Mineralization and alteration in theGreaterville district, Pima County, Arizona

Item Type text; Thesis-Reproduction (electronic); maps

Authors Gross, Michael Philip, 1939-

Publisher The University of Arizona.

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MINERALIZATION AND ALTERATION IN THE GREATERVILLE

DISTRICT, PIMA COUNTY, ARIZONA

by

M ichael P. G ross

A T hesis Subm itted to the Faculty of the

DEPARTMENT OF GEOLOGY

In P artia l Fulfillm ent of the Requirements For the Degree of

MASTER OF SCIENCE

In the G raduate C ollege

THE UNIVERSITY OF ARIZONA

1 9 6 9

ACKNOWLEDGMENTS

The author w ishes to acknow ledge w ith g ratefu l apprecia tion

the many people who have helped and encouraged him in th is project:

D r. John G uilbert for h is d irec tio n , co u n se l, and many helpful su g g es­

tions; M r. H arald Drewes of the U. S. G eological Survey for sug g est­

ing the problem; D rs . R. C . P e terson , S. R. T itley , D . L. Bryant, and

M r. J. S tew art for d isc u ss in g various a sp e c ts of the problem . The

author is indebted to M r. G . A. Barber of The Anaconda Company who

a lso d isc u sse d the geology of the area and opened new id eas and to

The Anaconda Company for p ro cess in g thin se c tio n s and po lished sur­

fa c e s . To my wife W ilm a, I w ish to ex p ress my g ratitude and app recia ­

tio n for her p a tien c e , encouragem ent and help in ed iting the m anuscrip t.

i i i

TABLE OF CONTENTS

LIST OF ILLUSTRATIONS ............................................................................... vi

LIST OF TABLES................................. .... .................................................. v iii

ABSTRACT................................................................................................................ ix

INTRODUCTION.......................................... .... ......................................... 1

Purpose and P r o c e d u r e s ...................................................................... 3M ining H is to ry ........................................................................................ 5

GENERAL GEOLOGY ......................................................................................... 8

Lower C re taceous (?) Sedim ents and M etasedim ents . . . . 8D escrip tion of Sedim ents and M etased im ents . . . . 8D isc u ss io n of Sedim ents and M etased im ents . . . . 17Summary ....................................................... 22

Igneous R o c k s ................................................... 23S um m ary .......................................................................... 28

Structural G e o l o g y .............................................................................. 29Regional S tru c tu re ....................... 29Local S t r u c tu r e .............................................. 29Sum m ary ......................................................................................... 36

DESCRIPTION OF ALTERATION AND MINERALIZATION........................ 41

A lte r a t io n .................................................................................................. 42A lteration in Igneous R o c k s ................................................... 42A lteration in Sedim ents and M e ta s e d im e n ts ................... 52

M inera liza tion in S ed im en ts, M e tased im en ts ,and Igneous R o c k s ................................ 56D issem inated M inera liza tion . . . . . . . . . . . 56Vein M in e ra l iz a t io n ................................................................. 60Z o n in g ................................ 63P arag en esis . .......................................... •................................ 64S um m ary ....................... 66

ANALYSIS OF STRUCTURE, MINERALIZATION, ALTERATION,AND METAMORPHISM........................... 67

Im plica tions of Folding and I n t r u s i o n .......................................... 67Im plica tions of M in e ra liz a tio n , A ltera tion ,

and M e ta m o r p h is m ................................................................. 70

P ag e

iv

V

TABLE OF CONTENTS—C ontinued

Page

M inera liza tion ............................................................................... 70A lteration ......................................................................................... 71M etam orphism .................................................................................... 72

In terp reta tion of S tru c tu re , M etam orphism , A lteration,and M in e ra liz a tio n ........................... 74

SUMMARY AND C O N C L U S IO N S ................................................................. 76

SELECTED BIBLIOGRAPHY............................................................................... 80

LIST OF ILLUSTRATIONS

1. Index M a p ..................................... .... .................................................. 2

2 . The Morning Star M in e ................................................................. .... 7

3 . Geology of the G ranite M ountain A r e a ............................ in pocket

4 . S eric itized M e ta - a r k o s e ................................................................. 11

5 . M eta -a rk o se in Thin S e c t i o n ........................................................ 12

6. Altered M eta -a rk o se in Thin S e c t io n ............................................... 12

7 . F e ld spa th ic Q uartz ite ...................................................................... 14

8 . Slightly R ecrysta llized Limestone . . . ............................ 14

9 . Veins of C a lc ite and Q uartz in L im es to n e ................... .... 20

10. Layered H o r n f e l s ................................................................ 21

11. R elatively Hom ogeneous H o r n f e l s ............................................... 21

12. A lteration Map of the G reaterv ille A re a ............................ in pocket

13. Porphyritic Q uartz M o n z o n ite ........................................................ 24

14. L east Altered Porphyritic Q uartz M o n z o n ite .............................. 24

15. Precam brian G ranodiorite . ............................................................. 27

16. G eologic S ections of the G reaterv ille A r e a ................... in pocket

17. Drag Folds in L im e s to n e ....................................................... 31

18. Drag Folds in M e ta s e d im e n t ........................................................ 31

19. Breccia D ike(?) .................................................................................... 35

20. Rose Diagram of Jo in ts in Southw est P o r t i o n ........................ 37

21. Rose Diagram of Joints in N ortheast P o rtio n ........................' . 38

22. Rose Diagram of Jo in ts in Three Stocks ofPorphyritic Q uartz M o n z o n ite ............................................... 39

F igure P age

v i

LIST OF ILLUSTRATIONS—C ontinued

Figure Page

23. Strongly Altered Porphyritic Quartz M o n z o n ite ........................ 44

24. C hem ical G ains and L osses in Porphyritic QuartzM onzonite w ith Increasing S eric itic A lteration . . . 51

25. S eric itized F eldspath ic Q u a r t z i t e ............................................... 54

26. Pyrite in V e i n l e t ............................................................ 58

27. Porous H o rn fe ls .................................................................................... 58

28. Sulfides D issem inated in H o rn fe ls ............................................... 59

29. Sulfides D issem inated in Porphyritic Q uartz M onzonite . . 59

' 30. Vein Quartz w ith S u l f i d e s ............................................................. 61

31. Sheared Outcrop Stained w ith L im o n ite ..................................... 62

32. Sheared Z o n e ........................................................................................ 62

33. Paragenetic Sequence of Hypogene M inerals inG reaterv ille A r e a ...................................................................... 64

34. C halcopyrite in P y r i t e ................................ 65

35. Boundary R e l a t i o n s ............................................... 65

v ii

LIST OF TABLES

1. M odal A nalysis of Six Sam ples of P artia lly AlteredPorphyritic Quartz M onzonite , R econstructed toO riginal C om position by W eight Percen t ............................ 46

2. M odal A nalysis of Five Sam ples of Altered PorphyriticQ uartz M onzonite by W eight P e r c e n t ..................................... 47

3 . C hem ical C om positions of Six Sam ples of P artia llyAltered Porphyritic Q uartz M onzon ite , R econstructed to O riginal Com position by W eight P e rc e n t . . . . . . 49

T ab le P age

4 . C hem ical C om positions of Five Sam ples of AlteredPorphyritic Q uartz M onzonite by W eight Percent . . . 50

v i i i

ABSTRACT

The G reaterv ille area is midway along the e a s t flank of the

Santa Rita M ounta ins.

N o rth east-so u th w est com pression during Laramide deform ation

folded a sequence of Lower C re taceous (?) sedim ents in to a large syn­

c line w ith a northw est-trend ing a x is . Three porphyritic quartz monzonite

s to ck s intruded the w eakened c en tra l part of the sy n c lin e .

M ost sedim ents show m etam orphism , which may have fa c ili­

ta ted la te r a lte ra tio n and m inera liza tion .

Hypogene q u a rtz -se r ic ite a lte ra tio n , p resen t in a ll rock ty p e s ,

v a rie s in ex ten t and in te n s ity . W eak and sparse p o ta s s ic a lte ra tio n ,

cen tra lly located in the s to c k s , grades through in creas ing ly in ten se and

ex tensive se r ic itic a lte ra tio n toward th e ir m arg ins. S e ric itic a lte ra tion

is pervasive and m oderate in m eta-a rkose and q u a rtz ite , w hile ranging

from pervasive and in ten se to nonex is ten t in h o rn fe lse s .

Q uartz-carbonate ve in s con tain sim ilar m ineralogy in a ll rock

ty p e s . Veins dip tow ard the cen te rs of in tru sive s to c k s , ind ica ting th a t

m ineralizing so lu tions were derived from the cooling magma.

M ost m ineralized ve in s are a sso c ia te d w ith in ten se quartz -

se ric ite a lte ra tio n ; m ineralization and a lte ra tio n are m ost in ten se a lone

sheared and b recc ia ted zones in c re s ts and troughs of folds th a t abut

s tocks than in folds th a t do not abu t s to c k s . Both m ineraliza tion and

a lte ra tio n were cau sed by hydrotherm al so lu tions em anating from the

cooling magma which formed the porphyritic quartz m onzonite in tru s io n s .

ix

INTRODUCTION

The G reaterv ille mining d is tr ic t i s on the e a s t flank of the

Santa Rita M oun ta in s, roughly midway along th e ir le n g th . This study

encom passes a roughly sym m etrical area around a porphyritic quartz

monzonite in tru s iv e , loca lly known as G ranite M ountain , w hich is s i t­

uated approxim ately one mile w est of the ghost town of G reaterv ille

(Figure 1). The study w as designed to determ ine the n a tu re , d is trib u ­

tio n , and derivation of a lte ra tio n and m ineraliza tion in the a re a .

The Santa Rita M ountains are a part of the Basin and Range

province of the Southw estern United S ta te s . Including the Patagonia

M ountains, a con tinuation of the Santa Ritas to the sou th , the range

is approxim ately 45 m iles long w ithin the United S ta tes and con tinues

further in to M exico. The range is long itud inal w ith i ts long ax is ori­

ented in a roughly no rth -sou th d irec tio n . H ow ever, the G reaterv ille

mining d is tr ic t is s itu a ted a t a m ajor change in the trend of the range;

south of the d is tr ic t the range trends approxim ately N20°W , w hile i t

trends approxim ately N20°E north of the d is tr ic t . This change in trend

may have been an im portant fac to r in the developm ent of the re la tiv e ly

com plex geology in th is part of the ra n g e .

The area is e a s ily a c c e ss ib le from Tucson over paved and

gravel roads th a t were w ell marked a t the tim e of th is fie ld w ork. One

drives e a s t on U .S . Highway 80 to i ts junction w ith Arizona S tate H igh­

w ay 83, turns south on Highway 83 for 18 m ile s , then tu rns right follow ­

ing the F orest Service road signs to G reaterv ille (Figure 1), From

1

0 Tucson

Figure I. Index Mop

3

G re a te rv ille , one proceeds up the tra il marked 'M elendrez P a s s .1 The

f irs t c a ttle guard on the tra il marks the m ap 's e as te rn boundary. Spe­

c if ic a lly , the map area is in the S l /2 s e c . 24, SE 1/4 s e c . 23, E l/2

s e c . 26, and se c . 25, T. 19 N . , R. 15 E. of the G ila and Salt River

M erid ian .

Topography is m oderately rough , co n sis tin g of ro lling h ills

cut by numerous w ash es and gu lches; e leva tions range betw een 5200

fee t and 6000 fe e t, w ith the h ig h es t e lev a tio n s on G ranite M ountain

and another h ill to the w e s t. D ata from the U niversity of Arizona Agri­

cu ltu ral Experiment S tation Bulletin 279 (1956) ind ica te th a t tem pera­

tu res reach an average high of 100°F in June and July and an average

low of 25°F in D ecem ber and January . Between th e se e x tre m e s , tem per­

a tu res are ra ther p le a sa n t w ith warm days and cool n ig h ts . Rainfall

averages 15 to 16 in ch es per year w ith April to July being the d rie s t

p e rio d . V egetation , h e a v ie s t on the north s lopes of a ll h i l l s , is char­

ac te rized by sm all sh rubs , c a c ti , and range g rass a t low er e leva tions

and scrub oak , m anzan ita , and jun iper a t h igher e le v a tio n s .

Purpose and Procedures

This study w as approached w ith the expec ta tion th a t an under­

standing of the re la tio n sh ip s betw een a lte ra tio n , m inera liza tion , and

igneous ac tiv ity in th is re la tiv e ly sm all area would y ie ld a more com­

p le te understanding of the d is tr ic t . In addition to G ranite M ountain , two

o ther porphyritic s tocks were found to intrude Lower C re taceous (?) sed i­

m ents of variab le litho logy . It w as proposed to determ ine the nature and

d is trib u tio n of a lte ra tio n and m ineraliza tion in re la tion to the in tru s io n s ,

and to d iscover the e ffe c ts of therm al, chem ical, s tru c tu ra l, or s tra ti­

graphic fac to rs upon the a lte ra tio n and m ineralization of the ho st ro ck s .

C ursory field exam ination showed d isce rn ib le m ineraliza tion

throughout the a re a , a lthough p ro spect p its and m ines were sm all and

sc a tte re d . M inera liza tion appeared prelim inarily to be re la ted to the

porphyritic in tru s ions; fu rther, m ineralization alw ays appeared to be

a sso c ia te d e ith e r w ith m etam orphosed or highly a lte red ro ck s .

A p ilo t study of sev era l random ly se lec te d rock types w as con­

ducted to fa c ilita te fie ld recognition of m etam orphic m inerals and vari­

ous types of a lte ra t io n . In an attem pt to e s ta b lish m egascopic c rite ria

for th e s e , hand specim ens w ere exam ined w ith a b inocular m iscroscope

and th in sec tio n s w ere studied w ith a petrographic m icroscope. How-,

ever, fine grain s iz e , va riab le litho logy , and pervasive q u a rtz -se r ic ite

a lte ra tio n made m egascopic recognition of m etam orphic m inerals and

varia tions in a lte ra tio n u n re lia b le .

