

in facies 2 . The larger plagioclase crystals in facies 3A correspond

to the inward pattern of change in tha t t he i r well-defined cores are

s l igh t ly higher in anorthi t e content than facies 2 plagioclase. The

principal di fference between the 1 arger pl agi ocl ase crystal s in faci es 3A

and the pl agioclase o f facies 1 B through 2 i s that the marginal band

i s condensed and the rim be t te r developed in the former.

The large K-fel dspar c rys ta l s a re s imilar to the perthi t e pheno-

crysts in f a c i e s 2 except tha t they are not pinkish and a r e s l igh t ly

larger. The f ine r grained crystals of facies 3A are d i s t inc t ly different

in several respects from the larger crystals of t h i s facies and together

const i tute another crystal population. This population contains

dominantly unzoned a1 bi t i c oligoclase, nonperthitic Or-rich K-feldspar,

quartz, and probably some b i o t i t e tha t for the most part form subhedral

or anhedral intergranular grains. The most important aspect of t h i s

population i s the clear ly different composition of the feldspars from

that of the coarser crystal population. The difference between the

two crystal populations in facies 3A explains why facies 3A i s very

similar to facies 2 in composition ( f i g s . 30, 31, 32) b u t modally

contains less K-feldspar ( f ig . 28). This diminished modal K-feldspar

percentage i s in keeping with the less a l b i t i c nature of the second-

generation K-feldspar, which i s more abundant than the e a r l i e r formed &

l a rge per thi t e c ry s t a l s of the f i r s t population.

Facies 3B i s allother f ac i e s t h a t contains two d i f f e r e n t crys ta l

populations, i n t h i s case c l ea r ly i den t i f i ed by t he grain s i z e con t ras t

between phenocrysts and groundmass. The phenocrysts include a1 bi t i c

01 i gocl ase , perthi t i c K-feldspar, quar tz , and b i o t i t e . The plagioclase

has a l b i t e overgrowths, and the coal se pe r th i t e has overgrowths of

nonper thi t ic K-feldspar. Allother important c h a r a c t e r i s t i c of t h i s

population i s the nature of the plagioclase zoning and composition.

The pl agf,ocl a s e phenocrys ts d l splay some gross zoni ng re1 a t ions t ha t

a re s imi l a r t o t he l a rge r plagioclase c ry s t a l s i n f a c i e s 3A, but they

a r e d i f f e r e n t i n two important ways. F i r s t , the overal l cornposi t ional

var ia t ion i s wi th in . the sodic o l igoc lase range, about Anl8 t o A n l 0 ,

and second, many phenocrysts have fea tures suggestive of we1 1 -developed

primary zoning and corresponding cornpo:i t ional heterogeneity but they

a r e ac tua l l y f a i r l y homogeneous in composition. The a1 1 otriomorphi c

mosaic of c r y s t a l s i n the a p l i t i c groundmass i s the second and c l ea r ly

l a t e r c rys ta l population i n f ac ies 36. I t includes unzoned a l b i t e -

o l igoc lase , nonperthi t i c K-feldspar, quar tz , and b i o t i t e . The fe ldspars

a r e commonly clouded and t h e modal data ( f i g . 28) show t h a t t he mosaic

has a low percentage of plagioclase.

Each of t he two f ac i e s i n zone 4 represents a c rys ta l population.

T h e equi granul a r hypidi omorphi c crys ta l s of faci es 4A i ncl ude a1 b i t e -

01 igoclase , s l i g h t l y per thi t i c K-feldspar, r e l a t i ve ly dark-colored

a quar tz , and b i o t i t e . This population i s c l ea r ly more f e l s i c than

those t ha t formed e a r l i e r (escepting f ac i e s lA) , i f only on the basis

of the petrographic ( f i g . 27) and mod21 data ( f i g . 28). The b i o t i t e

t race-elemnt data ( f i g . 32) show tha t some of the highest abundances

*of the fugi t ive elements a re present i n micas from the facies 4A

crystal population. Muscovite replacement of b i o t i t e , light-colored

quartz, and a decreased modal percentage of quartz distinguish the l a s t

crystal population, that of facies 48. The feldspars tend to be clouded

i n th i s facies and plagioclase superfi cia1 ly resembles that of faci es

4A, b u t the l a t t e r contains some crys ta l s tha t a re zoned, whereas

plagioclase in facies 4B i s c h a r a ~ t e r i s t i c a ~ l l y unzoned and has even

extinction. The trace-element data ( f i g . 31 ) show tha t t i n i s very

.low in th i s crystal population.

Geometry of the Complex -- Some exposed spa t ia l relationships provide a basis for modeling .

the s j ze and shapeof the complex. The spat ia l relationships permit

estimation of the overall pluton volume and of the volume relationships

of the internal zones. The volume relationships provide the data needed

t o determine bul k composition of the pl u t o n and t o estimate the degree

o f crys ta l l iza t ion tha t accompanied formation o f the successive internal

zones.

External Form

The general external form o f the complex appears t o be that of an

elongate tadpole or cyl i nder-shaped body tha t plunges shall owly t o the

southeast. This form and a t t i t ude i s suggested by the present exposed

out l ine and facies dis t r ibut ion as well as certain contact zone re la t ions .

The exposed surface approximates a horizontal section through the body.

The most conspicuous featur: shown by th i s section i s the oval out l ine

o f the external contact and the concentric zoning parallel t o i t in the

central and western parts of the pluton. This outl ine i s d is tor ted

along the northeast contact by an eastward projecting bulge. The

mapped facies dis t r ibut ion cl early shows tha t t h i s bul ge was formed

by intrusion of facies 3A and 38 rocks. A t the present level of exposure

the concentric and oval form of the body apparently existed in t ac t

throughout crystal 1 i zat i on of the margi nal zones, b u t the northeast

contact was a zone o f structural weakness t h a t localized upward dls-

placements of magma during the l a t e r stages of c rys t a l l i za t ion , The

exposed out l ine and facies d is t r ibut ion thus emphasize the ini t i a l oval

nature of the pluton, and these relationships a re believed t o dominate

the overall horizontal shape of the body a t depth.

a The general a t t i t u d e of the pluton can be inferred from several

relationships in the contact zone. The most important of these are

observed dips of the pluton-country rock contact , variations in the

width of the thermal aureole, and the spa t ia l re lat ionship of mineralized

areas t o the exposed pluton. The pluton-country rock contact has a

shallow dip, locally only about lo0 , a t several places along the south-

eas t margin. The thermal aureole i s widest aiong th i s par t of the

contact and, together w i t h the r e s t r i c t ion of mineral ization to country

rock areas southeast o f the exposed grani te , t h i s fur ther suggests

t h a t the upper surface of the pluton d i p s shallowly to the southeast.

The a t t i t u d e of the contact along other parts of the pluton margin

i s not c lear ly shown by any of the mapped f i e l d re la t ions . The only

indication that the northern contact may dip in the same direction

as the southeas tern contact- i s the d i s t r i buti on of sample 1 ocal i t i es

from which rocks of facies 1A were col lected. Rocks of t h i s facies a re

present only along or near the pluton-country rock contact, and are

found over a wider area long the northern contact than along any other

part of the margin. The wider band of faci es 1A rocks here i s on a

southeast-faci ng slope. This may indicate that the northern contact

dips semi -para1 1 el t o the ground surface, thereby i ncreas ing the exposed

width o f facies 1A rocks. This can only be a tentat i j fe conclusion,

however, because the samples were a l l collected from surface rubble

and downslope movement may have helped increase the width over which

facies 1A rocks a re present.

Because of the relationships noted above, the modeling of the

internal geometry of the complex tha t follows below assumes that the

i n i ti a1 ly emplaced pluton was d i s t inc t ly elongate, oval i n cross sect ion,

and had a shallow plunge to the southeast.

I n outlining the inferred crystal 1 ization history for the complex,

a se r ies of diagrams i s useful for r e a l i s t i c a l l y depicting the internal a

geometry of the plcton as i t evolved d u r i n g cooling and consol idation.

The diagrams a re based on a model tha t can be used t o calculate the

re la t ive volume relationships within the complex. The model i s a

concentrically zoned cylindrical mass that i s oval in cross section.

The radius r e l a t i onships of the concentric cylindrical she1 1s were

estimated from the distance between outer zone margins as exposed along

a 1 i ne trending N. 20' W . through the center of zone 4 , and the lengths

of the she l l s were estimated from the projections shown i n the cross

section of plate 1. These dimensions, together w i t h the calculated

. volumes and t h e i r re la t ive proportions, are l i s t ed in table 10.

As considerable variation in composi tion exis t s among the facies

o f zone 1 and zone 3 , the r e l a t ive proportions of these d i f f e ren t

a, (I]

a a,

3 (I] [O rd

facies have been estimated in order t o calculate a bulk composi tion for

the pluton. The estimated facies proportions a l so a re l i s t ed in table 10.

The calculated volume relationships were used to determine the bulk

major oxide composition that i s shown in table 7 . In addition, the

volume relationships show tha t the major proportion of the p,luton crystal-

lized to form the various facies of zones 1 and 2 . The internal

displacements that were followed or accompanied by crys ta l l iza t ion

o f the facies of zone 3 and 4 affected relat ively small volumes of

internal parts of the pluton, and they took place cnly a f t e r the

ent i r e complex was more than three-quarters crystal 1 i zed.

Crystal1 i zat i on of the Grani t e Complex -- The f i e ld relationships , the petrographic, textural , and modal

variations , and the major-elemen t and trace-el ement chemistry of the @ grani t ic rocks provide a framework within which a c rys ta l l iza t ion model

f o r the complex can be defi ned. This model, as here proposed, i ntegrates

a l l the available data toward a conceptual understanding of the magmatic

processes tha t were operative. The fol 1 owi ng discuss ion i s based upon

two principal assumptions : (1) following i n i t i a l emplacement of mgma,

most of the c rys ta l l iza t ion occurred in an essent ial ly closed system

without fur ther addition of rnagrr,a from below, and ( 2 ) the i n i t i a l magma

was compositionally homogeneous.

T h e most important single aspect of the granite complex i s the wide

spectrum of textural variations among rocks tha t everywhere are o f

grani t e compos i t i on. Study of the compl ex has been aided tremendously

by the recognition and mapping o f the textural 1y contrasting facies .

Di sconti nui t i e s between such faci es provide the primary basis for

subdividing the crystal1 i zation history o f the complex in to f ive d i f fe rent

s tages, as follows:

1 . Crystal 1 i z a t i on of f i ne-grai ned equi granul ar rocks (faci es lA)

a t and near the margin of the pluton,

2. Crystall i t a t ion of the outermost parts of the pluton to y ie ld

the completely transit ional textures and compositional trends observed

amcng rocks of facies lB, lC, and 2 ,

3. Emplacement and crys ta l l iza t ion of s e r i a t e rocks of facies 3A,

4, Emplacement and crystal 1 ization of a crystal - r i ch system t o

form cornposi te-textured rocks of faci es 38 ,

5. F i nal crystal1 ization of facies 4A rocks under vapor-saturated

resi dual -me1 t condi t i ons , and 1 ocal i zed formation of rocks of faci es .48.

e These crys ta l l iza t ion stages a r e discussed separately below, and the

principal magmatic processes tha t were important during the successive

stages are ident i f ied . The discussions u t i l i z e a s e r i e s of diagrams

t h a t depict the evolution of the granite complex, Though necessarily

schematic , they have been prepared approximately to scale .

Stage 1--Crystal 1 i za tion of Facies 1A

Stage 1 crystal 1 i zat i on developed the fine- t o medium-grai ned

equi granul a r rocks of facies 1A. The grani te magma was emplaced into

re,latively cool country rocks, and a t and near the contact t h e hea t loss

to surrounding rocks quickly lowered the magma temperature below i t s

sol idus, Relatively rapid and complete sol i d i f i ca t i an produced the

fine- and even-grained texture. The presence o f m i arol i t i c cavi t i es

i n facies 1A rocks shows tha t so l id i f i ca t ion was accompanied by vapor

saturat ion. A t t h i s time the relat ions in the pluton were probably

as shown in figure 33. The facies 1A rocks, composed of crystals and

coexi s t ing vapor, graded i nward i nto higher temperature domai ns where

crystals + melt + vapor existed. Farther inward, however, only c rys ta l s

+ melt existed.

The re la t ive ly rapid drop of temperature to subsol idus levels a t

and near the contact, accompanied by rapid crystal1 iza t ion , might be

expected to yield racks with quenchli ke character is t ics . B u t there a r e

complexities, most obvious i n the chemistry of these rocks, which suggest

tha t they a re not to t a l ly the r e su l t of simple quenchlike c rys ta l l iza t ion

of the i n i t i a l l y emplaced magma. The most apparent problem i s tha t the

facies 1A rocks do not have a bulk major-element or trace-element

composition that i s intermediate between the compositions of the most

f e l s i c and most mafic rocks of the complex. Indeed, these rocks are

similar i n many aspects of t h e i r chemistry to the most f e l s i c parts of

the pl uton except that t h e i r Na20/K20 r a t i o i s anomalously high. As

discussed in the section deal i n g wi t h crystal popul a t i ons , the general ly

anomalous nature of facies 1 A rocks i s probably due t o processes tha t

in part have s ignif icant ly affected t h e i r a lkal i content. The petro-

graphic, textural , and compositional relationships a1 l suggest t ha t t he '

facies 1 A rocks, and especially the i r feldspar populations, have been

a1 tered by hydrothermal processes. These processes must have affected

the rocks immediately fol lowing the i r complete so l id i f ica t ion in the

presence of a coexisting vapor phase and a t subsolidus b u t elevated

temperatures, What i s the nature of the hydrothermal processes tha t

should be expected in the contact-zone environment? Some experimental

data a re d i rec t ly pertinent to t h i s question.

k C G a m

E-r L O - v 3 0 m u

( U o r rci - w u-l 9 .r I .r F L m o o c n m a 0 !a Q-v z=

Tutt le and Bowen (1958, p. 89-91), Orvi l le (1963), and Martin and

Jahns (1976, pers. commun. ) have demonstrated experimentally tha t , for

vapor-saturated rock systems in thg presence of a temperature gradient,

s ignif icant and d i f fe rent ia l t ransfer of major rock constituents i s

to be expected. In general , f e l s i c ccnsti tuents , and especial ly K20,

migrate t h r o u g h aqueous vapor and precipi ta te a t regions of lower tem-

perature without mass t ransfer of vapor i t s e l f . That a sharp temperature

gradient existed in the vicini ty of the facies 1A rocks during and a f t e r

t h e i r crystal1 i zation i s indicated by the t ransi t ional increase of grain

s i ze inward from the contact and by the presence of hornfels in adjacent

country rocks, The compositional relationships in facies 1 A rocks -

therefore can be explained in terms of simultaneous ( 1 ) down-temperature

migration of K20 from facies 1A rocks into country rocks, and ( 2 ) possible

down-temperature migration of NapO from adjacent inner parts of the

pluton (areas where c rys ta l s + melt + vapor exis ted, f i g . 33) into

facies 1 A rocks.

Analytical data on the country rocks needed t o t e s t t h i s hypothesis

are not avai lable , b u t local replacement by muscovite, combined with a

the d i s t inc t ly a l te red nature of K-feldspar in many facies 1 A rocks,

support the conclusion that potash metasomatism was act ive in the contact

zone and tha t the K20 involved was derived by interaction of a coexisting

vapor phas'e with i n i t i a l l y crystal l ized K-feldspar c rys ta l s . The addition

of Na20 into facies 1A rocks seems t o be required because the i r total

a1 kal i content i s not as dras t ica l ly diffe'rent from tha t in other parts

of the pluton, as would be expected i f select ive K20 leaching had occurred.

The e f fec t of NapO addi t i on has been to i ncompletely a1 bi t i ze exi s t i rag

plagioclase c rys ta l s , as i s evidenced by the wide range in plagioclase

composi t i on from Ang t o Anp7.

In summary, stage 1 c rys ta l l iza t ion was not a simple process,

I t included rapid crystal l i za t ion of the i n i t i a l ly emplaced granite magma

and subsequent subsolidus reactions that resulted in s igni f icant a1 kal i

t r ans fe r , both in to and o u t of facies 1A rocks. The a1 kal i t ransfer

took place via a coexisting vapor phase in response to a sharp temperature

gradient tha t existed from points within to points just outside the

p i ~ t ~ n . The resul t ing rocks, facies 1 A rocks, therefore have complex

characteri s t i c s developed by two very di f fe rent processes. Thus they

are highly i r regular i n de ta i l s of petrography and composition, and

they represent a d i s t inc t subsystem of the granite complex,

Stage 2--Crystall i zation of Facies 1 B , 1 C , and 2

Stage 2 c rys ta l l iza t ion formed the rocks of facies 1B, IC, and 2 ,

which a re t ransi t ional inward from the contact-zone rocks of facies 1A.

During th i s stage about 75 percent of the complex was crystal l ized t o

yield rocks tha t a re characteri zed by completely transi t i onal rela t i on- G7

ships in v i r tua l ly every aspect of t h e i r petrology and chemistry. The

conditions inferred to have existed within the complex near the end o f

th i s stage are shown in figure 34. The materials of facies 1B, 1C,

and 2 had been added t o the walls of the pluton, and the remaining

melt-rich zone in the in t e r io r contained scattered suspended crys ta l s .

There i s no evidence tha t anything b u t crystal-melt equi l ibr ia were

involved in s tage 2 c rys ta l l iza t ion , except pqrhaps local ly in i t s

terminal parts.

The c o n t i n u i t y o f t h e inward t e x t u r a l g rada t i on i n d i c a t e s t h a t t h e

f a c i e s 1B-2 t r a n s i t i o n was formed by one process, Because of t he

composi t ional v a r i a t i o n s developed w i t h i n t h i s t r a n s i t i o n and t h e l a r g e

p a r t o f t h e complex i t represents, t h e c r y s t a l 1 i z a t i o n process t h a t

formed f a c i e s 10, l C , and 2 i s c r i t i c a l t o understanding t h e c r y s t a l l i -

z a t i o n of t h e complex as a whole. The nature o f every s y s t e m a t i c a l l y

v a r y i n g parameter between f a c i e s 1B and 2 d i r e c t l y r e f l e c t s the p r o x i m i t y

o f t h e immediate c r y s t a l l i z i n g environment t o t h e ex te rna l con tac t o f

t he p l u t o n . As temperature i s the on l y phys i ca l parameter o f the

c r y s t a l 1 i z i ng envi ronment t h a t can be expected t o have changed sys temat i - c a l l y inward from t h e con tac t , t h e present p e t r o l o g i c and chemical

r e l a t i o n s h i p s cou ld w e l l re f1 e c t t h e i n f l u e n c e o f a temperature grad i en t

inwdrd f rom t h e con tac t . T h i s g r a d i e n t would have been p o s i t i v e inward,

as schemat ica l l y i l l u s t r a t e d i n f i g u r e 34. I t must have had two major

i n f l u e n c e s on the c r y s t a l l i z i n g environment: (1 ) i t l e d t o s i g n i f i c a n t l y

d i f f e r e n t c r y s t a l 1 i z a t i o n r a t e s inward, and ( 2 ) i t determined the 1 i q u i dus

phases and t h e i r composi ti ons . That t he success ive ly h ighe r temperature regimes inward from f a c i e s

1A rocks c r y s t a l l i z e d more s l o w l y i s evidenced d i r e c t l y by the major

t e x t u r a l v a r i a t i o n s through t h e f a c i e s 1B t o 2 sequence: ( 1 ) i n c r e a s i n g

g r a i n s i z e , ( 2 ) i n c r e a s i n g aggregat ion o f anhedral qua r t z , and ( 3 )

( 3 ) i n c r e a s i n g aggregat ion and morphol og i ca l development o f K-fel dspar.

An i n t e r e s t i n g aspect o f t h e t e x t u r a l t r a n s i t i o n i s t h a t i t may preserve

a reco rd o f t h e t e x t u r a l adjustments t h a t occurred w i t h i n f a c i e s 2.

Because t h i s p a r t o f t h e p l u t o n presumably was ab le t o c r y s t a l 1 i z e more

s low ly , in-si tu t e x t u r a l evol u t i on was n o t stopped a t i n te rmed ia te stages

(facies 1B and 1C) b u t progressed t o the very homogeneous porphyritic

texture of facies 2. . .

Throughout stage 2 crystal 1 i za t i on the pri nci pal mineral s t ha t

can be readily assumed t o have been liquidus phases a re relat ively

anorthite-rich plagioclase, sodic K-feldspar, and b io t i t e . There i s

no c lear indication tha t quartz was a liquidus phase. Among the liquidus

phases, i t has been shown that the anorthi te content of plagioclase

increases i n ~ a r d within the complex, and i t i s probable tha t the a1 b i t e

content of K-feldspar does also. The systematic s h i f t s in b i o t i t e color

may mean tha t i t a lso changes composition .inward. All of these s h i f t s

in mi neral composi t i on could r e f l ec t an inward increase of temperature

within the pluton.