Field mapping w as done on Forest Service a ir photos blown up

to a sca le of 1:7920 or 1 inch = 660 fe e t. In the f ie ld , sec tion corners

w ere loca ted on the a ir photos in order to accu ra te ly tran sfe r field data

to the topographic base m ap. Seventy th in sec tio n s of the various rock

ty p es and a lte ra tio n products were analyzed p e tro g rap h ica lly . Twenty-

five sam ples of the sulfide m ineralogy, some of w hich dup lica ted thin

sec tio n sam p les , w ere p rocessed a s po lished su rfaces for m ineragraphic

a n a ly s is .

Both petrographic and m ineragraphic a n a ly se s were supple­

m ented by X-ray a n a ly s is using the pow der-d iffraction m ethod. X-ray

d a ta were used to iden tify and to confirm id en tifica tio n of fine-g ra ined

4

m inerals v is ib le in th in se c tio n s . X-ray da ta w ere a lso used to iden tify

minor su lfides th a t could not be determ ined w ith a m ineragraphic micro­

scope from po lished s u r fa c e s .

M ining H istory

Early in 1874, oxidized gold and s ilv e r ore w as d iscovered on

p roperties now known as the Yuba mine and the M orning Star m ine. The

M orning Star, mine w as formerly known a s the S t. Louis m ine. A p rospec­

tor named Smith (Schrader, 1915) d iscovered p lac e r gold la te r th a t same

y ea r. The gold p lacers soon brought about 200 men in to the camp and

stim ulated lode mining b ecau se th e ir very numbers le sse n ed the danger'

of Indian a t ta c k .

From 1875 to 1878, the p lacers were worked quite thoroughly

and by 1886 they w ere considered worked out; but from 1886 un til the

1 9 2 0 's , and probably even in to the 1930 's , there were s ti ll a few pros­

pecto rs working the g ra v e ls . Of la te , there has been litt le p lace r ac tiv ity .

Although the G reaterv ille d is tr ic t i s reported to have produced

over $7 m illion worth of ore (Schrader, 1915), i t i s estim ated th a t only

a sm all portion of ore w as produced from underground m ining. "The lode

d e p o s its occur a s q u a rtz -c a lc ite ve ins in the granite and the sedim entary

rocks surrounding G ranite M ountain . The ve in s are commonly banded ,

and where they occur in the granite or a sso c ia te d w ith the g ran itic

ro ck s , they u sua lly con ta in a s a gangue m ineral a lso barite" (Schrader,

1915, p . 153). V eins, pa rticu la rly th o se th a t developed horn s ilv e r in

th e ir oxidized z o n e , were opened w ith sm all m ines and many p rospect

p its ; m ost produced a t le a s t some ore (Schrader, 1915, p . 153). Gold

5

6

and silv er in sp ired m ost of th is early mining a c tiv ity , but the veins a lso

contained varying am ounts of p y r ite , g a len a , sp h a le r ite , and ch a lco -

p y r ite .

Since the early 1 9 2 0 's , alm ost no work has been done on any of

the m ines w ith the excep tion of the Morning Star mine where the h ead -

fram e, h o is t h ouse , and m achinery are s t i l l m aintained and operated

sporad ica lly by M r. Fred Harmes (Figure 2). Beginning in the la te

1950 's , severa l mining com panies have briefly surveyed the d is t r ic t 's

p o ten tia l. At p resen t, The Anaconda Company is doing exploratory work

in the a re a .

7

Figure 2 The Morning Star Mine

GENERAL GEOLOGY

The three re la tiv e ly large in trusions of porphyritic quartz mon-

z o n ite , which are of Laramide age (Mr. H arald D rew es, oral communi­

cation) , have been em placed in a folded sequence of Lower C re taceo u s (?)

sedim entary and m etasedim entary ro c k s . Sedim ents and m etasedim ents

re s t upon Precam brian g ran o d io rite , w hich m akes up the core of the

ran g e . Figure 3 (in pocket) show s the geologic re la tio n sh ip s of the a re a .

Nonigneous rocks c o n s is t of in terbedded a rk o se , san d sto n e , sh a le , and

impure lim estone th a t were m etam orphosed in m ost of the area because

of th e ir proximity to the three in trusive b o d ies . M etam orphism has con­

verted the arkose to m eta -a rk o se , the sandstone to fe ld sp a th ic q u a rtz ite ,

and the sha le and lim estone to hom fels and a rare skarn rock very near

the igneous co n ta c t.

Lower C re taceous (?) Sedim ents and M etased im ents

Although not w ithin the scope of th is s tudy , it is extrem ely un­

fortunate th a t the d e ta iled stra tig raphy of th is portion of the C re taceous

sec tio n is so im perfectly known, both here and e lse w h e re . This lack of

d e ta iled stra tig raphy p recludes any re liab le co rre lation of the a re a 's

estim ated 4000 to 5000 fee t of Lower C re taceous (?) sedim ents (Figure 3)

w ith o ther C re taceous sec tio n s in southern Arizona.

D escrip tion of Sedim ents and M etased im ents

M ost beds are re la tiv e ly th in (less than s ix feet); many are be­

tw een s ix inches and two fee t th ic k , making them very d ifficu lt to follow

8

9

for any d is ta n c e . A few th icker beds were observed to thin in a t le a s t

one d irec tio n . Field observation showed th a t many beds in th is sequence

are probably a lso very lo ca l in n a tu re , not ex tending more than two or

three hundred y a rd s . Because of th ese v aria tio n s in litho logy , it would

only be p o ss ib le to co n stru c t a very generalized and inadequate s tra ti­

graphic column; therefore th is w as om itted.

It should be em phasized th a t many, if not a l l , of the sedim en­

tary and m etasedim entary un its in the area are re la tiv e ly th in and of

lim ited la te ra l ex te n t. Thus, the sym bols for the Lower C re taceous (?)

sedim ents and m etasedim ents on the geolog ical map (Figure 3) rep resen t

' s truc tu ra l ra ther than litho log ic con tinu ity .

Schrader (1915), Stoyanow (1949), button (1958), and H eatw ole

(1965) report a reg ional northw est strike and n o rth east dip in the C re ta ­

ceous rocks along the e a s t flanks of the Santa Rita M ounta ins. Coupled

w ith th is , c ro ss-bedd ing and graded bedding, although it is s c a rc e , in ­

d ica te th a t the stra tig raph ic sequence goes up from w est to e a s t acro ss

the a re a . Very g en e ra lly , the sec tion changes from arkose a t the bottom

through sandstone and to sha le and lim estone as the h ig h es t exposed

un its on the e as te rn edge of the a re a .

Arkose and m e ta -a rk o se . The low est beds of exposed a rk o se ,

w hich seem to be Lower C re taceous (? ) , con tain rounded pebb les of

q u a rtz ite , rounded to angular fragm ents of p lag io c la se and o r th o c la se ,

and rounded to angular fragm ents of h o m fe ls , a ll presum ably derived

from the underlying Paleozoic and PreCambrian ro c k s . Rarely a p iece of

Precam brian granodiorite is found; one p iece m easured more than six

inches in d iam eter. Q uartzite pebb les from one to three inches in

10

diam eter are common (Figure 4), and sev era l larger pebb les are p re sen t.

In the low er part of the sec tio n , the arkose is tan to buff on the w eath ­

ered surface (Figure 4) and only sligh tly w hiter on the fresh su rface .

H ow ever, h igher in the se c tio n , the arkose beds become more red d ish ,

both on w eathered and fresh su rfa ce s .> .

M iheralog ical com position varies som ew hat, but thin sec tion

a n a ly s is in d ica te s average com position of the arkose beds a s follow s:

M ineral Percent

Q uartz 50-60

P lag io c lase 10-20

O rthoclase 10-20

Seric ite 10-15

C a lc ite 0 - 5

The co n stitu en t grains vary from rounded to angu lar, and rec ry sta llized

quartz grains appear to have filled in te rs ti tia l space (Figures 5 and 6).

O utcrops of a rk o se , pa rticu la rly the th ick er u n its , are abundant

and w ell ex p o sed . In the upper portions of the sec tio n , beds of arkose

tend to be finer grained and le s s abundant than in the low er p o rtions .

All sam ples of arkose th a t were exam ined contained some se ric ite ; how­

e v er, su lfide m inerals were very scarce and co n sis ted of anhedra of

pyrite (or lim onite a fte r pyrite) w hen p re sen t. M ost beds of arkose may

be c la s se d more accu ra te ly as m eta-arkose b ecause they show evidence

of m etam orphism .

Sandstone and q u a rtz ite . M ost sandstones have been metamor­

phosed to q u a rtz ite s and a lso seem to d ecrea se in abundance and grain

Figure 4. S ericitized M e ta -a rk o se .

Picture shows color of w eathered su rfa ce , shape and size of quartzite p e b b le s .

12

Figure 5. M eta-arkose in Thin Section.

Shows rounded to angular fragm ents of p lag ioclase (P) and ortho- c la se (O) both flecked w ith seric ite and se t in rec ry sta llized grains of quartz (Q). Plain ligh t, XI7.

Figure 6 . Altered M eta-arkose in Thin Section .

M eta-arkose showing se ric ite rep lacing p lag ioc lase (P) and ortho- c la se (O) as w ell as filling m icrovein lets in and around the constituen t g ra ins . X -n ico ls, XI7.

13

s iz e when going up the se c tio n , bu t excep tions to th is genera liza tion

occur. C om position v a rie s from alm ost pure quartz to q u a rtz ite s tha t

a lso contain 10 percen t p la g io c la se , 5 percen t c a lc i te , and 2 percen t

(?) o rthoclase (feldspath ic q u a rtz ite s ) .

In w eathered ou tcrop , the p u re , fin e-g ra in ed q u artz ite s are

ligh t gray to chalky w h ite . On the fresh surface they are ligh t g reen ish -

gray to a lm ost w h ite . They have conchoidal fracture and a po rce la in ­

like appearance . These q u a rtz ite s are com posed of subrounded, fine­

grained quartz th a t probably lo s t i t s rounded shape during m etamorphic

re c ry s ta ll iz a tio n .

C o arser-g ra in ed q u a rtz ite s and fe ld sp a th ic q u a rtz ite s are

m ainly reddish brown on w eathered su rfaces b ecau se of lim onite s ta in ­

ing; 'b lack manganese* sta in ing is p revalen t on w eathered su rfaces and

along jo in t p lan es w here quartz veining is abundant. These q u artz ites

are com posed of subrounded to anhedral grains of quartz th a t are se t in

a groundm ass of anhedral p la g io c la se , c a lc i te , o rth o c lase , and some­

tim es serf c ite (Figure 7).

O utcrops of th in -bedded fe ld sp a th ic quartz ite or fine-g ra ined

quartz ite are few and not w ell exposed . They are m ainly found in

w ashes th a t cu t a c ro ss the strike of the b e d s . The th ick er u n its , how­

ev er, outcrop cxi.ensi.vely and head up some h ills in the a re a .

Pyrite (or lim onite a fte r pyrite) u sua lly c o n s titu te s le s s than

one percen t of the sandstones and q u a rtz ite s . M ost of the pyrite is d is ­

sem inated , w ith le s s than 10 percen t of it occurring in sm all quartz

v e in s and along f ra c tu re s . Not uncommonly, a bed of re la tiv e ly unm eta­

morphosed sandstone w ill con ta in abundant cubes of pyrite along bedding

14

Figure 7. Feldspath ic Q u artz ite .

Shows the rec ry sta lliza tio n of quartz (Q) grains as w ell as grains of both p lag ioclase (P) and orthoclase (O). X -n ico ls , X15.

Figure 8 . Slightly R ecrystallized Lim estone.

Shows quartz (Q) and rec ry sta llized ca lc ite (C) filling v e in le ts and c av itie s in lim estone. P lain lig h t, XI5.

15

plane veins of quartz . C onsiderab ly le s s pyrite i s found in the more

strongly m etam orphosed and a ltered quartz ites than in the sa n d sto n es .

Another rock type w ith co lo r and tex ture much like both the

arkoses and fe ld sp a th ic q u arz ites is a lso p re sen t. How ever, i t con­

ta in s more p lag io c lase and se ric ite than the normal fe ld sp a th ic quartz­

i te s and rare ly con ta ins up to five percen t c a lc ite ; i t may more accu ­

ra te ly be ca lled subarkose (Krumbein and S lo s s , 1963, p . 171; and

H uang, 1962, p . 253). W ith com positional v aria tion in sim ila r-ap p ear­

ing b e d s , it is d ifficu lt to d iffe ren tia te a bed of subarkose from a bed

of fe ld spa th ic q u a rtz ite . In re a lity , both may be varia tions of sub­

arkose .

L im estone. L im estones vary from ligh t gray to b lack on w eath ­

ered su rfaces and are black on fresh su r fa c e s . M ost of the un its are

very th in bedded to lam inated . They c o n s is t of 80-98 percen t c a lc i te ,

1-19 percen t very fin e , rounded grains of q u a rtz , and 1-3 percen t or­

ganic m atter. Although m ost lim estones have undergone m etam orphism ,

even unm etam orphosed un its have undergone s lig h t re c ry s ta ll iz a tio n ,

a s evidenced by veins of c a lc ite and quartz which f ill frac tu res th a t

both p a ra lle l and c ro ss bedding p lanes (Figure 8).

M ineralization is more abundant in and around crushed and

sheared zones a sso c ia te d w ith the c re s ts and troughs of fo ld s . Pyrite

and lim onite a fte r pyrite are p re sen t, predom inantly in cub ic c ry s ta ls .