The relations discussed above imply that rocks of .facies 10, 1C,

and 2 crystal l ized essent ia l ly in place. I f the i n i t i a l magma was

compositional ly homogeneous, then some important charges in bul k composi-

tion through the facies 1B to 2 sequence remain t o be explained. A1 1

tne important composi ti onal paramezers show that t h i s sequence represents

a continuous systematic s h i f t toward more mafic compositions. Such a

s h i f t requires that sal i c consti tuents of the magma be removed from

facies 1 C and 2 rocks. A mechanism for producing the select ive removal

of sal i c components during in-si tu crystal 1 i zation has been proposed

by Vance (1961). He suggests that a migrating f ront o f vapor saturation

accompanies an inwardly advancing zone of to ta l so l id i f ica t ion . This

vapor contains residual a1 kal i and s i l i ca components and coul d redisso.1 ve

i n the i n t e r i o r melt-ri ch parts of the pluton adjacent t o the to t a l ly

sol i di f i ed zone. The end resul t , i f the process operates ef f ect i vely ,

i s a s a l i c , water-rich magma in the central parts of t he crys ta l l iz ing

complex. This mechanism i s consistent with the avai lable d a t a for the

Serpentine Hot Springs grani te complex, and i t could very we1 1 explain

the development of the re la t ive ly mafic rocks in facies 1C and 2 as

well as the more water-rich and more f e l s i c facies in in t e r io r parts of

the pluton.

In summary, the facies 10-1C-2 t rans i t ion probably formed as the

r e su l t of re la t ive ly slow and essent ial ly in-situ crys ta l l iza t ion .

Oli goclase, sodic K-fel dspar, and b io t i t e probably nucleated simultaneously

t h r o u g h o u t the marginal fac ies , b u t these minerals grew a t successively

hi gher temperatures and therefore a t slower rates inward. Residual

a lkal i and s i l i c a constituents were select ively transferred from facies 1 B

and 2 t o i n t e r i o r parts of the pluton by redi ssol vi ng of an in te rs t i t i a1

vapor phase in melt-rich parts of the pluton adjacent to an inwardly

advancing zone of total so l id i f ica t ion . By the end of stage 2 c r y s t a l l i -

zation about 80 percent ( facies 1 A + 10 + 1C + 2 ) o f the i n i t i a l 1y

emplaced magma was so l id i f i ed . This stage of crystal1 i za t i on therefore

represents the major fractionating event in the history of the complex,

during which the major segregation of f e l s i c ater rials t o the i n t e r i o r

of the pluton must have taken place,

Stage 3--Emplacement and ~ r y s ta 11 i zation of Faci es 3A

Stage 3 crys ta l l iza t ion was a d i s t inc t ly separate event in develop-

ment of the complex. I t followed marginal c rys ta l l iza t ion of facies 1B,

1C, and 2 ( s tags 2 crystal 1 iza t ion) and produced the seriate-textured

rocks of facies. 3A. A t the beginning of t h i s stage the pl uton was more

than 80 percent crystal1 ized, and the spat ia l relationships within the

complex probably were as shown i n figure 34. A t t he end of t h i s stage

the spa t i a l re la t ionships presumably had changed to those shown in

f igure 35; a , pa r t of the magma internal t o c ry s t a l l i z ed fac ies 2 rocks

was displaced s l i g h t l y upward throughout the complex but was preferen-

t i a l l y intruded along a s t r uc tu r a l l y weak zone a t the northeast contact .

This magma displacement probably was caused by an inward movement of

t h e pluton walls i n response t o t he vol ume decrease t h a t accompanied

major c rys ta l 1 i za t ion of the i n i t i a l ly emplaced magma.

The fac ies 3A rocks have some unique cha rac t e r i s t i c s , but they

a l s o a r e s imi la r i n some respects to fac ies 2 rocks. The s i m i l a r i t i e s ,

which a r e important t o an understanding of the nature of s tage 3

crysta l 1 i za t i on, a re in the bul k major-oxi de and trace-el emen t com-

posit ions ( f i g s . 30 and 31 ) and in the nature of the la rge feldspars

Large c ry s t a l s of plagioclase i n fac ies 3A rocks conform t o the trend

of inward increasing anorthi te content seen in the marginal faci es ,

and they a r e s imi la r i n gross zoning cha rac t e r i s t i c s t o the plagioclase

i n fac ies 2 . The large c ry s t a l s of K-feldspar a r e we1 1-developed

perthi t e s , s l i g h t l y l a rger and nonpink in color but otherwise apparkntly

s imi la r t o per thi t e phenocrysts of f ac i e s 2 . The s i m i l a r i t i e s between

rocks of fac ies 2 and 3A ind ica te t h a t the magma t h a t was displaced . .

and c rys ta l 1 i zed t o f ac i e s 3A rocks was c lose t o being a parent of

fac ies 2 rocks. Its bulk composition was s imi l a r t o t h a t of fac ies 2

rocks, and the sca t te red large fe ldspars i t contained were much l i k e

those accreted t o the walls during f ac i e s 2 c rys t a l l i z a t i on . T h i s

suggests t h a t t he fac ies 3A magma was a crys ta l + melt selvage adjacent

the so l i d i f i ed walls of the pluton, and t h a t i t s principal compositional

cha rac t e r i s t i c s were developed during the major f rac t iona t ion t ha t

accompanied s tage 2 c ry s t a l l i z a t i on .

L mr- 3 +J 0'4- m m W O

B u t s tage 2 crys ta l 1 i za ti on was terminated by inward movement

of the pluton walls t h a t forced magma adjacent t o the c ry s t a l l i z ed

fac ies 2 rocks t o move d i f f e r e n t i a l l y upward ( f i g . 35). c ry s t a l l iz'ation

accompanied t h i s displacement. and produced the unique character i s t i cs

of fac ies 3A rocks, including the development of a second c rys ta l

population and s e r i a t e texture . When the magma was i n i t i a l l y forced

to move i t contained c ry s t a l s o f ca l c i c oligocl~se, sodic K-feldspar, I

b i o t i t e , and quar tz . Upon displacement of the magma a second crysta l

popul a t ion comprisi ng sodic 01 i gocl a se , nonsodi c K-feldspar, b i o t i t e ,

and quar tz was nucleated. These minerals plated onto appropriate

ex i s t ing c ry s t a l s and a l so formed many new c rys t a l s . The crysta l 1 i za t ion

r a t e was f a s t enough t o prevent reaction between the early-formed

suspended , c ry s t a l s and t he melt, b u t slow enough t o prohibi t the develop-

ment of a d i s t i n c t granular i ty con t r a s t between the ear ly- and late-formed

populations. The f i na l resul ts of t h i s d isequi l i bri urn crys ta l l i z a t i on

were the se r ia te - tex tured , f ine- t o coarse-grained rocks of fac ies 3A

t h a t contain four d i f f e r en t fe ldspars .

In summary, major f rac t iona t ion during s tage 2 crys ta l 1 i za t ion

produced not only . the s o l i d i f i e d walls of the pluton b u t a l so an

inhomogeneous magma i n t e r i o r t o the c ry s t a l l i z ed zones. Near t he walls

t h i s magma was s imi l a r i n . bul k composi t i a n to t h e adjacent crys ta l1 i zed

zone, and i t contained early-formed higher temperature fe ldspars .

The temperature in the i n t e r i o r of the pluton evidently had lowered

enough so t h a t quartz was a l so a l iquidus phase. Inward adjustment of

the walls of the pluton forced the crysta l + melt selvage t ha t had

developed adjacent t o fac ies 2 rocks t o be displaced. Stage 3 dis-

equi l i briurn crysta l l i z a t i on was i n i t i a t e d by the displacement of the

magma, and i t produced the seriate-textured rocks of facies 3A. Par t

of the residual materials tha t had been concentrated toward the in t e r io r

of the pluton throughout most of stage 2 crys ta l l iza t ion was present

in the crystal + melt selvage. Local vapor saturation was common upon

near-complete so l id i f ica t ion , and i t produced pegmatitic pods and

miaroles containing euhedral a l b i t e , tourmaline, and quartz. Residual

magma in so l id i f ied facies 3A rocks produced discontinuous dikes of

apl i t e and apl i te-pegmati t e . +

Stage 4--Emplacement and Crystall ization of Facies 3B

The emplacement and crys ta l l iza t ion o f facies 3A rocks could be

likened t o a s l i g h t rupture of a toothpaste tube. The magma selvage

adjacent t o facies 2 rocks was able to move a re lat ively short distance

before i t t o t a l ly crystal1 ized and sealed u p the ruptured part of the

pluton. A t t h i s time the spat ia l relations may well have been as shown

in f igure 35. The seriate-textured rocks passed gradual ly downward

and inward into an i nhomogeneous core of part ia l ly crystal 1 i z e d residual

melt, Stage 4 c rys ta l l iza t ion occurred b o t h prior to and during the

intrusion of magma t o form the composite-textured rocks of facies 30.

A t the close of stage 4 crystal1 izat ion, the spat ia l relationships

within the complex could have been as shown in f igure 36. The intrusions

tha t formed facies 3B rocks produced small apophyses within ser ia te -

textured rocks, and a larger mass along the s t ruc tura l ly weak northeast

contact of the pluton.

The formation of facies 3B rocks marked the f i r s t s tep in the

c rys ta l l iza t ion of the residual mgma system tha t had been developed

in the i n t e r i o r of the pluton, largely during stage 2 c rys ta l l iza t ion .

Both chemically and petrologically these rocks represent a mixture

of residual magmil materi a1 s and ear l i e r crystal 1 i zed materi a1 s . The

major-oxide and trace-element data ( f i g s . 30 and 31 ) a l l show values

in the facies 3B rocks that a re d i s t i n c t . 1 ~ f e l s i c b u t * intermediate

between t h a t of the f ina l crystal l ized facies of zone 4 and t h a t of

e a r l i e r formed more mafic facies . Pe t ro lo~ ica l ly the facies 3B rocks

clear ly contain two d i s t i n c t crystal populations : the early-formed

phenocrys t s that probably represent crystal roots of the surrounding

seriate-textured rocks (ol igoclase, sodic K-feldspar, b i o t i t e , and

quar tz ) , and the mi neral s of the apl i t i c groundmass (a1 bi t e , K-fel dspar ,

quartz, and b i o t i t e ) . B u t the phenocrysts show some features tha t

indicate reaction with surrounding material before the magma was intruded

and completely crystal1 ized. These features a re the tendency of the

p l agi ocl ase phenocrysts t o show composi t i ons trending toward sodic

01 i gocl ase even though they characteri s t i cal l y have di s t i nct remnant

cores, and the presence of overgrowths on both plagi ocl ase and a1 kal i

K-feldspar phenocrysts. Wide a1 bi t i c rims occur on plagioclase and

nonperthi t i c K-fel dspar margins occur on large perthi t e c rys ta l s .

The development of a1 bi+tic plagi oclase phenocrysts probably explains

the low modal plagioclase i n - the ap l i t i c groundmass tha t crystal l ized

f rom the me1 t surroundi ng the phenocrys t s . The presence o f apl i t i c

groundmass clear ly indicates t h a t f inal crystal 1 i zation was very rapid

and d i s t inc t ly nonequi l i bri um, hence the reaction between phenocrysts

and surroundi ng me1 t must have preceded i ntrusi on and complete sol i d j f i - cation. Prior t o intrusion, therefore, the s i tua t ion in the pluton

(as shown in f ig . 36) must have involved a carapace o f crys ta l s and

melt surrounding an internal core t h a t was essent ial ly f r ee o f c rys ta l s .