S hale . Exposed only in re la tiv e ly deep w ashes and g u lch e s ,

sh a les are b lack and very fine g rained , w ith th in to lam inated bedding

th a t produces sp littin g w hich can b e s t be defined as shaley (McKee

and W eir, 1953). X-ray d iffraction w as needed to iden tify the very

16

fine-g rained m ineral co n ten t. Approximate proportions of the m inerals

fo llow , a s determ ined by X-ray a n a ly s is and petrographic study of the

m inerals.

M ineral Percent

Q uartz 15-20

C a lc ite 10-15

P lag io c lase 10

C hlorite 20

S eric ite 40

C arbonaceousm atter 0 - 5

All sh a les have generally undergone some form of metamor­

phism and commonly a lte ra tio n as w e ll, modifying the orig inal c lay

m ineralogy.

H ornfels and a rg i l l i te . W ith in c reased m etam orphic rank,

sha le grades through a rg illite to the far more abundant ho rn fe ls , which

is the dom inant m etam orphic product of both shale and lim estone .

A rgillites are dark gray to ligh t tan on w eathered su rfaces

but are invariab ly b lack on fresh su rfa c e s . Although they are covered

on gentle s lo p e s , a rg illite s generally outcrop m oderately w ell on s teep

s lopes and in w a sh e s . M ineralogy is sim ilar to the sh a le s , bu t th ese

m etasedim ents con ta in a few percen t more p la g io c la se .

W herever h o m fe lse s are p re sen t, outcrops are common. They

ty p ica lly show re lic t bedd ing , making re liab le s trike and dip read ings

p o ss ib le . H om felses vary from ligh t gray to b lack on w eathered

I

17

surfaces and from ligh t g reen ish gray to b lack to redd ish b lack on fresh

su rfaces . Because ho rn felses are so fine grained th e ir m ineralogy can ­

not alw ays be iden tified w ith the petrographic m icroscope .

X-ray d iffraction iden tified or confirmed the p resence of vary­

ing proportions of quartz , d io p s id e , s e r ic i te , p lag io c la se (An^_2Q),

c a lc i te , and su lfides in the h o rn fe lse s . P y rite , p y rrh o tite , and c h a lco -

pyrite are the su lf id e s . They are mainly d issem in a ted , but minor amounts

a lso occur along sm all v e in le ts and frac tu res in the h o rn fe lse s . The d is ­

sem inated su lfides are more abundant in h o m fe ls , co n stitu tin g up to 30

percen t of the rock , than in e ither sh a les or a rg il l i te s .

Under the b inocu lar m icroscope, many hand specim ens of

ho rn fe lses show a myriad of sm all, in terconnected (?) p o res . Horn­

fe ls e s com posed predom inantly of q u a rtz -d io p sid e are more porous than

th o se com posed predom inantly of q u a rtz -s e r ic ite .

D iscu ss io n of the Sedim ents and M etased im ents

Environment of d e p o s itio n . The c h a rac te r and sequence of

sedim ents and m etasedim ents su g g est tha t th ese rocks w ere deposited

by a tran sg ress in g se a . Arkose and sandstone are undoubtedly products

of an open co ast; sha le and lim estone w ere probably dep o sited in a

b rack ish lagoon or backw ater environm ent a s ev idenced by the lack of

f o s s i ls , p a rticu la rly marine fo s s i ls . Intim ate in terbedding in the se d i­

m ents and abrupt changes of rock type ind ica te a shallow environm ent

th a t responded to each fluctuation in sea lev e l or change in the c o a s t­

line .

18

Age and c o rre la tio n . Schrader (1915) co rre la ted th ese se d i­

ments w ith the Com anchean system (Lower C retaceous) on the b a s is of

h is fo ss il ev idence: O strea s p . , b ivalves of the Cham idae ty p e , and

an ammonite fragm ent probably of the genus A can thoceras. He believed

th a t th ese fo s s ils co rre la ted w ith those in the low er M ural lim estone a t

B isbee. Stoyanow (1949) found poorly p reserved lam ellib ranchs to the

south in F ish Canyon th a t were dated simply a s C re ta c e o u s , rather than

Upper or Lower C re ta ce o u s .

Lower C re taceo u s ro c k s , sim ilar to th o se in the G reaterv ille

area excep t for red co lo ring , have been described to the north by

H eatw ole (1965) and to the south by Lutton (1958). H ow ever, H eatw ole

(1965) a lso found dark -co lo red beds near the top of h is C re taceous s e c ­

tion a t Box C anyon Exam ination ind ica ted th a t the C re taceous se d i­

m entary rocks in Box Canyon are probably the same type a s th o se in the

G reaterv ille a re a . Loss of red coloring in the G reaterv ille area w as pos­

sib ly caused by metam orphism and (or) a lte ra tio n . S im ila rities a lso seem

to e x is t betw een G reaterv ille sedim ents and Lower C re taceo u s sed i­

m ents further e a s t as described by both Ran some (1904) and G illu ly

(1956).

The author c o n c lu d e s , on the b a s is of fo s s il and litho log ic

e v id e n ce , th a t sedim ents in the G reaterv ille area seem to be of Lower

C re taceous Age. H ow ever, recen t work has pushed back the age of

many unfos s ilife rrous rocks p rev iously thought to be C re taceous in age

(Dr. Donald L. Bryant, oral com m unication). C onsequen tly , the author

b e liev es th a t sedim ents in the map area can only questionab ly be ca lled

Lower C re ta c e o u s .

19

C harac ter and ex ten t of m etam orphism . M etamorphism ranges

from w eak to strong and a ffec ts a ll sedim ents in the G reaterv ille a re a .

In arkose and san d sto n e , quartz grains w ere re c ry s ta lliz e d , and any

a rg illaceous m aterial in the m atrix w as probably se r ic itiz e d . Arkose and

sandstone w ere m etam orphosed to m eta-arkose and f eld sp a th ic quartz ite

re sp ec tiv e ly .

Shale and lim estone grade from w eak rec ry s ta lliz a tio n of c a l-

c ite to th e ir dominant m etamorphic product: d e n se , w e ll-c ry s ta lliz e d

ho rn fe ls . M etamorphism in sha le and lim estone commonly began w ith

rec ry s ta lliza tio n of c a lc ite and developm ent of c a lc ite -q u a rtz ve ins

, (Figure 8). U ltim ately , th ese veins w ere sho t com pletely through the

rock , not n e ce ssa rily along jo in ts or bedding (Figure 9). D epending on

orig inal com position, sh a les developed p lag io c la se or se ric ite a s m eta­

morphic in ten sity in c re a se d . D evelopm ent of c a lc - s i l ic a te s , such as

diop side and trem olite , in the sh a les and lim esto n es marked the beg in ­

ning of hornfels form ation.

In some h o rn fe lse s , seg regated lay e rs of m etam orphic m inerals

appear to re flec t the o rig inal sedim entary b e d s , w hile in o ther horn­

fe ls e s , the m inerals appear to com prise a hom ogeneous m ix tu re . Figure

10 shows segregated layering in a hornfels formed from a sha le or a rg illa

ceous lim estone; X-ray a n a ly s is id en tifie s the m ineralogy as predom i­

nantly di op s id e , trem o lite , and p lag io c la se (Ang.^g), w ith le s s than

five percen t q u artz . Figure 11 show s a re la tiv e ly more hom ogeneous and

c o a rse r-g ra in e d .h o rn fe ls; th is type generally con ta ins varying propor­

tions of q u a rtz , d io p sid e , p lag io c la se (A n g ..^ ) , s e r ic i te , c a lc i te ,

py rite , p y rrh o tite , and c h a lco p y rite . M ineral com position in d ic a te s th a t

20

Figure 9. Veins of C alc ite and Quartz in L im estone.

The veins show no preference for either joints or bedding.

21

Figure 10. Layered H ornfels.

A hornfels in an early stage of form ation, layering presum ably re flec ts original bedding in the sedim ent. P lain ligh t, X15.

Figure 11. Relatively Homogeneous H ornfels.

Roughly equigranular grains of d iopside , q u a rtz , s e r ic i te ( ? ) , and pyrite seem to be fairly evenly d is trib u ted . X -n ico ls, X15 .

22

the hom ogeneous and co a rse r-g ra in ed h o m fe lse s are more in ten se ly

m etam orphosed than those w ith segregated layers of m inera ls .

The ex ten t of m etamorphism in the area can be seen in Figure

12 (in p o ck e t). The ou ter lim it of metamorphism roughly p a ra lle ls the

strike of the sed im en ts , p a ss in g re la tiv e ly c lo se to the e a s te rn side of

Granite M ountain . H o m felses th a t are as d e n se , s i l ic if ie d , and min­

era lized as th o se ad jacen t to the s tocks ex ten d , p a ra lle l to bedding ,

to the northern boundary of the a re a . Loughlin and Koschmann (1942),

La sky (1947), and C ooper (1957) have describ ed sim ila r p a tte rn s of

metamorphism in sed im ents surrounding in trusions in o ther pa rts of

Arizona and New M ex ico . This p refe ren tia l metamorphism along beds

rather than a c ro ss beds in d ica te s s tra tig rap h ica lly se le c tiv e metamor­

phism w hich w as the only m etam orphic zoning recognized in the a re a .

The pattern of se le c tiv e metamorphism is observed in beds e a s t and

no rtheast of G ranite M ountain . From inferred litho log ic con tinu ity

(Figure 3 ), beds th a t apparen tly abut the in tru sion show m etamorphism

of a h igher rank than beds w hich do not abut i t .

Summary

Sedim ents and m etasedim ents in the map area c o n s is t of th in ,

in terfingering un its of variab le litho logy w hose la te ra l ex ten t is prob­

ably quite lim ited . As a g en era liza tio n , the stra tig rap h ic sequence

changes from predom inantly a rk o s ic near the bottom to predom inantly

lim estone near the top; how ever, th in beds of a lm ost any com position

can be found sca tte red throughout the se c tio n . M ost of the rocks are

the m etamorphic equ iva len ts of th e ir o rig inal sedim entary beds tha t

have undergone various s ta g e s of m etam orphism . S tra tig raph ically

23

se lec tive m etam orphism , apparen tly a s so c ia te d w ith the G ranite Moun­

ta in in tru s ion , is the only m etam orphic zoning recognized in the a re a .

The ch arac te r of the sed im ents and m etasedim ents su g g ests s trand -line

or near-sho re dep o sitio n in a sh a llo w , tran sg re ss in g s e a . L itho log ically ,

th e se sedim ents seem to co rre la te w ith rocks reported a s Lower C re ta ­

ceous from other lo ca tio n s in the Santa Rita M ountains and sou theaste rn

Arizona, although the co rrec tn ess of defin ing th e se rocks a s Lower

C retaceous is now q u estio n ed .

Igneous Rocks

Two d is tin c t ty p es of igneous rocks w ere observed in the map

a re a . In trusive porphyritic quartz m onzonite of Laramide age (H. D rew es,

oral communication) is m ost abundan t. It in c lu d es the th ree major

s to ck s on a no rthw est-trend ing line in the cen tra l portion , two apoph­

y se s of G ranite M ountain , and an ex ten siv e ly and in ten siv e ly a ltered

dike (Figure 3). L ess abundant is the Precam brian granodiorite which is

exposed only in the sou thw est corner of the a re a .

Porphyritic quartz m onzon ite . Porphyritic quartz m onzonite

w eathers to a ligh t ta n , tan n ish brow n, or commonly to a b la c k -sp o tte d ,

w h itish surface on w hich the phenocrysts of p lag io c la se are read ily

ev iden t (Figure 13). On fresh su r fa c e s , the rock is w hite w ith b lack

specks of b io tite or c h lo rite . For s ix of the le a s t a lte red porphyritic

quartz m onzonite th in s e c tio n s , w hich re ta ined recogn izab le ou tlines of

orig inal m in e ra ls , modal a n a ly se s of orig inal com position w ere d e te r­

m ined.

24

.Jp

Figure 13. Porphyritic Q uartz M onzon ite .

Shows ch a rac te ris tic w eathered appearance and porphyritic te x tu re .

Figure 14. L east Altered Porphyritic Quartz M onzonite .

Sericite occurs as flecks and as m icroveinlets rep lacing p lag io - gro^e (p) P henocrysts. C hlorite (C) rep laces b io tite (B) . The intergrown (q\ ^ s s (G) com posed of quartz and K -feldspar corrodes the quartz

; Phenocryst. X -n ico ls, X18.

25

The follow ing p e rc en ta g es , based on an average of the six

sam ples, rep resen t the b e s t p o ss ib le determ ination of o rig inal porphyr-

i t ic quartz monzonite com position:

M ineral Percen t

P lag io c lase 45.0

O rthoclase 24.5

Quartz 24 .4

Biotite 4 .9

C linopyroxene (?) 0 . 6

Sphene trace

Opaque M inerals 0 .7

The rock i s p h an eritic , h o lo c ry s ta llin e , and porphyritic , w ith

a com pletely intergrow n groundm ass. P lag ioc lase and o rthoclase pheno-

c ry s ts are subhedral to euhedra l, b io tite phenocrysts are subhedral to

anhedral, and quartz phenocrysts are anhedra l. Phenocrysts to ta l le s s

than 50 percen t of the rock . The groundm ass is e s se n tia lly com posed of

irregularly intergrow n quartz and o rthoclase c ry s ta ls , in nearly equal

proportions. They are more than 0 .1 mm in diam eter; therefore by defi­

nition (Stringham, 1966, p . 37), they are not ap h an itic . C om position

and ch arac te r of the rock ind ica te th a t i t is a g ran ito id , porphyritic

quartz m onzonite.