The pluton was almost wholly c rys ta l l ized , and i t appears l ikely tha t

vapor saturation was reached i n the i n t e r i o r a t t h i s time. The presence

of a vapor phase would have aided the reaction of the phenocrysts w i t h

the surrounding melt. Intrusion of th i s crystal-melt-vapor system may

have been in i t i a t ed i n part by the increase i n internal pressure that

could well have accompanied vapor saturat ion. In any case, upward

displacement of the crystal-rich carapace d i d occur, and a r e l i e f of

confining pressure evidently accompanied t h i s magma movement. The vapor

phase escaped rapidly and produced quartz veins, greisen selvages,

and porphyry d i kes along joi nts in surrounding crystal 1 i zed rocks.

Some o f the vapor may have escaped into the country rocks, b u t pertinent

evidence i s not available. Release of the vapor quenched the melt in

the faciks 3B magma, thereby producing the d i s t i n c t apl i t i c groundmass.

As t h i s magma was frozen, the rupture accompanying intrusion was healed

and stage 4 c rys ta l l iza t ion was terminated.

Stage 5--Crystall izat ion of Facies 4A and 4B

S taqe 5 crystal 1 i zati on completed sol i d i f ica ti on of the granite La

complex by forming the fine- and even-grained f e l s i c rocks of zone 4

from the residual melt i n the core o f the pluton. This stage is-considered

to be a separate one mostly fo r convenience, b u t the important processes

involved in th i s part of the c rys ta l l iza t ion history a re closely related

t o those of stage 4 . several d i f fe rent physical changes in the crystal-

1 i z i ng envi ronment occurred d u r i n g the transi t ion from stage 4 crystal 1 i - zation to complete so l id i f ica t ion of the complex, and they resulted

in important differences in the final products, b u t the changes are a1 1

interrelated and fo r the most part were e i ther simultaneous or closely

spaced in time. The spat ia l relationships in and adjacent t o the

complex a f t e r f inal c rys ta l l iza t ion probably were as shown in figure 37 .

Final crystal 1 i zation was a combination of two di f fe rent processes,

in-s i tu crys ta l l iza t ion a t or s l igh t ly below solidus temperatures for

a1 1 the mi neral consti tuents, and 1 ocal i zed 1 ower temperature i nteracti on

of coexisting vapor and crys ta l l ine core a f t e r the pluton margin was

di splaced by normal f au l t s .

Crystall ization a t o r s l igh t ly below the solidus for a l l the mineral

constituents occurred durins o r soon a f t e r emplacement 0,' the magma

tha t formed facies 3B rocks. The lowering o f confining pressure marked

by th i s intrusive event probably also had the ef fec t of moving the

internal residual me1 t below i t s sol idus through exsol u t ion of aqueous

vapor. A t t h i s time the in t e r io r of the pluton was everywhere vapor

saturated, and the textural and mineralogic homogeneity of the facies

4A rocks i s the resu l t of complete c rys ta l l iza t ion of the residual melt

under these condi tions . The l a s t important event in the crys ta l l iza t ion history was the

f r x t u r i ng of the pluton by normal fau l t s that displaced the marginal

zones b u t terminated as a ser ies of splays in the in t e r io r of the yluton.

This faul t ing occurred very soon a f t e r complete so l id i f ica t ion of the

residual me1 t, and these throughgoing s t ructures must have tapped and

localized the t in-rich vapor phase t h a t coexisted with the to t a l ly

~ r y s t a l l i zed , s l igh t ly subsolidus facies 4A rocks. This faul t ing i s

very l ikely t o have been jus t one aspect of the dynamic adjustments

within and near t h e pluton as i t s volume decreased near the end of

crystal 1i zation. When viewed in th i s context, i t becomes apparent

tha t the intrusion of magma t o for,,i faci es 38 rocks, the complete

in-s i tu crystal1 i zat i on of faci es 4A rocks under vapor-saturated

' I - L U y ) a J o a 7 Q J

hC, c 3.7 ~ a , a J m a u

F

m a r r f r r*r L E U aJ 3 o m u r r c C n U ~ r n U O

a 7

LL

conditions, and the faul t ing t o release the confined vapor phase a r e

a l l related events, closely sequential in time, t h a t almost terminated

crys ta l l iza t ion of the complex.

These events did n o t end the c rys ta l l iza t ion history because facies

48 rocks s t i 11 remained t o be formed. I t has been shown that facies 4B

rocks are essent ia l ly equivalent to facies 4A rocks in texture and

composition except tha t they show d i s t i n c t effects of reaction with a

vapor phase a t temperatures below those of fac ies 4A crys ta l l iza t ion .

The facies 48 rocks are spa t i a l ly related to the crosscutting f a u l t

splays, which a re clear ly defined by even lower teniperaturc argi 11 i c

a l te ra t ion and replacement by s i l i c a . I t appears t h a t the formation

of facies 40 rocks was the l a s t c rys ta l l iza t ion event, and that i t

was accompl i shed under total ly subsol idus conditions by reaction of a

coexisting vapor, a t t h i s time only present in the vicini ty of the cross-

cutting f au l t s t ruc tures , with rocks tha t , before reaction, probably

were typical facies 4A rocks. The principal chemical changes that

accompanied these reactions were caused by leaching o f s i l i c a and cer tain

t race elements, par t icular ly t i n , from t h e parent faci es 4A rocks.

In summary, the principal events of stage 5 crystal l i za t ion were

those that closely followed intrusion of facies 38 rocks (stage 4

crystal 1 i za t ion) , including ( 1 ) crystal1 i zation of faci es 4A rocks under

vapor-saturated condi t i ons and a t temperatures near the sol i dus , and

( 2 ) localized reaction of a vapor phase with previously crystal l ized

facies 4A rocks to produce the leached rocks of facies 4B. This was

the l a s t event in crystal l i t a t i o n of the complex.

A1 terna t i ve Interpreta t i ons

The crys t a l l i t a t i on model presented above i s consistent wi t h the

available data and seems required i f the principal assumptions a re

valid. These assumptions a re tha t t he i n i t i a l l y emplaced magma was

compositionally homogeneous and tha t no fur ther addition of magma

(from depths where the magma was generated) occurred during c r y s t a l l i -

zation. Should one or both of these assumptions be inval id , a1 ternat ive

i ntarpretations of the crystal 1 izat ion hi story a r e possible. The forma-

tion of the t ransi t ional marginal faci es remains as the cr i t ical part

o f the c rys ta l l iza t ion history in any a l te rna t ive explanation.

The gradual s h i f t toward more mafic compositions inward through

the marginal facies of zones 1 and 2 could be explained by a n i n i t i a l l y

i nhornogeneous magma. This i nhomogenei ty coul d have been produced by @ progressive tapping of a ver t ica l ly inhomogeneous magma source (ba thol i th)

or by flow di f fe rent ia t ion as the magma rose to i t s level of emplacement.

The magma source could have been segregated so tha t i t became progressively

more s a l i c a t higher leve ls . This vertical segregation could have been

preserved in the separated and risi?ig magma column so tha t the f i r s t -

emplaced magma ( a t the contact) was s a l i c and graded inward to more

femic compositions. ~ o w b e r , t h i s does n o t seem l ike ly because the

processes that might produce a vertical ly ,segregated magma source do

not appear to be applicable here. Vapor saturation and subsequent

migration, along w i t h dissolved a1 kali and s i l i ca const i tuents , t o higher

levels of the parent batholith i s unlikely because the pressures a t the

depths of the parent bath01 i t h would prohibit vapor saturation unless

the magma were extraordinarily rich in volati l e s . Crystal s e t t l i n g i s

discounted because the grani t e compl ex of the Serpentine Hot Spri ngs

area does not contain minerals, such as amphibole, t ha t might have

segregated vert i cal ly i n a parent ba tho1 i t h .

Flow different iat ion during r i s e of the granite magma may have

occurred even though the magma.appears to have been dominantly molten

upon emplacement. This i s possible because the mafic inclusions in the

complex may not be homogeneously dis t r ibuted. The dis t r ibut ion of these

xenol i ths was not studied i n detai 1 , b u t the larger ones appear t o be

concentrated in facies 2 rocks where they locally form screens and pipe-

l i k e swarms. Ef the percentage of mafi c xenoliths does increase inward

through the marginal facies i t could have been caused by segregation of

the xenol i ths toward the in t e r io r par ts of the magma during i t s r i s e

t o the level of emplacement. These xenoliths appear t o be refractory

remnants, and materi a1 s pa r t i a l ly assimi lated from t h e m could have been

responsible, a t l eas t in par t , for the progressive s h i f t t o more mafic

compositions through the margi nal faci es . Another possi ble interpretat ion does not require an i n i t i a l ly

inhomogeneous wgma t o explain development of the marginal facies .

I t could be argued tha t conditions within the i n i t i a l ly ernplaced magma

chamber were conducive to 1 arge-scal e t ransfer of residual coqs t i tuents

from in te r io r parts of the marginal facies ( fac ies 1 C a n d 2 ) in outward

direct ions. This interpretat ion ,requires that the process considered

responsible for stage 1 crystal 1 i za t i on was operative throughout zones

1 and 2 . I f conditions of vapor saturation existed simultaneously

throughout zones 1 and 2 and i f the postulated posit ive temperature

gradient inward d i d ex i s t , then some residual consti tuents could have

been select ively transferred from facies 1 C and 2 rocks toward the

pluton margin, resul t ing in the increasingly more f e l s i c cornpasi t ions

outward.

Regardless of the a l te rna t ive in t e rp re t a t i an ( s ) favored for

development of the marginal facies , i t appears tha t they formed by

dominantly in-si tu crystal 1 izat ion. Further evolution of the in t e r io r

of the complex would have to be explained by crystal 1 i zat i on processes

a t depth and subsequent intrusion of magma tha t was less d i rec t ly related

to the marginal facies than suggested in the detailed five-stage

crystal1 ization model presented ear l i e r ,

Economic Geology --

The Serpentine Hot Springs area includes placer deposits of gold

and cassi t e r i t e and several bedrock areas w i t h anomalous concentrations

of base metals, s i l v e r , and t i n . The presence of c a s s i t e r i t e in placers

was f i r s t reported by Collier (1304, p. 28) ind l a t e r confirmed by

Knopf (1908, p . 63 ) . Moxham and West (1953) outlined the occurrence

of mineralization , 1 ode prospects, and placer deposits in the Serpentine-

Kougarok area and summarized the resu l t s of i nvesti gations fo r radio-

act ive minerals i n t h i s area. Sainsbury and others (1968) presented

data on the nature of c a s s i t e r i t e i n the gold placers of Humbol t Creek

and suggested nearby areas favorable for lode deposits of t i n . The

presence of s ignif icant amounts of c a s s i t e r i t e i n the Humbolt Creek

drai nage 1 ed to reconnaissance. and detailed geochemical surveys tha t

showed the dis t r ibut ion of many anomalous metal concentrations i n b o t h

a l l uvi a1 and bedrock materi a l s (Sai ns bury and others, 1970). The

following discussion o f the nature of the mineralized areas i s based

on the geologic mapping by the author and geochemical data presented

by Sai nsbury and others (1970), supplemented by an addi tional detai led

geochemical soi 1 survey i n one of the bedrock areas. The main purpose

of t h i s part of the report i s t o sumar i ze the nature of the mineral i zed

areas and to discuss t h e i r spat ia l and genetic relationships to the

granite complex. These relationships a re s imilar t o those found in

many other tin-mineralized areas of the world.

Geochemi cal Surveys

The geochemical surveys i ncl uded spectrographic analyses of stream

sediments, pan:ied concentrates, some s lu ice concentrates, and bedrock

and soi 1 samples. The geochemical resu l t s ident i fy many anomalous

metal concentrations in bedrock and a l luvia l materials from a large

area d i rec t ly southeast of the grani te contact. No element associations

a re obviously correlated wi t h the t i n anomalies in a l l uvial mater ials ,

b u t 1 e a d , t i nc, s i l v e r , a rsenic , and antimony a re commorly associated

w i t h t i n anomalies in bedrock materi a'l s .

A1 luvial Materials

A reconnaissance stream sediment survey o f the ent i r e area i den-

t i f i e d a few weakly anomalous t i n concentration-s, mostly in areas within

or d i rec t ly adjacent to the grani te complex. The f a i lu re of these

sediment samples (-80 mesh) t o ident i fy the t i n concentrations known

to e x i s t along Humbolt Creek (Sainsbury and others , 1968) led t o a

second more detailed examination of the al luvial materials in t he

streams draining the eastern part o f the area. In th i s study (Sainsbury

and others , 1970), panned concentrates were found to be part icular ly

useful in detecting important metal concmtrations i n the stream

sediments and i n tracing the metal concentrations to the'ir possible

source areas.

The detai led' study showed tha t anom1 ous concentrations o f t i n

a re present in sediments of both Humbol t and Ferndal e Creeks. The

t i n anomalies were ident i f ied local ly along the en t i r e lengths of these

streams i n the study area, and they are headed i n the hi1 ly areas

just southeast of the granite complex. Other metals commonly present

in anomalous concentrations are go1 d , z inc, molybdenum, and local ly lead,

b u t the panned concentrates did not reveal any strong correlations

between these el ements and the t i n a noma-l i es .

Bedrock Materi a1 s

A search f o r lode occurrences of c a s s i t e r i t e was undertaken in

the bedrock areas between the granite complex and the headwaters of

Ferndale and Humbolt Creeks. This search was seriously handicapped

by the extensive accumulations of f ros t - r i ven rubble that m n t l e s

almost a1 1 of th i s area. The geochemical investigations were therefore

,res t r i cted t o the sampling of surface materi a1 s i ncl udi ng rock frag-

ments, vein materials, and soi 1s from a1 tered zones. The a1 tered zones

were usually ident i f ied by the surface accumulations of discolored,

general ly i ron-hydroxide encrusted rock or gossan fragments, and

especially local ,zed concentrations of f i ne-grained soi 1 materials in

place of the usual rock .fragments. Three mineral ized bedrock areas

w i t h anomalous concentrations of t i n were i denti fied , a1 though no

def in i te lode occurrences of cassi t e r i t e were found.

The three mineralized bedrock areas a re shown in plate 1 . The

two easternmost areas ( loca l i t i e s A and B, pl . 1 ) a re discussed by

• Sainsbury and'others (1970). The t h i r d ( l o c a l i t y C, p l . 1 ) i s nearer

t he eas tern con tac t o f the g r a n i t e complex, and data supplemental t o

those o f Sainsbury and o the rs (1970) concerning t h i s area a re presented

here. This zone i s de f i ned by d i sco lo red and r u s t y s o i l m a t e r i a l s and

i ron-hydroxi de encrusted fragments t h a t occur adjacent t o a very

f ine-gra ined g r a n i t e d i ke t h a t t rends N. 80' E. Th is d i k e appears

t o have been emplaced a long a f a u l t , pnd s l i ckensided fragments of

a l t e r e d rocks present i n t h e sur face m a t e r i a l s suggest t h a t movement

a long t h e f a u l t has been r e c u r r e n t and i n p a r t postdates m i n e r a l i z a t i o n .

Soi 1 samples, i n c l u d i n g se lec ted a1 te red rock fragments, were c o l l ected

a long the l e n g t h o f t he a1 t e r e d zone, and the r e s u l t s of spec t rograph ic

analyses o f these samples a r e l i s t e d i n t a b l e 11. The samples con ta in

@ h i g h l y anomalous amounts o f s i l v e r , a rsen ic , lead, antimony, t i n , and

z inc . This assoc ia t i on o f elements i s p resent i n a l l t h e bedrock

minera l i zed areas w i t h anomal ous concent ra t ions of t i n , Of these

elements, the most abundant a r e l e a d and z i n c .

Spat i a1 and S t r u c t u r a l Cont ro ls

The geochemical surveys c l e a r l y show t h a t m i n e r a l i z a t i o n i n t h e

Serpent ine Hot spr ings area i s l o c a l i z e d i n t h e h i l l y a rea southeast o f

t h e g r a n i t e contact . Th is p a r t of t h e area i s unique i n t h a t i t l i e s

d i r e c t l y above t h e grani t e s tock . The i nd i v i dual minera l i zed areas

a1 1 occur a long f a u l t s , as i s evidenced by the presence o f s u b t l e

t o conspicuous topographic d i s c o n t i n u i t i e s , d i s t i n c t l y 1 i n e a r t r a i n s

o f a1 tered rock o r s o i 1 ma te r i a l s , and t h e common occurrence of b recc ia ted

o r s l i c k e n s i d e d rock fragments i n t h e surface rubb le . The most impor tan t

l o c a l i z i n g f a u l t s a r e those t h a t t r e n d northwest, and the major one o f

these appears t o be the throughgoing s t ructure tha t extends from the

headwaters of Humbolt Creek to the central parts of the exposed granite

complex; This f a u l t terminates as a se r i e s of splays in hydrothermally I

a1 tered zone 4 rocks. The splays localized the devellopment of the

leached, leucocratic rocks of facies 4B. I t i s probable that additional

work woul d 1 ocate other mineral ized s t ructures , ei ther para1 1 el o r sub-

s idiary to the northwest-trending set of f a u l t zones.

Simi lari t i es t o Other Ti n-Mi neral i zed Areas

The t i n and related mineralization present in the Serpentine

Hot Springs area has certain character is t ics indicating that the area

i s simi l a r t o other t i n-mi neral i zed areas even though commercial lode

deposi t s of c a s s i t e r i t e have not yet been ident i f ied. The simi l a r i t i e s

a re those tha t characterize the general geologic environment and the

processes tha t have concentrated, mobi 1 ized, and 1 ocal ized t i n . Th@

processes control 1 i ng t i n dis t r ibut ion in the Serpentine Hot Springs

area a re believed t o be typical of those responsible fo r other t i n

deposits associated w i t h grani te plutons, i . e . , most of the major t i n

deposits of t h e world.

The most important s imilar i ty- i s the close spatial association o f

cassi t e r i t e and other anomalous concentrations of t i n with an epi zonal ,

composite granite 71 uton that i s geochemical ly speci a1 i red fo r ti,n and

other fugi t ive elements (Sainsbury and others , 1968; ?temprot, 1971 ;

Groves, 1972) . The consistent spat ia l association o f most of the world's

major t i n deposits with such granite plutons, mostly b i o t i t e granites

b u t including some two-mica granites , has long been recognized and has

led Sainsbury and Reed (1973, p . 647) t o conclude tha t the consistency

of t h i s association i s comparable to tha t of chromi t e deposits and t h e i r

host rocks.

The second important charac ter i s t ic of the mineralized areas i s

t he i r 1 ocal ization by f a u l t s , par t icular ly f au l t s t ha t e i kher t ransect

the granite o r local ize granite dikes. This type of s t ructure i s impor-

tan t in localizing t i n deposits in many areas (Hosking, 1967; Sainsbury

and Ree8, 1973). In the major t i n d i s t r i c t s where faul t s ' z r e known

to play an important role in localizing the deposits, the timing of

mineralization, faulting , and final crystal 1 i zation of associated

grani tes has been a perplexing and general ly confusing problem. This

i s discussed l a t e r b u t , as in the Serpentine Hot Springs area, the main

problem i s that mineralization has followed movement on f au l t s that

transect the main granite pluton, ye t .they commonly localize fine-

grained ( "rhyol i t i c " ) d i kes . The th i rd important aspect of the mineralized zones in the Serpentine

Hot Springs area i s that the element su i t e associated with the bedrock

t i n anomalies i s character is t ic of the lead-zinc zone developed in

many t i n-mineral ized areas. Metal dis t r ibut ions in t i n-mineral ized

areas c o m n l y show a preferred spat ia l relationship to areas mineralized

with c a s s i t e r i t e or the associated granite complexes (Dewey, 1925;

Sainsbury ana Hamilton, 1967). The metal sui ze present i n anomalous

concentrations in the bedrock areas southeast of the granite complex

i s charac ter i s t ic of the lead-zinc zone of t i n d i s t r i c t s , a zone

character is t ic of the fringe or outer areas of mineralization in the

d i s t r i c t . The implication for the Serpentine Hot Springs area i s that

the major tin-mineral ized areas have not been exposed. I t i s possible,

i f not probable, t ha t the principal t i n mineral ization l i e s down-dip

on the mineralized s t ructures , a t depths that a re near or within the

granite complex.

Therefore, based on simi l a r i t i e s in the general geologic envi ronme~t

and the nature of mineralized areas , i t i s concluded tha t metal -concen-

t r a t ing processes that have been operative in the Serpentine Hot Springs

area are typical of major tin-mineralized areas elsewhere in the world.

For t h i s reason, the fol lowing discussion of the origin of the mineral i -

zation i n the study area has several important implications concerning

the or igin of t i n deposits in general .

Ori gi n of Mineral i zation

The spat ia l association o f t i n mineral ization with the granite

a complex i s a l so believed t o be a genetic association. This study has

shown tha t the grani te complex: ( 1 ) i s geochemically specialized f o r

t i n and other fugi t ive elements, ( 2 ) underwent f r a c t ~ onal crystal 1 i za-

tion and concentrated t i n and related elements to an evolving residual

me1 t , and ( 3 ) evol ved a separate aqueous phase d u r i n g the l a s t stages

of crystal 1 i zation that concentrated the t i n and associated elements . The bedrock mineralized areas a re localized along s tructures that

were e i ther developed or reactivated during the l a t e stages o f c r y s t a l l i -

zat ion, and because these areas are rnineaSalized with a su i t e o f elements

tha t i ncl udes those known t o have been concentrated duri ng crystal 1 i za-

tion of the grani te complex, i t i s be1 ieved tha t the f lu ids responsible

f o r development of the t i n mineralization are those tha t evolved d i rec t ly

from the associated granite complex a t the end stages of c rys ta l l iza t ion .