Figure 14 shows ty p ica l exam ples of th is g ran ito id , porphyr­

itic quartz m onzonite. These are the le a s t a lte red specim ens observed ,

and b est i llu s tra te the o rig inal com position and tex ture before a lte ra ­

tio n . S eric ite and ch lo rite are a lte ra tio n products ra ther than orig inal

26

m ineral com ponen ts. The groundm ass, particu la rly the o rth o c lase , cor­

rodes the quartz and p lag io c la se p h en o cry sts .

The magma w as ev iden tly intruded from a deep source in w hich

large phenocrysts of p lag io c la se had grown. After in tru sion to i t s p res­

ent p o sitio n , more rapid cooling of the liquid forced the phenocrysts

out of equilibrium , and the m elt corroded the p h en o cry sts . Many of the

phenocrysts are now sca llo p ed .

All sec tio n s of the porphyritic quartz m onzonite s to ck s show

q u a rtz -se ric ite a lte ra tio n , but some show much le s s than o th e rs , a s

described below . The porphyritic dike rock and the apophyses south­

w est of G ranite M ountain are probably a lte red porphyritic quartz mon­

zonite of e s se n tia lly the same com position a s the s tocks and were

probably intruded a t approxim ately the sam e tim e. Their extrem e a lte ra ­

tion m akes modal a n a ly se s of th e ir o rig inal com position u n re liab le .

C h lo ritized b io tite s from near the top of G ranite M ountain have

given a po tassium -argon date of 6 3 + 1 .5 m illion years (H. D rew es, oral

com m unication). In trusion thus occurred during the Laramide orogeny,

as is c h a ra c te ris tic of many o ther porphyritic p lu tons in Arizona.

Precam brian g ranod io rite . Redish-brow n granodiorite crops

out in the sou thw est corner of the area (Figure 3). It is h o lo cry sta llin e

and equigranular w ith subhedral to euhedral c ry s ta ls of p lag io c la se

(Angg) and subhedral c ry s ta ls of b io tite and m icrocline. Q uartz and the

rem aining m icrocline and o rth o c lase occur a s anhedral c ry s ta ls (Figure

15). The rock h as been abundantly se ric itiz ed ; m ost of the b io tite has

been a lte red to a com bination of pennin ite ch lo rite and e p id o te . The

follow ing approxim ate pe rcen tag es are a modal a n a ly s is based on 930

27

Figure 15. Precam brian G ranodiorite.

An equigranular rock showing flecks of se ric ite in p lag ioc lase (P). M icrocline (M) and quartz (Q) are the other m in e ra ls . A vein of se ric ite (S) runs N-S on the left side of photograph. X -n ico ls , XI6 .

28

point counts:

M ineral Percent

Q uartz 3 3 .6

P lag io c lase 15 .2

M icrocline 8 . 2

O rthoclase 5 .0

Seri c ite 2 1 .9

Biotite 3 .5

Epidote 2 .3

C hlorite 7 .1

C a lc ite 0 .5

Opaque m inerals 2 .7

Lower C re taceo u s (?) rocks in Box C anyon to the north have a

sedim entary co n tac t w ith the g ranod io rite , although i t has been dated

a s Laramide by the lea d -a lp h a method (H. D rew es, oral com m unication).

He th inks th a t the rock i s probably of Precam brian a g e . Sim ilarity to

o ther Precam brian g ran ites and the sedim entary co n tac t in Box Canyon

ind ica te th a t the granodiorite predated the C re ta c e o u s , thus supporting

M r. D rew es 's co n c lu sio n . By im p lica tion . Lower C re taceo u s (?) rocks

in the map area a lso have a sedim entary co n tac t w ith the g ranodiorite .

The porphyritic quartz m onzonite i s not known to cut the g ranod io rite .

Summary

Both Precam brian granodiorite and g ran ito id , porphyritic quartz

monzonite of Laramide age are p resen t in the map a re a . Precam brian

g ranodiorite , the basem ent rock , i s not observed to be cu t by the

29

porphyritic quartz m onzonite . Porphyritic rocks intrude m etasedim ents of

Lower C retaceous (?) age th a t were deposited on the PreCambrian grano-

d io rite . The porphyritic rocks occur a s s to c k s , apophyses of the s to c k s ,

and d ikes; the apophyses and d ikes are in ten se ly and ex ten siv e ly

a lte re d .

S tructural Geology

Regional Structure

The G reaterv ille d is tr ic t i s located a t a m ajor sh ift in trend of

the Santa Rita M ountain ra n g e . To the sou th , the ax is of the range

trends approxim ately N20°W and to the north i t trends approxim ately

N20°E. button (1958, p la te 2) mapped many fau lts in the northern Santa

Ritas a s strik ing northw est w ith le f t la te ra l d isp lacem en t. The Pima

County geologic map (W ilson, M oore, and O 'H aire , 1960) show s a

strong northw est o rien ta tion of struc tu res in the Santa R itas . M any fold

a x e s , fa u lts , and shears throughout southern Arizona trend northw est

(button, 1958, p la te 1; and W ilso n , 1962). This trend is a ttribu ted to

two major forces im portant during baram ide deform ation in southern

Arizona: (1) n o rth east-so u th w est com pression (button, 1958; and W ilson ,

1962) and (2) the Texas lineam ent, which has le ft la te ra l d isp lacem ent

(button, 1958; and M ayo, 1958).

Local Structure

Based on com plexity of s tru c tu re , an im aginary d iagonal line

with a N45°W trend can be used to b ise c t the map (Figure 3) in to two

roughly equal a re a s . H ereafte r, reference to the n o rth east portion of

the map w ill designate the more s truc tu ra lly com plex area above the

30

im aginary line ; s im ila rly , " the sou thw est portion" w ill designa te the

le s s s truc tu ra lly com plex area below the im aginary l in e .

F o ld ing . In the sou thw est portion , m etasedim ents have a re­

gional strike of approxim ately N 2 0 ° - 25°W and a regional dip of approx­

im ately 6 0 ° - 70°NE. The a ttitu d e is referred to as regional b ecause (1)

ae ria l photographs show th a t the trend con tinues to the south and (2)

Stoyanow (1949) d e sc rib e s the C re taceous rocks in th is part of the

Santa Ritas a s having reg ional northw est s tr ik e s and no rtheast d ip s .

M inor v aria tio n s in the sou thw estern portion probably re su lt from loca l

deform ation in the m etased im en ts.

In th e n o rth east portion , sedim ents and meta sedim ents are ex­

ten s iv e ly folded and con ta in many minor f a u l t s . F au lts are generally

found near and on c re s ts and troughs of the fo ld s . M any fau lts d ie out

w ith in six fee t of ou tcrop , and they rare ly show d isp lacem en ts g rea ter

than one in ch . Almost a ll fau lts c ro ss bedding p la n e s . Although bedding

plane fau lts e x is t, they are extrem ely d ifficu lt to rec o g n ize . W hile map­

ping the sedim ents and m etased im en ts, a com plicated pa ttern of strikes

and dips emerged (Figure 3 ). This pa tte rn is caused by the fo ld s , some

of w hich die out in re la tiv e ly short d is ta n c e s and m ost of w hich have

sinuous a x e s . Subsid iary drag(?) fo lds develop in the lim bs of larger

fo lds (Figure 16, north end of sec tio n C - C , in p o ck e t) . M any outcrops

show intraform ational drag fo lds (Figures 17 and 18). U ndulation of the

bedding, pa rticu la rly in unm etam orphosed lim estone and sh a le , is ev i­

dent near fold a x e s . This undulation p o ss ib ly exp la in s anom alous strike

and dip read ings in the n o rth ea s t portion . Although undulation may be a

31

F igure 18 D rag F o ld s in M e ta se d im e n t

32

factor in the structure of the sou thw est portion , in traform ational drag

folding in lim ey and h o m fe ls ic beds i s probably more im portant.

T ightening along the a x ia l p lan es of fo ld s , p a rticu la rly along

c re s ts , cau sed shearing , b rec c ia tio n , and rad ica l changes in the s trik es

and dips of b e d s . C lo se to the a x e s , ind iv idual beds commonly strike

p a ra lle l to the fold ax is and dip nearly v e rtic a lly . Small m ines and

p rospect p its expose abundant quartz -ca rbonate v e in s , w ith and w ith­

out su lf id e s , in th e se b e d s . The v e in s are a sso c ia te d w ith sheared and

b reccia ted zones on c re s ts and troughs of fo lds and extend p a ra lle l to

bedding along th e ir f la n k s . B recciated zones are ty p ica lly recem ented

by fin e -g ra in ed , a lm ost c ryp tocry sta lline s i l ic a . B recciated and sheared

zones are generally the m ost in te n se ly m etam orphosed and a ltered por­

tions of the country rock . E vidently , sheared and b recc ia ted zones fur­

n ished open channelw ays a long w hich m ineralizing and a ltering flu ids

gained a c c e s s to the sedim ents b ecause abundant quartz -carbonate

veins are a sso c ia te d w ith th e se zo n es .

North of the northw est in tru s io n , s tr ik es and d ips vary . Be­

cau se metamorphism has destroyed re lic t bedding , it is probable th a t

some read ings may have b een taken on jo in t p la n e s . Aerial photographs

showed th e true a ttitu d e of the beds (Figure 3).

At th is p o in t, two problem s are particu la rly obvious: (1) why

the rocks in the n o rtheast portion are more s truc tu ra lly com plex than

th o se in the sou thw est portion and (2) w h a t, if any , re la tio n sh ip s e x is t

betw een structu re in the sed im ents and the em placem ent of igneous in ­

tru s io n s . As described below , th ree hypo theses offer answ ers to these

33

problem s: (1) a gravity f a u l t , (2) a basem ent sh ear zone, and (3) d ila ta ­

tion or w eakness in the cen tra l portion of a large sy n c lin e .

In tru s io n . The in tru s io n s can be c la s s if ie d as predom inantly

p a ss iv e b ecau se the sedim ents seem to wrap around and abut them w ith

l i t t le apparent d isrup tion (Figure 16, sec tio n A-A' and sec tio n B -B ') .

How ever, minor d iv erg en cies in s trike su g g est th a t the s tocks caused

minor deform ation, e sp e c ia lly around the northw estern stock w here the

m etasedim ents seem to have been pushed as id e (Figure 3). The northern

lobe of G ranite M ountain (Figure 16, sec tio n A-A') seem s to have e ith e r

domed the m etased im en ts , contributing to the form ation of an a n tic lin e ,

or to have been em placed a t a fold in the m etased im en ts. Along G ranite

M ountain 's north s id e , one strike and dip (Figure 3) in d ic a te s some

dragging as the in trusion w as em placed .

The igneous dike in the w e st portion of the map (Figure 3) is

ex ten siv e ly a lte red and becom es a s i l l in p la c e s . It w as not observed

to have cu t o ther in tru s iv e s , so i ts time of in trusion can only be specu ­

la ted a s having been re la tiv e ly la te in the in tru s iv e seq u en ce . The dike

m ostly like ly occupied a frac tu re or frac tu res w hich developed in the

sedim ents and m etased im ents a s they ad ju s ted to the em placem ent of

the s to c k s . O ther d ik e s , too sm all to map a t the se lec ted s c a le , are

ad jacen t to the igneous s to c k s .

F au ltin g . The fau lt betw een G ranite M ountain and the north­

w estern stock w ill be referred to a s the Alpha fau lt (Figure 3). I t seem s

to have resu lted from read justm en t of the m etasedim ents to th e se two

in tru s iv e s . D isp lacem en t on the Alpha fau lt cannot be m easured b ecau se

of metamorphism and a lte ra tio n in the m etased im en ts . A ssocia ted

34

m ineralization c o n s is ts of p y rite , p y rrh o tite , c h a lco p y rite , and qu artz -

carbonate v e in s . Several p ro sp ec t p its are on or near the Alpha f a u l t .

The a re a 's only o ther s izab le fau lt is the Beta fau lt in the

southw estern corner (Figure 3). An oblique fa u lt, one to s ix fee t of

b reccia and gouge mark i t s zone. The Beta fau lt s tr ik es approxim ately

N55°W and dips 5 5 ° - 60°SW . S lick en slid es trend S70°W and plunge

3 5 °. Abundant quartz -ca rbonate vein ing contain ing p y r ite , g a le n a , ch a l­

copyrite , and sphalerite is a sso c ia te d w ith the f a u l t . Schrader (1915)

reported the galena a s a rg en tife rro u s .

B re e d a tio n . A b reccia d ike(?) e n te rs the a rea from the w e s t .

• I t i s a very prom inent feature in p la c e s but apparen tly does not extend

acro ss the area (Figure 3). The b reccia is com posed of fragm ents of

hornfels and quartz ite th a t could range in age from Cam brian to C re ta ­

c eo u s , arkose th a t is probably Lower C re taceous (? ) , and Precam brian

g ranod io rite . These fragm ents are angular to rounded , vary from boulder

to sand s iz e , and are cem ented w ith a very f in e -g ra in e d , a lm ost crypto­

c ry s ta llin e s il ic a (Figure 19).

Along part of the b re c c ia 's le n g th , a porphyritic quartz mon-

zonite dike is a sso c ia ted along one or both s id e s . W here the dike is

found on both s id e s , the b recc ia is cu t by unbreccia ted s trin g ers of the

porphyritic quartz m onzonite, ind ica ting th a t the b recc ia w as formed

prior to the d ik e . W here country rocks crop out on both s id es of the

b rec c ia , some o ffse t appears to have occurred; the amount is d ifficu lt

to determ ine for the c h a rac te r of the m eta-arkose m akes co rre la tion a l­

m ost im p o ssib le .

35

Figure 19. Breccia Dike (?).

Shows shape and size of fragments that are re cemented with very- fine-grained almost cryptocrystalline silica.