Such an origin i s generally accepted by most other workers on t i n deposits,

, even though some opposition to th i s relationship ex i s t s (?temprok,

1967). The development o r r e a c t i v a t i o n of f a u l t i n g a t t h e end stages

o f c r y s t a l l i z a t i o n and the subsequent l o c a l i z a t i o n of d ikes and m ine ra l i za -

t i o n a1 ong them i s common i n t i n depos i ts ( e .g . Hosking, 1967). .The

v a r i a t i o n s i n t he pressure regime of t h e c r y s t a l 1 i zi'ng g r a n i t e complex

are be l i eved t o have impor tan t i m p l i c a t i o n s concerning t h e t i m i n g

o f f i n a l c r y s t a l 1 i z a t i o n , f a u l t i n g , and mineral i za t ion . The c lose

t i m i n g between these events i s probably n o t co inc iden ta l , and i s

discussed f u r t h e r below.

I t should be noted here t h a t t h e r e i s evidence f o r two t i n - m i n e r a l i -

z i n g processes. F i r s t , i t has been shown t h a t the vapor t h a t coex is ted

a t near -so l idus temperatures w i t h f a c i e s 4A rocks was probably enr iched

i n t i n . Escape of t h i s vapor immediately upon f a u l t i n g of t h e margins

o f t h e complex would l e a d t o m i n e r a l i z a t i o n a t h igher , coo le r l e v e l s

i n the c r o s s c u t t i n g f a u l t system. Second, t h e remnant vapor t h a t

occupied t h e r o o t s of t h e f a u l t system reacted w i t h c r y s t a l l i z e d fac ies

4A rocks and, among o t h e r t h ings , a1 lowed removal o f t r a c e e l ernents

as t h e temperature lowered. The l a t t z r t i n - m i n e r a l i z i n g process,

leach ing o f t i n - r i c h b i o t i t e g r a n i t e , i s favored by some workers

(T ischendor f and o thers , 1971 , p. 19), b u t i t i s probable t h a t t he f i r s t ,

d i r e c t escape o f h ighe r temperature t i n - r i c h vapor, i s a l s o a major

t i n - m i n e r a l i z i n g process.

CHAPTER I11

IMPLICATIONS OF THE STUDY

The granite complex i s believed to be representative of a large

number of epi zonal grani t e p l u tons , part icular ly those associated with

t i n deposits. Because of t h i s , the implications of th i s study touch

on major problems of igneous petrology as well as the genesis of ore

deposi ts . These a re discussed separately below. The di scussions

present many ideas that are in the formul a t ive stage and, therefore,

actually out l ine major areas in which research i s needed.

Crystal 1 ization ---- of Zoned Granitic Complexes

The general c rys ta l l iza t ion pattern determined fo r the granite

complex can now be compared t o tha t of other grani t ic complexes to see

whether the c rys ta l l iza t ion processes ident i f ied in th i s study a re

more generally operative in the formation of zoned plutons. The general

crystal 1 izatiorppattern o f importance here i s : (1 ) the gradual' inward

change of texture and composi tion that t e s t i f i e s to slow marginal

accretion of increasingly mafic materials, and ( 2 ) termination of marginal

accretion and development of divergent c rys ta l l iza t ion patterns as a

resu l t of the displacements of inte.rior more f e l s i c residual magma.

Of less importanc~ to the general c rys ta l l iza t ion pattern are the

metasomatic a l te ra t ions of rock compositions and some petrographic

a relat ions tha t may occur a t both the beginning and end stages of

crystal l i zation,

A preliminary survey o f the l i t e r a t u r e on zoned g r a n i t i c complexes

c l e a r l y shows t h a t the general c r y s t a l l i z a t i o n pat tern i s of more than

1 imi ted occurrence and t h a t many plutons have c r y s t a l l i z e d i n a manner

s im i l a r t o t h a t of the g r an i t e complex of the Serpentine Hot Springs

area . The most apparent a r e the o the r t in -gran i tes of t he Seward

Peni nsul a (Knopf , 1908; Steidtmann and Cathcart , 1922), but many

addit ional g ran i te complexes a s s o d a t e d w i t h t i n deposi ts a r e probably

a l so s imi la r . Similar plutons a r e the White Creek ba tho l i th of British

Columbia (Ressor, 1958) and several bathol i ths in eas t -centra l Texas

(Keppel, 1940). One of the Texas occurrences, the Enchanted Rock

bathol i t h , has been extensively s tudied (Hutchinson, 1956; Rag1 and and

o thers , 1967, 1968). A1 1 of these bathol i ths a r e of intermediate and

f e l s i c composition and have well-developed thermal aureoles. They show,

on the bas is of f i e l d , mineralogic, o r chemical da ta , evidence of

marginal accre t ion t h a t developed inwardly gradational t ex tu res and

increas ingly mafi c compositions through the major pa r t of t h e i r

c r y s t a l l i z a t i o n h i s t o r i e s . The inward gradational r e la t ionsh ips a r e

terminated by the in t rus ion of c en t r a l l y loca ted , r e l a t i v e l y f e l s i c

magmas. The Enchanted Rock ba tho l i th a l s o has an anomalous contact

fac i es w i t h a composition t h a t appears to have been affected by a1 kal i

metasomati sm,

I t appears then t h a t th-e crys ta l1 i z a t i on pat tern of the g ran i te

complex of the Serpentine Hot Springs area i s duplicated in several

plutons o the r than those of northern Seward Peninsula and other t i n -

mineralized areas . This pat tern has been developed on both stock and

bath01 i t h i c sca i es , and t he crys ta l 1 i za t ion processes responsible seem

t o r e s u l t commonly when intermediate t o f e l s i c magmas are intruded

into somewhat s tab le epizonal environments. Such conditions enabl e

the development of temperature gradients within the plutons and are

conduci ve t o protracted crystal 1 iza t i on h is tor i es . The mode of

crystal 1 i ta t ion documented fo r the granite complex of the Serpentine

Hot Springs area i s , therefore, be1 ieved t o be of much more general

occurrence than now recognized.

Ori gi n of T i n-Beari ng Grani tes --

I t i s generally accepted tha t t i n-grani tes are b i o t i t e or two-mi ca

granites that form epizonal, composite plutons and have la te-crystal-

l i z ing facies . B u t there i s considerable discussion in the l i t e r a t u r e

concerning whether or n o t t i n-grani t e s a re s ignif icant ly d i f fe rent

from othqr grani tes . Most of the discussion has focused on the geo-

chemistry of the plutons and differ ing conclusions have been reached

(Rattigan, 1963; Hosking, 1967; !temprok and Zkvor, 1974). Much of

the confusian in understanding the geochemical data i s believed t o

ex i s t because the d a t a have been generated by sampling programs without

fu l l knowledge of the f i e l d relations necessary to understanding the

observed compositional variations. Even so, the overwhelming evidence

i s t ha t t in-granites a re geochemically specialized ; they contain high

concentrations of such elements as t i n , l i thium, f luorine, beryl1 ium,

and niobium (Ratti gan, 1963; Sai nsbury and others , 1968; 5temProk,

1971 ; Groves, 1972; Tischendorf , 1973).

The trace-el ement data from the grani t e compl ex of the Serpentine

Hot Springs area show especially well the nature o f t h i s geochemical

special izat ion. Because these data apply t o a large su i t e of elements,

i t has been shown tha t the specialization i s a systematic enrichment

i n fugi t ive elements and a depletion in those elements expected to

enter mafi c mineral s t ructures . This systemati c enri chment-depletion

re1 ationship not only characterizes the geochemical special i zation but

also implies tha t the bulk trace-element composition i s the product of

crystal -me1 t processes.

How does such a geochemical ly speci a1 i zed grani t e ori g i nate?

Granite melts can be formed as a resul t of two very di f ferent processes:

anatectic me1 ting or fractional c rys ta l l iza t ion . f n high-gicade metamor-

phi c environments tha t remai n closed systems an i ni t i a1 granite me1 t might

be expected to scavenge fugi t ive trace elements such as those associated

in t i n-grani tes. However, metamorph ; c environments are not characteri s-

t i ca l ly cl osed systems and dispersal of fugi t ive elements during progressive

metamorphism i s to be expected. Therefore, i t i s believed that tin-grani t e

melts are more 1i kely to be the product of fractional c rys ta l l iza t ion of

larger bathol i thic masses than of anatect ic melting. Such an origin pro-

v i des the necessary i n i t i a l me1 t temperatures and imposes the cons t r a i nts

of crystal-l iquid equi l i bri a on the evolving trace-element composition, a

composition tha t i s strongly suggestive of such an origin. An interest ing

possi bi 1 i ty i s t h a t , i f the progeri to r bath01 i t h were granodi ori t i c i n

composition and therefore precipitating s igni f icant amounts o f K-feldspar

and possibly b i o t i t e , the resulting residual granite magma m i g h t be

expected t o have the low K20/Na20 t h a t t in-granites apparently have.

Regardless of the choice o f origin as discussed above, i t may be

important here to mention that t in-granites appear t o be the resu l t

o f crustal processes. The nature of the i n i t i a l geochemi cal special iza-

tion and the l a t e r dis t r ibut ion within the crys t a l l i z i ng granites

clear ly indicate tha t crystal-melt equi l i bri a control the main trace-

element dis t r ibut ion. Such regular and systematic relationships would

n o t be expected t o be produced by the most assuredly complex interaction-

of streaming vol a t i les from subcrustal sources ( M i tchell and Garson,

1972, p. 822).

The Concept of Metal logeneti c Provi nces - -

The inhomogeneous dis t r ibut ion of ore deposits i n rocks of the

earth Is c rus t has fostered the concept of 'metal 1 ogeneti c provi nces . The basic premise of t h i s concept i s that the observed inhomogeneous

dis t r ibut ion i s due t o one, o r a combination o f , the following two

factors: (1 ) certain el ements a re inhomogeneously dis t r ibuted in the

e a r t h ' s c rus t o r mantle, and ore deposi t z are the r e su l t of general

geologic processes acting in areas of higher metal concentrations, and

( 2 ) the processes chiefly responsible fo r determining cer tain element

concentrations are of a special i zed nature and of variable occlrrence

in space and time. If inhomogeneous metal dis t r ibut ions e x i s t they

probably developed early in the e a r t h ' s histo-y and may be in the mantle

(Noble, 1970, p. 1619) o r in the c rus t (Krauskopf, 1970, p . 657) or both.