36

Because of angu lar ch a rac te r of the fragm ents, a sso c ia te d

quartz m onzonite , and apparen t o ffse t, the b reccia may be a fau lt b rec­

c ia . If so , the change in the b re c c ia 's outcrop d irec tion (Figure 3) may

ind ica te an extrem ely irregu lar fa u lt.su rfa c e , p o ss ib ly a th ru s t fau lt. In

conclusion , the b reccia i s o lder than the a s so c ia te d dike and e ith e r

ea rlie r than or contem poraneous w ith em placem ent of the s to c k s .

Tointinq. Strike and dip readings w ere tak en a t a ll outcrops

in w hich bedding or re l ic t bedding w as o bserved . Jo in t read ings were

taken only a t the la rg e r outcrops th a t showed c le a r-c u t jo in t develop­

m ent. Rose diagram s w ere construc ted from th ese read ings even though

they were barely enough for adequate s ta t is t ic a l a n a ly s is . A Rose d ia ­

gram of the sou thw est portion of the area show s the developm ent of a

prom inent s e t of c ro ss jo in ts o rien ted approxim ately N45°E and N80°E

(Figure 20). The o rien ta tion of th e se c ro ss jo in ts conforms to the north­

e a s t-so u th w e s t com pression proposed by button (1958) and W ilson

(1962). A Rose diagram of the n o rth east portion is much le s s defin itive

(Figure 21) and probably re f le c ts add itional fo lds th a t ro ta ted the jo in ts

out of th e ir orig inal o rie n ta tio n s . Although a Rose diagram of the three

stocks su g g ests a s e t of jo in ts com parable to those in the sou thw est

portion , i t a lso su g g ests ano ther se t a t righ t ang les (Figure 22).

Summary

The map area is d iv ided in to a re la tiv e ly undeformed sou thw est

portion and a deformed n o rth east portion in w hich a com plex se rie s of

fo lds have developed . A Rose diagram of jo in ts in the sou thw est portion

of the map in d ica te s th a t they w ere formed by n o rth ea s t-so u th w e st .

37

NA

Scale 1/5 inch = I Reading 201 Readings

Figure 20. Rose Diagram of Joints in Southwest Portion

H

Scale 1 /5 inch = I Reading 168 Readings

Figure 21. Rose Diagram of Joints in Northeast Portion

39

NA

Scale 1/5 inch = I Reading

158 Readings

Figure 22. Rose Diagram of Joints in Three Stocksof Porphyritic Quartz Monzonite

40

com pression . Shearing and b recc ia tio n are common along the c re s ts and

troughs of the fo lds in the n o rth east portion .

In trusions of porphyritic quartz m onzonite are m ainly p a s s iv e ,

although minor ac tive em placem ent caused lo ca l deform ation around mar­

g ins of the s to c k s . An a lte red d ik e , not observed to cu t the other in tru ­

s io n s , is a sso c ia te d w ith a b recc ia in the w estern part of the a re a .

Because unbreccia ted s trin g ers of the dike cu t the b re c c ia , the b recc ia

w as formed prior to the dike and may or may not have been contem por­

aneous w ith em placem ent of the porphyritic s to c k s . The b recc ia may

p o ss ib ly occupy a fau lt zone .

DESCRIPTION OF ALTERATION AND MINERALIZATION

Two ty p es of hypogene a lte ra tio n have been recognized in the

G reaterv ille area: p o ta s s iu m -s il ic a te a lte ra tio n and q u a rtz -se r ic ite

a lte ra tio n .

P o ta ss iu m -s ilic a te a lte ra tio n , a s defined by C reasey (1966,

p . 61), c o n s is ts of m uscovite (se r ic ite ) , b io tite , and K -feldspar or any

two of th ese th ree p h a s e s . In p o ta s s ic a lte ra tio n of in term ediate to

g ran itic ro ck s , K -fe ldspar and se ric ite generally rep lace p la g io c la se ,

se ric ite rep laces K -fe ldspar, and b io tite generally forms in a h o st such

as primary m afic s i l i c a t e s .

Q u a rtz -se ric ite a lte ra tio n con ta in s q u a rtz , s e r ic i te , and pyrite

a s the p rincipal p h ases (C reasey , 1966, p . 62). In a quartz m onzonite,

for exam ple, "seric ite may rep lace p la g io c la se , b io ti te , c h lo rite , and

K -feldspar in am ounts th a t vary w ith the in te n s ity of a lte ra tio n . Altera­

tion q u a rtz , a s in troduced by the a lte rin g so lu tio n s and a s a by-product

of the a lte ra tio n re a c tio n s , is commonly d ifficu lt to d iffe ren tia te from

q u a rtz .

D iscern ib le m inera liza tion of both the ve in and d issem ina ted

types is p re se n t. Primary m inerals c o n s is t of p y rite , py rrho tite , ch a lco -

p y rite , b o m ite , g a le n a , sp h a le r ite , a member of the te n n a n ti te - te tra -

hedrite so lid so lu tion s e r ie s , m ag n e tite , and h em atite . N ative gold is

reported by Shrader (1915), F . H arm es,and J . S tew art (oral communica­

tions) . Secondary m inerals c o n s is t of angle s i te , cerru site (? ) , lim on ite ,

c o v e llite , a z u rite , m a lach ite , and c h ry so co lla . All m inerals excep t

41

42

pyrrho tite , m agnetite , and hem atite are p re sen t in the q u artz -ca rb o n ate

v e in s , but only p y rite , py rrho tite , c h a lco p y rite , m agnetite , and hem a­

tite have been observed a s d issem inated m in e ra ls .

A lteration

A lteration in Igneous Rocks

P o ta ss iu m -s ilic a te a lte ra tio n is found only in the porphyritic

quartz m onzonite. Q u a rtz -se ric ite a lte ra tio n is found in the Precam brian

granodiorite and in the porphyritic quartz m onzonite.

Precam brian g ran o d io rite . In the Precam brian g ranod io rite , the

• only a lte ra tio n product ev iden t is pervasive and m oderately strong se ri­

c ite th a t could be of any age from Precam brian to Laram ide. Porphyritic

quartz monzonite is not known to cu t the granodiorite in the G reaterv ille

a re a . H eatw ole (1965) reports th a t granodiorite two m iles to the north

in Box C anyon, where there is no known a sso c ia tio n w ith Laramide in ­

tru s io n s , is a lso se r ic itiz e d . S e ric itiza tio n of the granodiorite is thus

assum ed to be unrela ted to the porphyritic quartz m onzonite in tru sions

and no further m ention of i t w ill be m ade.

Porphyritic quartz m onzon ite . P o ta ss iu m -s ilic a te a lte ra tio n

i s w eak and rarely seen in the porphyritic quartz m onzonite s to c k s , ap­

pearing only near the c en te rs of two of the th ree exposed s to c k s . P o ta s-

s ic a lte ra tion is seen as the p a rtia l rep lacem ent of sev era l p lag io c la se

phenocrysts by o rthoc lase ; unaltered portions of p lag io c la se pheno-

c ry s ts show a lb ite tw inning w hich is continuous w ith w eak but s t i l l

v is ib le a lb ite tw inning in dom inantly rep laced p o rtio n s . C oncom itant

replacem ent of ch lo rite by b io tite is not se en .

43

Q u a rtz -se ric ite a lte ra tio n of varying in ten sity pervades the

s to c k s . I t is le s s abundant and w eaker in p o ta s s ic zones and becom es

more ex ten siv e and in ten s iv e tow ard m argins of the s to ck s (Figure 12).

S eric ite rep la ce s p la g io c la se , occurring both in m icrovein lets and a s

d issem ina ted minute flakes (Figure 14). C hlorite a fte r b io tite is rep laced

along c leav ag es by s e r ic i te . W ith in c reased in te n s ity of q u a rtz -se r ic ite

a lte ra tio n , p lag io c la se becom es ex ten siv e ly a lte re d , pseudom orphs of

se ric ite a fte r ch lo ritized b io tite deve lop , o rth o c lase develops d issem i­

nated f la k e s , phlogopite rep la ce s quartz and o rthoclase in the ground-

m ass , and m icrovein lets of se ric ite cu t phenocrysts of quartz (Figure

23).

In the dike of po iphyritic quartz m onzonite, se r ic itiz a tio n is

a lso ex tensive and in ten s iv e w ith sp a rse and w eak p o ta s s ic a lte ra tio n

appearing in one of th ree th in se c tio n s . The apophyses sou thw est of

G ranite M ountain show sim ilar ex ten siv e and in ten siv e se r ic itiz a tio n

w ith sp a rse and w eak p o ta s s ic a lte ra t io n (?); one apophysis w as in itia lly

m istaken for a m etasedim ent u n til m icroscopic exam ination iden tified i t

a s ig n eo u s.

During m icroscopic exam ination of th in se c tio n s from the por-

phyritic quartz m onzonite s to c k s , i t w as noted th a t a lte ra tio n had not

com pletely destroyed o rig inal m ineral o u tlin e s . Thin sec tio n s of the

le a s t a lte red rocks w ere ch o sen for modal a n a ly s is in an attem pt to re­

construct the orig inal com position of the porphyritic quartz m onzonite

toward determ ining both ex ten t of a lte ra tio n and re su lta n t m ineralogical

and chem ical c h a n g e s .

44

Figure 23. Strongly Altered Porphyritic Quartz Monzonite.

Shows pseudomorphs of sericite (S) after both plagioclase (?) and chlorite (C) after biotite. Sericite also appears to fleck orthoclase in the groundmass. X-nicols, XI8 .

45

The reconstructed o rig inal m ineral com position and the p resen t

m ineral com position of the porphyritic quartz m onzonite s to c k s , as de­

rived from modal a n a ly se s of sev era l th in se c tio n s , are shown in Tables

1 and 2 . Point coun ts varied from approxim ately 750 to 2000 per s lid e .

Grain s iz e averaged 0 .5 mm in the groundm ass, but phenocrysts ranged

from 0 .5 cm to 1 .5 cm in d iam eter. R econstructed orig inal m ineral com­

p osition of the porphyritic quartz m onzonite w as computed from thin

sec tio n s of the s ix le a s t a lte red rocks (Table 1). A lteration w as not

strong enough to d estroy o u tlin es of o rig inal m inerals (Figure 14), and

p o ta s s iu m -s ilic a te a lte ra tio n w as m inor, so m inerals w ere counted as

if they were u n a lte red . V ariations in m ineral com position betw een the

s ix sam ples are sm all enough to assum e th a t th e ir average re liab ly

rep re sen ts the o rig inal rock com position . P resen t m ineral com position

of the porphyritic quartz m onzonite s tocks w as computed from poin t

counts of five th in s e c tio n s , ranging from le a s t a lte red to m ost a lte red

(Table 2). A com parison of the re la tiv e a lte ra tio n of sam ples th a t were

analyzed for the ta b le s fo llow s:

Sample Number R elative A lteration

0 -2 8 -1J-0 -1M -5 -4O - 3 - l

1-32-1

F -15 -1 0 —4— 1

L east a lte red

L east a lte red L east a ltered

L east a lte red L east a lte red L east a lte red

J-2-1P -1 1 -1

M oderately strongly a ltered

Strongly a lte red Strongly a lte re d .

46

TABLE 1 .•—M o d a l a n a ly s i s o f s ix s a m p le s o f p a r t ia l ly a l te r e d p o rp h y r iticq u a r tz m o n z o n ite , r e c o n s t r u c te d to o r ig in a l c o m p o s itio n by w e ig h t p e rc e n t

Sample Number

M ineral F -15 -1 J-0 -1 1-32-1 0 -4 -1 0 -2 8 -1 2 i cn 1 Average

O rthoclase 22.8 21 .3 21.0 26 .0 22.0 26 .6 23 .3

P lag ioc lase 4 2 .7 48 .9 46 .7 43 .8 44 .8 44 .3 45 .1

Quartz 2 6 .4 18.8 22.0 2 6 .0 27 .3 24 .0 24 .1

Biotite 6.0 8 .7 8 .7 3.5- 4 .6 3 .0 5 .7

Clinopyroxene (?) 1.0 0 .3 0 . 6 0 . 1 • • 0 .3 0 .4

Sphene 0.01 • • • ♦ 0 .3 0 .0 4 0 .05

Opaque M inerals 0 .9 2.0 0 .9 0 .4 1 .7 1 .9 1 .3

TABLE 2 . — M o d al a n a ly s i s o f f iv e sa m p le s of a l te re d p o rp h y r itic q u a r tzm o n z o n ite by w e ig h t p e rc e n t

47

Sample Number

M inerals 1-32-1 F -15-1 O -3 - l J-2 -1 P -11-1

O rthoclase 25 .9 16 .4 21.1 4 .8 9 .4

P lag ioc lase 19 .7 19 .6 14 .7 11.8 3 .2

Quartz 25 .6 25 .0 36 .1 35 .7 36 .1

Biotite 0 . 8 0 .9 • • • •

Phlogopite • • • • 2 .7 1 .7 1.0

C hlorite 4 .0 7 .2 • • • • • •

Sericite 19 .4 28 .3 24 .5 5 3 .2 4 9 .2

M ontm orillonite 0 . 2 0 . 2 • • • • • •

Z oisite 2 .3 1 . 1 - • • • • • •

C a lc ite 1.2 • • • • • • • •

Opaque M inerals 0 . 6 1 .3 0 .9 2 .9 1.1

48

A com parison of Table 2 w ith Table 1 shows the quan tita tive m ineralog-

ic a l changes a sso c ia te d w ith in c reased q u a rtz -se r ic ite a lte ra tio n d e s ­

cribed q u a lita tiv e ly ab o v e .

T ables 3 and 4 show chem ical com positions of the porphyritic

quartz monzonite ca lcu la ted from the modal a n a ly se s in Tables 1 and 2 ,

re sp ec tiv e ly . Table 3 show s the reconstructed orig inal chem ical compo­

sition of the porphyritic quartz m onzonite , and Table 4 g ives the p resen t

chem ical com position of five rep resen ta tiv e sam ples of a lte red porphyr­

itic quartz m onzonite. A com parison of the ta b le s in d ic a te s chem ical

changes in the rocks as they becom e p rog ressively more a ltered a s

.shown graphically in Figure 24. Im portant changes e n ta il lo s s e s of

AI2O3 , C aO , NagO, and FeO, w ith concom itant gains of KgO and HgO.

Although not c o n s is te n t, MgO seem s to in c rease in the w eakly a ltered

rocks and to d ecrease in the in te n se ly a ltered ro ck s .

Small am ounts of z o is ite , ty p ic a lly le s s than one p e rcen t, ap­

pear in a lm ost every th in sec tion ; s ligh tly more zo is ite is p resen t in

sec tio n s showing p o ta s s iu m -s il ic a te a lte ra tio n . This z o is ite generally

show s anom alous deep blue in terference c o lo rs , and i t norm ally rep laces

chlorite and c linopyroxene(? ) . At the top of the northw est in tru s io n ,

zo is ite is m ost abundant and a lso rep la ce s p la g io c la se .

Summary of igneous a lte ra t io n . S e ric itic a lte ra tio n in the P re-

cam brian granodiorite is probably unrela ted to a lte ra tio n in m etased i­

m ents and porphyritic quartz m onzonite in tru s io n s . In the in trusive

ro ck s , pervasive q u a rtz -se r ic ite a lte ra tio n is m oderate to strong , w hile

p o ta s s ic a lte ra tio n is w eak and rare ly se e n . As the in te n s ity of q u a rtz -

se ric ite a lte ra tio n in c re a s e d , p lag io c la se and ch lo rite a fte r b io tite w ere

TABLE 3 .—C hem ical com positions of s ix sam ples of p a rtia lly a ltered porphyritic quartz m onzon ite , reconstructed to orig inal com position

by w eigh t percen t

49

Sample Number

O xides F -15-1 J-0 -1 1-32-1 O -4 - l 0 -2 8 -1 M -5-4 Average

S i0 2 7 7 .4 7 3 .6 74 .7 7 8 .6 77 .1 77 .3 76 .5

a12 °3 8 .5 9 .1 8 .7 8 . 8 8 .5 8 .9 8.8

Fe2 °3 • • • « • • • •

FeO 1.1 1.6 1.6 0 . 6 0 . 8 0 .5 1.0

MgO 1 .4 1 .7 1.8 0 . 6 0 . 8 0 . 6 1.2

K2° 3 .3 3 .2 3 .1 3 .5 3 .1 3 .5 3 .3

N a20 3 .9 4 .4 4 .2 3 .3 4 .0 4 .0 4 .0

CaO 2 .9 3 .1 3 .0 2 .7 2 .7 2 . 8 .2 .7

H 2° 0 .7 1.1 1.1 0 .4 0 . 6 0 .4 0 .7

co2TiQ2 • • • • • • 0 . 1 • • • •

Opaque M inerals 0 .9 2.0 0 .9 0 .4 1.7 1 .9 1.3

50

TABLE 4 . — C h e m ic a l c o m p o s itio n s of f iv e s a m p le s o f a l te re d p o rp h y r iticq u a r tz m o n z o n ite by w e ig h t p e rc e n t

Sample Number

O xides 1-32-1 F -15-1 O -3 - l 1- 2-1 P -11-1

S i0 2 73 .3 7 1 .3 77 .3 70 .2 75 .2

>

t—' to oCO

8 . 0 7 .7 7 .1 7 .4 6 . 6

Fe 2°3 0 .4 0 .4 • • • •

FeO 0 . 2 0 . 1 • • • •

MgO 1.8 2 .9 1.0 0 . 6 0 .4

* 2° 5 .6 4 .9 5 .3 6.0 6 . 2

N azO 1.8 1.8 1.3 1.1 0 .3

CaO 2 .4 1 .5 0 .9 0 .7 0 . 2

H2° 5 .4 8 . 1 5 .2 10.8 9 .9

co2 0 . 6 • • • • • •

H o to • • • • • • • •

Opaque M inerals 0 . 6 1 .3 0 .9 2 .9 1.1

51

Fresh Lst Alt Lst Alt Mod Alt Str Alt Str Alt

Fresh Lst Alt Lst AltMod AltStr AltStr Alt

FreshLst AltLst AltMod AltStr AltStr Alt

Fresh Lst Alt Lst Alt Mod AltStr AltStr Alt

Fresh Lst Alt Lst Alt Mod AltStr Alt Str Alt

Fresh Lst Alt Lst Alt Mod Alt Str Alt Str Alt

AloO2^3

k2o

N d 2 0

CaO

MgO

H2O

T ~ I ~ f 8 9 10 11 12

Figure 24 . Chemical Gains and Losses in Porphyritic Cuartz Mon- zonite with Increasing Sericitic Alteration.

Fresh = average of 6 samples from Table 3.Least altered (Lst Alt) = samples 1-32-1 & F-15-1 from Table 4. Moderately altered (Mod Alt) = samples 0 -3 -1 from Table 4. Strongly altered (3tr. Alt) = samples J-2-1 & P-11-1 from Table 4.

52

f irs t rep laced ; rep lacem ent of o rthoclase fo llow ed . Al, C a , N a, and Fe

w ere lo s t during a lte ra tio n , w hile K and H+ were added .

A lteration in Sedim ents and M etased im ents

Q u a rtz -se ric ite and p o ta s s iu m -s ilic a te (?) a lte ra tio n are p resen t

in the m etased im en ts, but no hypo gene a lte ra tio n w as observed in un­

m etam orphosed sedim ents (Figure 12). S e ric itic a lte ra tio n is observed

in the m eta-arkose and q u a rtz ite . Because added quartz is extrem ely

d ifficu lt to d is tin g u ish from rec ry s ta llized grains of q u a rtz , a lte ra tio n

w ill herein generally be described in term s of se ric ite a lo n e . H ow ever,

s ilic a -r ic h h o m felses and abundant sm all quartz v e in le ts th a t f i l l jo in ts

c lea rly dem onstrate the add ition of quartz e s se n tia lly everywhere th a t

se ric ite a lte ra tio n occurred . D ick ite w as found ad jacen t to one qu a rtz -

carbonate vein in the m etased im en ts, and ch lo rite w as found ad jacen t

to another v e in .

M e ta -a rk o se . M oderate (?) q u a rtz -se r ic ite a lte ra tio n is perva­

sive throughout beds of m e ta -a rk o se , apparen tly unrela ted to proxim ity

of con tac t w ith the in tru s io n s . S eric ite occu rs a s m inute shreds rep la c ­

ing both p lag io c lase and o rthoc lase and as sm all c ry s ta ls filling m icro-

v e in le ts in and around fe ld sp ars (Figure 6) . In te rs titia l m ateria l in the

m eta-arkose is a lso strongly se r ic itiz e d .

As in ,the porphyritic quartz m onzonite, p o ta s s iu m -s ilic a te a l­

te ra tio n (?) appears rare ly and is very w eak . In the w estern part of the

a re a , not more than th ree to four p lag io c lase fragm ents p artia lly re ­

p laced by o rthoclase w ere found in each of three th in s e c tio n s . No

developm ent of new b io tite is ap p aren t.

53

Q u a rtz ite . Q u a rtz -se ric ite a lte ra tio n in quartz ite is sim ilar to

th a t in m eta -a rk o se , but no p o ta s s iu m -s ilic a te a lte ra tio n w as recogn ized .

The ex ten t of se ric itiz a tio n in the sparse g ra ins of p lag io c la se and ortho-

c la se v a rie s from a few shreds to com plete replacem ent (Figure 25). Ser­

ic ite f i lls minute v e in le ts in the q u a rtz ite , and se ric itiz a tio n of quartz­

ite m atrices is not uncommon (Figure 25). S eric ite in m icrovein lets is

thought to rep resen t a lte ra tio n se r ic i te , but se ric ite w hich rep laces

fe ld spars and se ric ite in m atrices cannot be defin ite ly recognized as

e ither sedim entary or a lte ra tio n se r ic ite .

H o rn fe ls . As noted ab o v e , ho rn felses are the p rincipal m eta-

morphic products of lim estones and sh a le s . A lteration in the nornfelses

i s re s tric te d to the q u a rtz -se r ic ite type . Altered and unaltered ho rn fe lses

have many s im ila ritie s : co lo r, frac tu re , very fine grain s iz e , in te rbed ­

ding in the m etased im en ts, and the p resence of se ric ite e ith e r from a l­

te ra tion or from metamorphism of o rig inal sedim entary m ateria l. Because

m egascopic d ifferences betw een a ltered and unaltered h o rn fe lses could

not be recognized in the f ie ld , they were not mapped se p a ra te ly .

D ifferen tia ting a lte red from unaltered h o rn fe lses required X-ray

a n a ly s is to supplem ent th in sec tio n a n a ly s is for m ineral id en tifica tio n

and confirm ation. X-ray a n a ly s is showed th a t h o rn fe lses from the area

are generally com posed of q u a rtz , d io p s id e , s e r ic ite , p la g io c la se , and

c a lc ite ; the rocks form a continuum ranging from predom inantly quartz -

d iopside a t one end to predom inantly q u a rtz -se r ic ite a t the o ther. From

q uartz -d iopside to q u a r tz - s e r ic ite , the con ten t of p lag io c la se and c a l­

c ite d e c re a s e s .

54

Figure 25. Sericitized Feldspathic Quartzite.

Sericitic matrix (S) surrounds all grains. Some grains of plagio- clase (P) and orthoclase (O) are altered while others are apparently un­affected. X-nicols, XI7.

55

Although they vary in orig inal com position, lim estones and

sh a les would seem un likely to y ie ld more than 20 percen t metamorphic

se ric ite b ecau se abundant s i l ic a and calcium in th e se sedim ents would

have fac ilita ted the developm ent of calcium s il ic a te s and fe ld sp ars in

the normal co n tac t m etam orphic environm ent (Barth, 1962, p . 264-265).

H ornfelses com posed of predom inantly qu artz -d io p sid e w ith a few per­

cen t of p la g io c la se -s e r ic ite w ere probably formed by metamorphism

which w as isochem ica l excep t for lo s s of C O g. Q ualita tive d ifferen ti­

a tion of m etam orphic from a lte ra tio n se ric ite i s d if f ic u lt , so only quan­

tita tiv e d ifferen tia tion w as made; where the nonsulfide portion of hom -

fe ls e s con ta ins le s s than 20 percen t s e r ic i te , the se ric ite w as consid ­

ered m etam orphic. As a working defin itio n , unaltered ho rn fe lses were

assum ed to be those w ith nonsulfide portions th a t to ta l a t le a s t 80 per­

cen t quartz , d io p sid e , and p la g io c la se , w hile se ric ite and c a lc ite to ta l

le s s than 20 p e rcen t. H ornfe lses to ta llin g more than 20 percen t se ric ite

in th e ir nonsulfide portion are assum ed to have undergone se r ic itic a lte r­

a tion a fte r m etam orphism . Predom inant q u a rtz -se r ic ite con ten t in horn­

fe ls e s im plies com plete rep lacem ent of a ll o ther m inera ls . As observed

a b o v e , d ecreasin g porosity in the hornfels is a s so c ia te d w ith in creas in g

in te n s ity and ex ten t of a lte ra tio n .

S tra tig rap h ica lly , se le c tiv e m etam orphism , a s d isc u sse d ab o v e ,

caused dense fin e-g ra in ed h o rn fe lses to develop from beds of lim estone

and sha le th a t e ith e r abut the s to ck s or c ro ss sheared and b recc ia ted

zones th a t abut the s to c k s . S ilic ifica tio n and se ric itiz a tio n are m ost in ­

ten se in beds w hich abu t the s to c k s , and beds a sso c ia te d w ith sheared

and b recc ia ted zones w hich abu t the stocks; how ever, a lte ra tio n has

56

been s e le c tiv e , rep lacing only favorable beds of th ese h o rn fe lse s . The

fac to rs w hich determ ined favorable beds for a lte ra tio n are unknown, but

one may have been chem ical su sc e p tib ility . S ilic ifica tio n and s e r ic i t i-

zation p refe ren tia lly a lte r beds of ho rn fe lses to the lim its of the a re a ,

although in te n s ity of a lte ra tio n seem s to d ecrease w ith increas ing d is ­

tan ce from the s to c k s .

Summary of sedim entary and m etasedim entary a lte ra tio n . Mod­

era te ly (?) in ten siv e q u a rtz -se r ic ite a lte ra tio n is pe rvasive in m eta-

arkose and q u a rtz ite , occurring in sm all v e in le ts and rep lac ing grains

of p lag io c la se and o rthoclase in varying am ounts. A se ric ite m atrix is

not uncommon in both m eta-arkose and q u a rtz ite . S e ric itic a lte ra tio n in

the h o rn fe lses is se lec tiv e and ranges from nonex isten t to in te n se .

S e lec tiv ity and in ten sity probably vary w ith s tru c tu re , d is tan c e from

the in tru s io n s , and , p o ss ib ly , chem ical su sce p tib ility to the a lte ring

flu id s .

M ineralization in S ed im en ts, M etased im ents .

and Igneous Rocks

Both vein and d issem ina ted m ineraliza tion occurs in a ll rock

ty p e s , as determ ined by fie ld and laboratory exam inations using the

hand le n s , b inocu lar m icroscope, and m ineragraphic m icroscope.

D issem inated M inera liza tion

P yrite , py rrho tite , and chalcopyrite are the only d issem inated

su lfid es recognized ; hem atite and m agnetite w ere found in one po lished

surface of porphyritic quartz m onzonite from the top of G ranite M ountain.