Important regional variations in the dis t r ibut ion of t i n deposits

have long been recogni zed. Ti n metal 1 ogenetic provinces , commonly

called t i n be l t s , have been mapped on global scales (Schuiling, 1967;

Sainsbury and others , 1969) . The important question concerning th i s

d is t r ibut ion pattern of t i n was asked by Goldschmidt (1958, p . 393):

"Is th i s unquestionable difference in the abundance of t i n . . . in

various regions of the globe due to regional differences in the e f f i -

ciency of the processes that concentrate t i n in such rocks as grani tes?"

This questicn i s ent i rely valid today and s t r ikes a t the heart of the

metallogenetic concept: are the observed t i n distributions due to the

occurrence of special processes or do they reflect special primitive

t in distributions? The probable origin of the Serpentine Hot Springs

granite compl ex and the genesis o f associated mineral izat i on provi de

some important insights into th is question.

If the granite me1 t t h a t crystal1 ized t o form the granite complex

was i t s e l f a residual me1 t developed by bathol i th ic fractionation,

then the origin of associated t in deposits can be visualized as the end

result of a complex mu1 t istage process : (1)generation of a ba-Lhol i thic

me1 t that ( 2 ) fractionally crystal1 ized and ( 3 ) separated a residual

granite me1 t t h a t was (4 ) emplaced in an epi tonal mvironment and

(5 ) fractionated so t h a t i t ( 6 ) developed a residual melt-aqueous

phase system that was enriched in fugitive trace elements, and the , . . . .

aqueous phase was then ( 7 ) localized along timely structures or in

favorable parts of the.granite complex. This multistage process indicates

t h a t maps showing the distribution of t i n deposits directly reveal

places in the earth 's crust where a rather unique sequence o f events

must have occurred. The tin-grani te--ti n deposi t envi ronmer~t does

n o t directly shed 1 i g h t on general crustal or subcrustal characteristics

b u t i t may do so indirectly. This question i s unresolved b u t poten-

t ia l ly resolvable; the cr i t ica l questions of special process versus

special element distribution must be asked of the geologic environments

in which bathol i thic melts are generated and crystal1 i zed. With respect

t o the progenitors of tin-grani tes some of the important questions are:

1 . Are the bathol i th ic melts generated in crustal or subcrustal

environments?

2 . What i s the nature and influence of the tectonic environment

that accompanied bathol i t h i c generation and crystal 1 i zation?

3. Did the batholi ths have a specialized crystal 1 izat ion history?

Did s.uch factors as mineral stabi 1 i t i e s (hornblende vs. b i o t i t e )

play a role in aiding the i n i t i a l concentration of fugi t ive elements?

4 . Were the prebatholi t h i c rocks enriched in t i n?

The evaluation of the data pertinent to these questions i s beyond

the scope of t h i s report b u t , on the basis of t i n occurrences on the

Seward Peninsula, i t appears t h a t progeni tor bathol i ths a re bioti t e

granodiori tes generated in crustal environments ( f o r the most part in

o ld , Precambrian crus t ) not d i rec t ly re la tab le t o act i ve continental

margins. The major point of t h i s discussion i s that the origin of

t i n deposits i s very l ike ly a resu l t of crustal processes, and tha t

therefore the fundamental questions concerning the metal 1 ogenetic

concepts of special process or special eiement dis t r ibut ion (o r both)

a re probably answerable by d i rec t observation of the pertinent geologic

envi ronmen ts . This orf g i n of t i n deposi ts conforms we1 1 with the ideas of

. . Krauskopf (1970), which strongly support a mu1 t i s t age origin for the

- world's principal ore deposits. The geochemical d a t a yet to be gained

may a lso confirm his tentat ive conclusion tha t early geochemical

specialization of crustal materials plays an important role in deter-

mining the potentlal of these materials f o r fostering mineral deposits ,

In e i the r case, i t seems c l ea r t h a t t i n deposits a r e the r e su l t o f

crustal processes; there i s no need to resort t o subcrustal processes

t o explain the i r or igin even though physical interaction of crustal-

subcrustal materials may play a role in creating environments where

crustal bath01 i thi c me1 t s can be generated (Barker and others, 1975).

Ti mi ng of Fracturing , Final Crystal 1 i zation , - and Mineral i zati on -

A common feature of many of the world's tin-mineralized areas i s

the close b u t sequential development in space and time of fracturing

of the main consolidated granite mass, crystallization of the final

residual magma, and mineralization. A few examples of this relationship

are: ( 1 ) the elvan dikes of Cornwall which are coincidental with

"emanative centers" in a zone of crustal weakness of deep-seated

fissuring t h a t f i r s t allowed release of the elvan magma and later the

ore-bearing solutions (Dines, 1956, p. 7 ) ; ( 2 ) the locali tation of

i ntrus'ions , extrusions, d i s 1 ocati ons , and mineralization a1 ong deep

faults in the Erzgebirge (Ti schendorf and others, 1971 , p . 18) ; and

( 3 ) the association of rhyolite dikes, lode deposits, and f a u l t zones

a t Lost River, Alaska (Sainsbury, 1964) . T h e mineralized areas of the

Serpentine Hot Springs area seem t o be simi lar iri' a general way t o

the above examples because the mineralizing fluids and a t least one

porphyry dike w.ere localized a l o n g fractures, the most important of

which transects the mi n mass of consol idated granite and ,dies o u t as

a series of hydrothermally altered splays in the part of the comfjlex

occupied by the evolved residual magma. The timing of fracturing, final

crystal 1 i zation, and mineral i zation i s an important problem because

their obvious sequential development has led t o some confusion in .

understanding the relationship between the magmatic processes and the

ore-formi ng processes.

The problem are in te rpre t ive and come from trying to decide just

how f a r apart the d i f fe rent steps are in time and whether or not they . .

are the resu l t of one continuous geologic process or of d i s t inc t ly

d i f fe rent processes tha t coincidentally occur in the same space a t

about the same time. For example, Boyle (1970, p . 4 ) considers the

f a c t tha t mineralized veins commonly cut and o f f se t grani t ic bodies

and related rocks such as porphyries d s very good evidence t h a t hydro-

thermal ore deposits , in general ,, are unrelated t o magma'ti c prncesses.

Important "post-grani te" local i zi ng f au l t s in t i n-mi neral i zed areas

have been interpreted as parts of regional systems tha t a re unrelated

t o the local epizonal plutonic environment. Mineral ization along these

s t ructures is. considered t o be evidence that the mineral izing f luids

or iginate a t deep levels below tha t of the exposed granite bodies and,

in part a t l e a s t , a r e unrelated t o them (Sainsbury and Hami l ton, 1967) .

I t i s the purpose of t h i s discussion to show that the sequence of

f ractur ing , final crystal 1 i zation, and mineral ization represents the

record o f an essent ial ly continuous process that i s not coincidental il,

in space or time and i s to be expected in most of the cpi zonal magmatic

envi Fonments tha t characterize t i n-mineral i zed areas. The key to

understanding these re1 a t i onships l i e s in a conceptual ization o f the

general physical environment tha t ex is t s in the vicini ty o f a frac- '

t ionating epizonal pl uton tha t approaches, o r reaches, vapor saturat ion

d u r i n g the f inal stages of crystal1 izat ion.

The typical epi zonal t i n-gran i t e pl uton i s post- tectoni c and i s

emplaced into crustal levels where rocks are capable of b r i t t l e f racture.

Consequently pre-emplacement fau l t s a re 1 i kely to e x i s t , and the pl u t o n

may actual ly be localized in part by them. After emplacement,

crystal 1 i za tion proceeds and i s accompani ed by two important changes. @ The f i r s t i s t ha t the to ta l volume occupied by granite material decreases .

throughout crystal 1 i zation and the second i s that the water (and

other valati l e ) content of the' developing residual magma increases.

Both of these changes are important influences on the physical envi ron-

ment because they increase the ins t ab i l i t y of the local pressure regime.

In the f i r s t case, the decrease in the to ta l volume occupied by granite

materials means tha t adjacent country rocks must adjust inward. I f .

they have suf f ic ien t strength, inward adjustment may not be gradual

nor may i t happen u n t i 1 near the final stages of crystal l i za t ion .

In th i s s i tua t ion , fractures that a re essent ial ly analogous to col lapse

s t ructures can develop, and the dislocations may be along new fau l t s

or a1 ong reactivated pre-emplacement faul t s . Development o f these

s t ructures would have the immediate e f fec t of lowering the local pressure

regime in the v ic in i ty of the residual magma. I n the second case, the

build-up of water content i s important because i f saturation occurs,

the local pressure regime can i n some cases increase dras t ica l ly because

of the increased volume of the separated aqueous phase. Therefore, as

f i na l . crystal 1 i zat i on approaches, the combi nation o f the above two major

changes in the local physical environment creates a dynamically urktable

space i n the vicini ty of the c rys ta l l iz ing pluton. Under these condi-

tions i t i s to be expected tha t f ractur ing, probably by some combination

of col lapse1 i ke movements and displacements caused by forceful expulsion

o f the high-pressure aqueous phase and associated melt will occur.

When the confining nature of the country rocks and of the consolidated

granite margins i s terminated by th i s f ractur ing, then the local pressure

regime in the residual melt system in dras t ica l ly lowered and the final

c rys ta l l iza t ion process begins.

The process of f inal c rys ta l l iza t ion i s essent ia l ly one of con-

tinuous transformation from a dominantly melt system t o a completely

aqueous vapor system. When the pressure i s lowered in the residual

melt system, rapid separation and expansion of the aqueous phase occurs

and portions of the residual me1 t a re forced into the country rocks,

typically as porphyry dikes, along w i t h portions of the aqueous phase.

Almost imedia te ly the residual me1 t i s quenched, both in the expelled

dikes and in deeper parts of the magma chamber. Throughout freezing

of the residual me1 t i t i s l ikely tha t aqueous-phase evolution continues

both from internal parts of the residual me1 t system and fron the

externally expel1 ed dikes . As the temperature 1 ow~rs a f t e r quenching .

of the residual me1 t , reaction of the evol ved aqueous phase with adjacent

rocks, including those which have j u s t been formed by quenching, creates

the common a1 tered and mi neral, i zed zones. T h u s , during f i nal crystal - 1 izat ion of many t i n-grani tes the most important physical changes a r e

those t h a t d i rec t ly a f fec t the local pressure regime, As f inal

crystal 1 izat ion approaches, the physical envi ronment i s dynamically

unstable and t h e commonly encountered sequence of f ractur ing, residual-

me1 t freezi ng , and mineral i za t i on represents re1 ated aspects of a con-

tinuous and .in part virtual ly simultaneous process tha t transforms a

residual , water-rich melt system to a system dominated by an evolved

aqueous phase.