H ornfe lses genera lly con ta in the most d issem inated su lfide m ineralization,

57

followed by lim estone , porphyritic quartz m onzonite, s h a le , san d sto n e ,

q u a rtz ite , a rk o se , and m eta-a rk o se in order of d ecreasin g su lfide con­

te n t. Pyrite is d issem inated throughout a ll rock ty p es , although only a

trace i s commonly found in m eta -a rk o se , q u a rtz ite , and san d sto n e .

C halcopyrite is found in h o m fe lse s and porphyritic quartz m onzonite,

and pyrrhotite is re s tr ic te d to h o m fe lse s .

Sedim ents and m etased im en ts . In sedim ents and m etased i­

m en ts, pyrite occu rs as grains of le s s than 2 mm in d iam eter, ranging

from perfec t cubes to anhedra . C ubic to anhedral pyrite occurs in the

groundm ass; pyrite a long frac tu res and in quartz v e in le ts , w hich are

rarely w ider than 2 mm, is m ostly cubic (Figure 26). D issem inated an­

hedral pyrite g rains appear to f ill pore spaces throughout the sedim ents

and m etased im en ts, e sp ec ia lly in the h o m fe lses (Figure 27). Pyrrhotite

is d issem inated as anhedral grains and chalcopyrite is d issem ina ted as

anhedral grains w hich are generally m icroscopic in s ize (Figure 28).

Porphyritic quartz m onzonite . In the porphyritic quartz mon­

zo n ite , approxim ately 25 percen t of the pyrite appears to be a sso c ia te d

w ith ch lorite a fte r b io ti te . The rem aining pyrite and the much le s s abun­

dan t chalcopyrite are d issem in a ted throughout the in tru sive rocks as

anhedral grains (Figure 29).

Limonite a s a w eathering product of p y rite , py rrho tite , and

chalcopyrite f i lls jo in ts , forms coatings on ou tcrops, and occurs as

pseudom orphs a fte r pyrite and py rrho tite . Penetra tion of lim onite seem s

to vary proportionally w ith rock po rosity . On rocks where shearing ,

quartz ve in ing , and ex ten siv e d issem inated m ineralization e x is t .

58

Figure 26. Pyrite in V einlet.

Subhedral pyrite (py) in ve in le t of quartz (q) cu tting fine-g rained hornfels with finely d issem inated pyrite and ch a lco p y rite . Plain ligh t, X26.

Figure 27. Porous H ornfels.

Shows porosity of hornfels and grain of subhedral pyrite (py). Plain ligh t, X26.

59

Figure 28. Sulfides D issem inated in H orn fe ls .

Anhedral grains of pyrrhotite (po) and chalcopyrite (cp) are d is ­sem inated in very fine-grained ho rn fels. Plain ligh t, X26.

D issem inated subhedral pyrite (py) and anhedral chalcopyrite (cp) in igneous rock . Plain lig h t, X26.

60 -

lim onite co a tin g s up to 1 /8 inch th ick are accom panied by 'b lack man­

g a n ese 1 s ta in in g .

Vein M ineralization

Q uartz-carbonate ve in s and v e in le ts w ith sim ilar su lfide min­

eralogy cut a ll rock ty p e s , a lte red and u n a lte re d , but not a ll ve ins con­

ta in the su lfide m inera liza tion . Veins can be found throughout the a re a ,

although they are le s s abundant farther from the s to c k s . The v e in s are

normally th ree to nine inches th ic k , and v e in s up to two fee t th ick are

not uncommon, but ve in s more than two fee t th ick are sc a rc e . V einlets

and m icrovein lets le s s than a few m illim eters th ick are commonly a s s o ­

c ia ted w ith the v e in s . The abundance of v u g s , comb q u a rtz , and euhedral

pyrite in d ica te s deposition in open sp a ce s (Figure 30). Recurrent open­

ings along the ve in s are ind ica ted by m ultiple generations of quartz -

carbonate in some of the veins (Figure 30).

Pyrite is the m ost abundant hypogene su lfide in the v e in s ; i t is

alw ays a sso c ia te d w ith the o ther su lfides; g a le n a , sp h a le rite , ch a lco -

p y rite , bo rn ite , and a member of the ten n a n tite - te tra h ed rite so lid so lu ­

tion s e r ie s . N ative gold is reported (Schrader, 1915; F. Harmes and

J . S tew art, oral com m unication). Schrader (1915) a lso reported a rgen tif­

erous galena and na tive s ilv e r , but ne ither w as iden tified during th is

s tudy . Supergene m inerals in the ve ins are c o v e llite , lim onite , an g le -

s i te , m alach ite , a z u rite , ch ry so co lla , and c e rru s ite . In add ition to the

ubiquitous quartz and c a lc i te , v e in s in or near the in tru s io n s a lso carry

b a rite as a gangue m ineral.

Sedim ents and M e tased im en ts . In the sedim ents and m etased i­

m en ts, v e in s are genera lly a sso c ia te d w ith sheared (Figures 31 and 32)

Figure 30. Vein Quartz with S u lfides.

Euhedral pyrite (py) and anhedral chalcopyrite (cp) are d issem i­nated in two generations of quartz-carbonate veining (designated q l and q2 ). Plain ligh t, X26.

62

Figure 31. Sheared Outcrop Stained with Limonite.

Shows sheared , a ltered m etasedim ent w ith severa l ve ins cutting rock . Vein to right of hammer is approxim ately p ara lle l to bedding.

63

and b reccia ted zones on the c re s ts and troughs of fo ld s . C re s ts and

troughs of fo lds th a t abut the various in trusions are the m ost ex tensively

veined areas in the sed im ents and m etased im en ts . From the c re s ts and

tro u g h s, ve ins appear to extend away from the ax ia l p lane along the

flanks of the folds p a ra lle l to bedding . In the M orning S tar m ine, lo­

ca ted on the c re s t of an a n tic lin e , ve ins are su b p ara lle l to each other

and to bedding, a lm ost forming a stockw ork of varying th ick n ess (J.

S tew art, oral com m unication, M .S . th e s is in p rep ara tio n ). Sheared

zones north of the northw est in tru s ion and so u th eas t of the so u th eas t

in tru sion a lso con ta in abundant m ineralized quartz -ca rbonate v e in s .

M ost ve in s in sh ear zones seem to p a ra lle l bedding, but a few cu t bed­

ding (Figure 9).

Porphyritic quartz m ohzonite . In the porphyritic quartz mon-

zonite in tru s io n s , ve ins f ill jo in ts and minor fa u lts . M ost v e in s range

from on e-h a lf inch to one foot in th ick n ess and appear to have maximum

la te ra l ex ten t of not more than 300 fe e t. M any ve ins have more lim ited

la te ra l ex te n t. Only th ick e r ve in s w ere genera lly opened by mining;

a lm ost a ll of th ese ve in s dip toward the cen ters of the in tru s io n s , and

a ll w ere observed to p inch out w ith in 5 to 175 fee t down d ip .

Zoning

Various groupings of the d issem inated p y rite , p y rrh o tite , and

chalcopyrite are randomly sc a tte re d throughout the a re a . No apparent

zoning of th ese d issem in a ted su lfid es w as recogn ized . In a ll m ineral­

ized qu artz -ca rb o n ate v e in s , a ll vein su lfides w ere p resen t in varying

64

proportions. Because th e se v aria tio n s were not c o n s is te n t in any d irec ­

tio n , no zoning of the vein su lfid es seem s to e x is t .

Para g en esis

B ecause d issem in a ted su lfides give no ind ica tions of th e ir re la tive

tim es of d e p o sitio n , Figure 33 is determ ined from the p arag en esis of vein

Early Late

Q uartz C a lc ite

Pyrrhotite

Pyrite

C halcopyriteG alena

SphaleriteBornite

T ennan tite- te trah ed rite

Figure 33. P aragenetic Sequence of Hypogene M inerals in G rea ter- v ille A rea.

m ineralogy. Found only as a d issem ina ted m ineral, pyrrhotite i s early

and may in d ica te deposition a t tem peratures of approxim ately 600°C

(Yund and Kullerud, 1966). In the quartz -ca rbonate v e in s , pyrite c u ts

and is cut by a ll su lfid es (Figure 34). C halcopyrite is d issem inated w ith

pyrite in sev era l genera tions of ve in quartz (Figure 30), f i lls frac tu res

in pyrite (Figure 34), is ex so lved from sphalerite (Figure 35), and m ain­

ta in s m utual boundary re la tio n s w ith spha lerite and galena; spha lerite

65

Figure 34. C halcopyrite and Pyrite.

Shattered pyrite (py) is filled w ith chalcopyrite (cp) which is a lso exsolved in sphalerite (s i) . Pyrite cu ts through sphalerite beyond range of photograph. Plain l ig h t , X26.

Figure 35. Boundary R elations.

G alena (gn) and sphalerite show mutual boundary re la tions while chalcopyrite (cp) is exsolved in sp h a le rite . Plain ligh t, X26.

66

and galena a lso have m utual boundary re la tio n s (Figure 35). T etrahedrite-

tennan tite appears to be dep o sited along the m argins of p y rite , sphaler­

i te , and g a len a . Bornite appears to have m utual boundary re la tio n s w ith

cha loopy r i te .

Summary

Both vein and d issem inated m ineraliza tion are found in a ll rock

ty p e s . The h ig h es t p e rcen tag es of the d issem ina ted su lf id e s , p y rite ,

py rrho tite , and c h a lc o p y rite , are found in the h o m fe lse s . S ignificant

amounts of pyrite and chalcpy rite are a lso found in the porphyritic

quartz m onzonite. Q uartz -carbonate ve in s f ill openings in the sed im en ts,

m etased im en ts, and porphyritic quartz m onzonite. In addition to quartz

and c a lc i te , the v e in s con ta in p y rite , g a len a , sp h a le rite , chalcopyrite ,

b o rn ite , te tra h e d r ite - te n n a n tite , and native gold . N either vein nor d is ­

sem inated su lfides show zoning. P aragenesis in d ica te s th a t among the

su lfides pyrite w as dep o sited a t in te rv a ls over the longest time sp an ,

with chalcopyrite n ex t. Pyrrhotite w as early ; galena and sphalerite were

deposited toge ther. T e trah ed rite -ten n an tite deposition w as probably la te .

ANALYSIS OF STRUCTURE, MINERALIZATION,

ALTERATION, AND METAMORPHISM

M etasedim entary rocks in the G reaterv ille area have been in ­

truded by three s to ck s of porphyritic quartz m onzonite . S alien t fea tu res

described above include s tra tig rap h ica lly se lec tiv e metamorphism in the

sedim ents; a lte ra tio n of. favorable m etasedim entary beds; zoned a lte ra ­

tion in the in tru s ions; sim ilar vein su lfide m ineralogy throughout sed i­

m ents , and igneous rocks; and the apparen t re la tion of structure to

m inera liza tion , m etam orphism , and a lte ra tio n . To th ese observations

are added two stru c tu ra l problem s introduced above; 1) why rocks in

the no rtheast portion are struc tu ra lly more com plex than those in the

sou thw est portion , and 2) w h a t, if any , re la tio n sh ip e x is ts betw een

structure in the m etasedim ents and sed im en ts , and the em placem ent of

the igneous in tru s io n s .

Im plica tions of Folding and In trusion

As described above, the sedim entary and m etasedim entary

rocks in the map area form a re la tiv e ly undeformed southw est portion

and a com plexly folded n o rth east portion . Three hypo theses can be pro­

posed to exp la in re la tio n s betw een deform ation in the sedim ents and

m etased im en ts, and em placem ent of the th ree s to ck s along the observed

northw est a lignm ent. These hypo theses are 1) a gravity fau lt, 2) a b a se ­

ment shear zone, and 3) d ila tio n or w eakness in the cen tra l portion of a

large sy n c lin e .

67

68

The f irs t hy p o th esis p o s tu la te s a gravity or glide fa u lt. The

upper part of the stra tig raph ic se c tio n , contain ing considerab ly more

lim estone and sha le than the low er p a rt, is struc tu ra lly le s s com petent

than the low er p a rt. After s teep til t in g , a s ind ica ted by the a ttitu d es of

rocks in the sou thw est portion , the upper and low er parts may have sep ­

ara ted along a plane of w e a k n e ss , the upper part gliding downward and

folding upon i ts e l f . The plane of sep ara tio n , a w eakness in the outer

portion of the c ru s t, would have served as a loci for the in trusions a fte r

they reached the sha llow er portion of the c ru s t.

A second hyp o th esis p o s tu la te s th a t the se rie s of fo lds re fle c ts

movement a long a w ide basem ent shear zone oriented approxim ately

N45°W . This o rien tation is proposed b ecause both the C anelo H ills and

the northern end of the H uachuca M ountains so u th eas t of the area have

a northw est s truc tu ra l alignm ent w hich p ro jec ts in to the G reaterv ille

a re a . The change in trend of the Santa Rita M ountains in the G reaterv ille

d is tr ic t m ight then have been cau sed by righ t la te ra l d isp lacem en t along

th is proposed sh ear zo n e . In the map a re a , repeated d iffe ren tia l move­

ment w ith in the shear zone could have cau sed the com plex se rie s of

fo lds in the overlying ro c k s . The igneous in tru s ions w ere then em placed

in th e se w eakened and folded rocks along a northw est lineam ent.

The th ird hypo thesis p roposes th a t a re la tiv e ly large sy n c lin e ,

w ith a no rthw est-trend ing a x is , w as formed a s a re su lt of n o rth east-

sou thw est com pression . Subsid iary folds w ere developed e ith e r along

the n o rth east limb or w ith in the c en tra l portion of the m ajor fo ld . Ac­

cording to th is h y p o th es is , in tru s ions w ere em placed near or in the

trough of th e sy n c lin e . P a rtia lly ac tiv e em placem ent of in tru s ions and

69

intraform ational slippage w ere responsib le for loca l deform ation in the

sedim ents and m etased im en ts.