The main point of th i s discussion i s t o emphasize the element of

continuity between the magmatic and ore-forming processes in the t i n -

granite envi ronment. The essenti a1 mechani sms i nvol ved have been

di scussed i n the 1 i terature and appl i ed to unders tandi ng i ndi vi dual

deposi ts for some time. For example, E m n s (1 934) considered the

importance o f volume changes accompanying aqueous phase separation,

Raymond and others (1971 ) applied t h i s concept to an understanding of

the s t ructural controls of t i n mineralization a t Wheal Jane, Cornwall,

and P h i 11 ips (1973) examined the possible s t ructural effects of retro-

grade boiling in shallow crustal environments. The data on the molal

vol ume properties of water reported by Burnham an> Davis ( 1 971 )

fur ther emphasize the tremendou's ef fec ts that an evolved aqueous phase

can i ni t i a t e i n a magmatic envi ronment under cer tain conditions . Inhomogeneous and rapidly, even viol entiy , changi ng pressure regimes

a re to be expected in these environments,

Importance of Depth in Environments of Tin Mineral ization - -

The search for s imi l a r i t i e s among t i n deposits i s useful b u t much

can also be 1 earned by examining t h e i r differences. The most obvious

differences a re those tha t a re defined by the spat ia l and structural

nature of the tin-bearing areas. In t h i s regard, the depth of formation

of t i n deposits i s believed to be direct ly re5ponsible f o r more of the

major differences than any other s ingle factor . To help c l a r i fy t h i s ,

t i n deposi ts can be grouped into f ive general categories which , according

to t h e i r increasing depth of formation, are: ( 1 ) deposits formed a t

the surface, ( 2 ) deposits formed in near-surface environments, ( 3 ) deposits

formed mostly exter ior to parent granite bodies, ( 4 ) deposits formed

within parent granite bodies, and ( 5 ) cer tain pegmati tes . The general

character is t ics of these groups are as follows: a

1 , Deposits formed a t the surface occur in rhyol i te flows and

domes and consist of hemati te-cassi t e r i t e veinlets , veins, encrusta-

t ions , and disseminations tha t are localized along jo in t s , ' fractures,

f;ow bands, i ntraformational breccias, o r in cavi t ies . The host rocks

a re commonly devi t r i f ied and may be replaced by zec l i tes and s i l i c a .

Production i s nbstly from placers tha t a re derived from the commonly

low-grade lodes. Numerous examples of t h i s type of deposit are found

in Mexico, New Mexico, and Nevad?.

2. Deposits formed i n near-surface envi ronments a re structural ly

and m i neralogi cal ly compl ex polymetal 1 i c veins and s tockworks . They

a r e associated with intrusive dike complexes and plugs tha t commonly

grade in to coeval volcanic rocks, Breccia s t ruc tures , a t l e a s t in part

diatremes, are common. Hydrothermal a1 terat ion of the host rocks can

be extensive and includes ch lor i t iza t ion , s e r i c i t i z a t i o n , and s i l i c i f i -

cation. The exceptionally rich t i n-si 1 ve.* deposits of Bol ivia a re

commonly ci ted as examples of th i s group, b u t others ex i s t and the group

may a lso include deposits such as those associated with ring-di ke

complexes in Brazi 1 and Africa. 3

3. Deposits formed mostly ex ter ior to parent granite bodies are

probably the most abundant and economically important t i n deposi t s of

the worl d . They are commonly tin-tungsten-sul f ide deposi ts , and quartz

i s the most important gangue mineral . The veins tend t o be localized

along porphyry dikes and can have a preferred or ientat ion within a

par t icu lar mineralized area. Zoning of metal dis t r ibut ion within

mineral ized d i s t r i c t s o r within individual veins i s commn. A 1 terat ion

c lear ly i s spa t i a l ly related to mineralized s t ructures and includes

such types as greisenization, s e r i c i t i za t ion , a1 bi t i za t ion , and

tourmal i ni ta t ion. Postmi neral ization argi 1 l i zation i s a d i s t inc t

charac ter i s t ic of some of these deposits. Some of the best examples

of deposits in th i s g roup a re the veins of Cornwall.

4. Deposi ts formed within parent grani t e bodies a re characteri s-

t ica l ly low-grade deposi ts i n which cassi t e r i t e occurs i n di ssemi na-

ti ons , vei nl e t stockworks, and 1 ocal pegmati t i c pods or 1 enses wi thi n

a composite p l u t o n . Contacts between early- and late-crys t a l l i zi ng

facies may a c t as s t ructural t raps and control local izat ion of greisen

and ore. In th i s s i tua t ion , the greisen and ore are preferent ial ly

developed i n the l a t e r crystal 1 i zing, generally f i ne-grained faci es .

Wolframite may occur i n the deposit as well as f l u o r i t e , 1 i thium-rich

micas, and tourrnal i ne. Examples of t h i s group are the grei sen deposits

of the Erzgebi rge and several deposits, such as the Anchor mine, i n

Tasmani a and Austral i a.

5. Pegmati t e s that form t i n deposits a re comnonly large discrete

bodies that may or may not be direct ly associated with granite plutons.

They character is t i cal l y occur in h i gh-grade metamorphic terranes and

- are Precambrian in age. The grade of these deposits i s low b u t t he i r

value may be increased by the presence of minerals other than cassi t e r i t e .

In f a c t , col umbi te-tantal i t e i s d is t inc t ly more comnon i n cassi t e r i te-

beari ng pegmati tes than in any other type of t i n deposi t . Important

ti n-bearing pegmati tes occur in the shield areas o f Africa and Brazi 1 .

Most of the world's t i n deposits fa1 1 into the above groups b u t

not a1 1 o f them. A n important exception i s the "geosyncl i nal " deposi t s

(Baumann, 1970, p. 868) t ha t are stratabound and associated wi t h sub-

marine volcanic rocks.

The r o l e of depth i n determin ing t h e na tu re of t he f i r s t two groups

i s r a t h e r c l e a r : deposi ts formed a t t he sur face i n r h y o l i t e f lows and

domes a r e e s s e n t i a l l y the r e s u l t of in-s i tu processes t h a t a r e l a c k i n g

i n s i gni f i c a n t t i n - c o n c e n t r a t i n g mechanisms, and depos i ts formed i n

near-sur face envi ronments can have a1 1 the compl e x i t i e s t h a t charac-

t e r i ze an e s s e n t i a l l y open magmatic-hydrothermal system. I n t h e nex t

two deeper groups t h e depth o f formation i n f l uences the m o b i l i t y o f t he

r e s i d u a l me1 t and/or aqueous phase system. I n depos i ts formed e x t e r i o r

t o the parent g ran i tes , dynamic i n s t a b i l i t y e x i s t s i n t he v i c i n i t y

o f t h e c r y s t a l 1 i z i n g p l u t o n and l a t e i n t r u s i o n s commonly accompany

expu ls ion o f t h e evolved aqueous phase. I n depos i ts formed w i t h i n t h e

pa ren t g ran i tes the evolved aqueous phase may be mobi le w i t h i n the hos t

i n t r u s i ve compl ex, where i t i s commonly I ocal i zed i n s t r u c t u r a l t r aps ,

b u t t h e increased pressure regime i n the v i c i n i t y o f t h e evolved aqueous

phase does n o t exceed the con f i n ing a b i l i t i e s of surrounding rocks and

i t s escape t o h ighe r 1 evels i s prevented. I n t he deeper 1 eve ls where

t i n - b e a r i n g pegmati tes a r e formed, t h e aqueous phase i s a medium t h a t

f a c i 1 i t a t e s t r a n s f e r o f m a t e r i a l s i n t h e r e s i d u a l environment, bu t i t

i s n o t p h y s i c a l l y separated from t h e s i l i c a t e components o f t h ? f i n a l

r e s i dua 1 sys tem.

The above f i v e - f o l d s u b d i v i s i o n i s a r b i t r a r y . Even though i t s

purpose i s t o h e l p c l a r i f y d i f fe rences between t i n depos i ts , i t a l s o

helps t o show t h a t c o n t i n u i t y e x i s t s among them. A1 1 g radat ions o f

s p a t i a l and s t r u c t u r a l r e l a t i o n s h i p s , from those of the sur face envi ron-

ment t o those o f pegmati tes, a r e t o be expected. F o r example, the d c p o s i t

a t M t . -B i scho f f , Tasmania (Groves and o thers , 1972) i s associated w i t h

a r a d i a l l y o r i e n t e d d ike system and may have formed a t a dep th intermediate

between the near-surface environment and t h a t of most deposits formed

exterior t o granite plutons. Some mineralized cupolas or cusps on

grani te plutons may be examples of deposits intermediate between those

formed exterior and those formed interior t o the parent granites.

This would appear t o be the case i f the cupola represents mobile residual

melt t h a t i s structurally trapped a t the pluton-country rock contact,

and i f stockworks and greisen develop both within i t and in adjacent

country rocks. Unfortunately, i t i s not always clear whether mineral i zed

cusps are developed in residual faci es or simply represent localization

of the mineralizing aqueous phase in a structural trap formed by the

contact between earlier granite facies and the country rocks. The

deposit a t Lost River, Alaska (Sainsbury, 1964), i s a possible example

of a deposit that i s intermediate between those formed interior and

those formed exterior t o the parent. granite pluton.

The above emphasis on the role of depth in determining major

spatial and structural differences between tin deposits i s n o t unique

t o this discussion. I ts i kson (1960) uses the depth of formation as

one useful aspect o f a categorization of regional relationships in P a

t i n-mi neral i zed be1 t s . Rudakova and T i khomi rov (1970, p. 89) state

the "The reason for diversity of the types of t in deposits associated

with intrusives i s the formation of the la t te r a t different depths

in the earth," and Varlamff ('1974) has sketched o u t a rather complete

classification of t in deposits t h a t i s based primarily upon their depth

of formation. B u t even though depth of formation may be the most

important single factor responsible for spatial and structural dif ferences

among t in deposits, i t i s not the only factor. Some others are. the -

pre-emplacement physica: nature of the country rocks, the influence of

t e c t o n i c displacements un re la ted t o the c r y s t a l 1 i z i n g p l u ton, and t h e

abso lu te amount o f water and other v o l a t i l e s con ta ined i n t h e g r a n i t e

me1 t. The i n t e r a c t i o n o f f a c t o r s such as these and o t h e r s he lps t o

c r e a t e an element of uniqueness f o r each d e p o s i t and, i n l i g h t o f t h e

a c t u a l comp lex i t i es of t h e magmatic-hydrothermal envi ronments t h a t .form

t i n depos i ts , i t i s remarkable t h a t so many s i m i l a r i t i e s and i r . 3 i c a t i o n s

of c o n t i n u i t y among them e x i s t . Because o f t h i s c o n t i n u i t y , f u r t h e r

c l a r i f i c a t i o n o f t h e s p a t i a l and s t r u c t u r a l n a t u r e o f t i n depos i ts

i s l i k e l y t o r e s u l t i n impor tan t s teps toward understanding t h e dynamic

n a t u r e o f ep i zonal magmati c env i ronments .

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