Emplacement of the in tru sions is m ost reasonab ly explained by

a com bination of basem ent sh ear and su rfic ia l fo ld ing . Evidence from

the fie ld , a e ria l p ho tog raphs, and a sm a ll-sc a le map (button, 1958,

p la te 2) defin ite ly show a large syncline w ith subsid ia ry fo lds e ith e r

along its n o rth east limb or w ith in i ts c en tra l p o rtio n . Figure 3 su g g ests

th a t sp ec ific beds wrap around the northw est side of G ranite M ountain ,

but th is con tinu ity cannot be p o sitiv e ly a sse r te d b ecau se of cover and

the len so id a l nature of ind iv idual b e d s . H ow ever, the author b e liev es

th a t th ese beds do wrap around the northw est side of G ranite M ountain

b ecau se s tr ik es show c o n sis ten cy over the w hole a re a . A re la tiv e ly

large sync line can a lso be inferred from s te re o -p a ir ae ria l pho tographs.

button (1958, p la te 2) m apped C re taceo u s rocks betw een Box Canyon

and Fish C anyon, w hich in c lu d es the G ranite M ountain a re a , and h is

reco n n a issan ce sc a le map a lso shows the large syncline w ith su b s id i­

ary folding on i ts no rtheastern limb or w ithin i ts cen tra l portion . Rose

diagram s (Figures 20, 21, and 22) show a s e t of c ro ss jo in ts w hich w ere

probably formed from n o rth east-so u th w est com pression . Although the

syncline is re la tiv e ly la rg e , it is un likely to extend deep in to the c ru s t

and is probably re s tric te d to the C re taceous (?) ro c k s .

A basem ent sh ear zone could have allow ed magmas to reach

shallow er portions of the c ru s t w here the syncline might a c t a s a lo c i

for em placem ent of the in tru s io n s . The bend or o ff-se t of the Santa

R itas in the G reaterv ille d is tr ic t su g g ests some so rt of right la te ra l

m ovem ent. The struc tu ra l alignm ent of the C anelo H ills and the

70

H uachuca M ountains th a t can be pro jected in to the G reaterv ille area is

further evidence for a shear zone.

I t i s in te res tin g to note th a t if the struc tu ra l alignm ent of the

C anelo H ills and H uachuca M ountains is continued N45°W through the

G reaterv ille a re a , i t would a lso p a ss through or near the Twin B uttes,

Pim a, M iss io n , and S ilverbe ll d e p o s its . This may be add itional ev i­

dence for a large ex tensive sh ear zone p ass in g through the G reaterv ille

a re a .

It i s concluded th a t a northw est sh ear zone w as the avenue

along w hich magma m igrated in to shallow er portions of the c ru s t. After

reaching the shallow er ro ck s , the magma w as lo ca lized in the w eakened

cen tra l portion of the syncline formed by n o rth ea s t-so u th w e st com pres­

sion .

Im plications of M inera liza tion , A lteration, and M etam orphism

The re la tio n sh ip s betw een the in tru s io n s , m inera liza tion , a lte r­

a tio n , and metamorphism may be a t le a s t p a rtia lly explained by exam in­

ing vein m ineraliza tion in sed im en ts , m etased im en ts, and igneous

stocks; zoned a lte ra tio n in the stocks; and s tra tig rap h ica lly se lec tiv e

isochem ica l metamorphism in m etased im en ts .

M ineralization

Sim ilar su lfide m ineralogy in a ll quartz -ca rbonate ve ins in d i­

c a te s th a t they were em placed under sim ilar co n d itio n s. M ineralization

appears to be re la ted to the in tru s io n s b ecau se sheared and b recc ia ted

zones tha t abut the s to ck s are much more ex ten siv e ly veined than sim i­

la r s truc tu res w hich do not abut the s to c k s . Q uartz -carbonate veins in

71

the porphyritic quartz m onzonite s to ck s generally dip tow ard the cen ters

of the in tru s io n s , ind ica ting form ation of a se t of co n e-sh ap ed jo in ts

and fau lts in the c ry s ta lliz e d outer sh e ll th a t probably capped the in tru ­

sion as it co o led . M ineralized ve in s occupying th ese jo in ts and fau lts

show th a t they formed channelw ays along w hich hydrotherm al so lu tions

could m igrate ou tw ard , depositing the q u a rtz , c a lc i te , and su lfides now

observed in a ll v e in s . In the sedim ents and m etased im en ts, form ation of

v e in s appears to be stru c tu ra lly contro lled by sheared and b recc ia ted

zones along the tigh tened c re s ts and troughs of subsid ia ry fo ld s . These

zones ev iden tly furn ished a c c e s s for the hydrotherm al so lu tions to mi­

grate from the igneous rock in to the host rock .

A lteration

In ten se q u a rtz -se r ic ite a lte ra tio n around most m ineralized

quartz -ca rbonate veins in d ic a te s th a t the so lu tions w hich m ineralized

the veins w ere a lso resp o n sib le for q u a rtz -se r ic ite a lte ra tio n in the a re a .

In the s to c k s , an inner zone of rare and w eak p o ta s s ic a lte ra ­

tion in te rfingers w ith pervasive and w eak q u a rtz -se ric ite a lte ra tio n .

The h igher tem perature p o ta s s ic a lte ra tio n seem s to have occurred in

the cen tra l pa rts of the in tru s io n s , changing to se r ic itic a lte ra tio n

toward the m arg in s, where both in ten sity and ex ten t of se ric itiz a tio n

in c re a se s (Figure 28).

S e ric itic a lte ra tio n in the m etasedim ents is sim ilar to th a t in

the in tru sions; se ric ite i s observed to rep lace grains of p lag io c la se and

o rthoclase and to f ill m icrovein le ts th a t cu t and surround a ll grains

(Figures 6 and 25). The m etased im ents show th e ir m ost in ten se a lte ra tio n

72

in beds ad jacen t to sheared and b recc ia ted zones w hich abut the s to c k s ,

again ind icating tha t th ese zones furnished a c c e s s for the hydrotherm al

so lu tions (Figure 12). M igrating in to the m etased im en ts , a lte ring flu ids

followed th ese struc tu res and m igrated along beds p a ra lle l to s tr ik e ,

p referen tia lly a ltering favorable b e d s . In favorable beds th a t abut the

s to c k s , in ten se a lte ra tio n becom es le s s in ten se farther away from the

in tru s io n s .

In the a lte red porphyritic quartz m onzonite , Figure 24 shows

tha t AlgOg, NagO, C aO , and MgO(?) w ere lo s t , w hile KgO and HgO

w ere added . This type of exchange is defined as hydrogen m etasom atism

and base leach ing (Meyer and H em ley, 1967). S eric ite rep laces p lag io -

c la s e , b io tite , ch lorite a fte r b io ti te , and o r th o c la se . A lteration in ­

c reased in ex ten t and in ten s ity from the cen te r of the s to ck s outward;

apparently se ric ite becam e more and more in so lu b le , re la tiv e to o ther

m inera ls , a s the so lu tions m igrated outward through the s to c k s . This

type of reaction is expectab le of a s ligh tly ac id solution; " . . . a

b a s ic so lu tion would give ca tion to hydrogen ion ra tio s too high for

hydrogen m etasom atism " (Meyer and Hem ley, 1967, p . 214). A ltering

so lu tions in the porphyritic quartz monzonite w ere ev iden tly sligh tly

ac id and reacted w ith the w all rocks to cau se hydrogen m etasom atism

and base leach in g .

M etam orphism

M ineral com position of unaltered h o m fe lse s , a s described

above, su g g ests th e ir form ation by metamorphism which w as isochem i­

ca l except for lo ss of C O g. Isochem ical metamorphism is a lso seen in

the sim ple re c ry s ta lliz a tio n of arkose to m eta-arkose and sandstone to

73

q u a rtz ite . All metamorphism w as probably isochem ical b ecause m eta-

a rk o se , q u a rtz ite , and some h o m fe lses were formed by isochem ica l

m etam orphism . The m etam orphosing agent probably co n sis ted e s s e n ­

tia lly of H zO. If there w ere v o la tile s p re se n t, they were ev iden tly un-

re ac tive w ith c o n stitu en ts of the sedim entary ro ck s .

A h igher m etam orphic rank is observed in beds th a t abut the

s tocks or c ro ss sheared and b recc ia ted zones th a t abut the s tocks than

in sim ilar s tru c tu res th a t do not abut the s to c k s . This stra tig raph ica lly

se lec tiv e metamorphism appears to have been controlled by flu ids w hich

m igrated p refe ren tia lly along bedding ra ther than ac ro ss bedding. These

flu ids may have orig inated from 1) early hydrotherm al w aters em anating

from the cooling magma, or 2) c ircu la ting m eteoric w aters moving toward

the s tocks p a rtica lly under the im petus of a therm al convection c e l l , or

3) a com bination of or in te rac tion betw een th ese two f lu id s , both of

which c o n s is t m ainly of w a ter.

C ooper (1957) h a s shown that rem oval of COg during otherw ise

isochem ical metamorphism of sha les and lim estones c a u se s lo ss of vo l­

um e. H igher porosity of unaltered h o m fe lses in the G reaterv ille a re a ,

a s described a b o v e , probably re fle c ts such lo ss of volum e. D evelop­

ment of porosity could have fa c ilita te d m igration of la te r hydrotherm al

so lu tions th a t caused a lte ra tio n and m inera liza tion , a s in d ica ted by

in ten se ly a lte red h o m fe lse s in w hich pores are apparently filled w ith

a lte ra tio n m in e ra ls .

M ineraliz ing flu id s may have been sligh tly la te r than a lte ra tio n

f lu id s , b ecause d issem in a ted su lfid es in the sedim ents and m etased i­

m ents are m ainly dep o sited in unaltered or sligh tly (?) a lte red ro ck s .

74

How ever, d issem inated su lfid es could have a lso been formed by su lfoda-

tion of sedim entary iron and copper, if they were abundant enough. "Dis­

sem inated su lfides in the sedim ents and m etasedim ents are p refe ren tia l­

ly deposited in carbonate beds and th e ir m etam orphic eq u iv a le n ts . This

observation conforms to a conclusion drawn from a survey of the l ite ra ­

ture; many d ep o sits are described in w hich the su lfides are se lec tiv e ly

deposited in lim estone and c a lc - s i l ic a te rocks ra ther than ad jacen t

rocks of another com position .

In terp re ta tion of S tru c tu re , M etam orphism .

A lteration , and M ineralization

Sheared and b recc ia ted zones which abut the stocks seem to

have p layed a m ajor part in contro lling m etam orphism , a lte ra tio n , and

m ineralization in the m etased im en ts. W here th ese zones abut the s to c k s ,

a ll three phenomena generally occur. All three phenomena penetrate the

sedim ents and m etasedim ents on e ith e r side of th ese struc tu res and a ll

three are a lso m ost in ten siv e and ex tensive a long them .

Zoned a lte ra tio n in the in tru s ions and sim ilar vein m ineraliza­

tion in a ll rock types offer strong ev idence for a common hydrotherm al

origin of a lte ra tio n and m inera liza tion . The cau se of metamorphism is

le s s c lear; perhaps a com bination of h ea t from the magmas or early

hydrotherm al w aters in te rac tin g w ith connate or m eteoric w aters in itia ted

m etam orphism , w ith the m ost in te n se metamorphism cau sed by hydro-

therm al so lu tions m igrating away from the s tocks p refe ren tia lly along

beds p a ra lle l to s tr ik e .

In summary, a predom inantly hydrotherm al origin for the ob­

served phenomena is supported by major field and laboratory evidence:

75

1) progressive q u a lita tiv e change from p o ta s s ic a lte ra tio n in the cen ters

of the in trusive s tocks to se r ic itic a lte ra tio n toward th e ir m ag in s, 2)

increasing in ten s ity and ex ten t of se ric itic a lte ra tio n from the cen ters

of the stocks toward th e ir m arg ins, 3) sim ilar types of a lte ra tio n in both

stocks and ho st ro c k s , 4) v e in s in the porphyritic quartz m onzonite

w hich dip toward the cen te rs of the s to c k s , 5) s im ila ritie s of sulfide

vein m ineralogy in a ll rock ty p e s , and 6) s tru c tu ra l control of metamor­

phism , a lte ra tio n , and m ineraliza tion in sedim ents and m etasedim ents

around the s to c k s .

A lteration in the G reaterv ille area appears sim ilar to d e sc rip ­

tio n s of a lte ra tio n in some porphyry copper d e p o s its . C opper m ineraliza­

tion is p resen t, and the porphyritic quartz m onzonite m ight y ie ld more

copper a t dep th . G alena and spha lerite are known to form replacem ent

d ep o sits in lim esto n es . Exploration e a s t of G ranite M ountain m ight lo ­

ca te a con tac t m etasom atic d ep o sit contain ing some com bination of

copper, le a d , or z in c .

SUMMARY AND CONCLUSIONS

In the G reaterv ille a re a , g ran ito id , porphyritic quartz m onzo-

n ite of Laramide age in trudes sedim ents and m etasedim ents of Lower

C re taceous (?) a g e . The porphyritic quartz m onzonite is not observed

to cut the basem ent rock , a Precam brian g ranod io rite . The sedim ents

and m etasedim ents are be lieved to have a sedim entary co n tac t w ith the

g ranod io rite .

Lower C re taceo u s (?) sedim ents and m etasedim ents are com­

posed of th in , in terbedded un its of lim ited la te ra l ex te n t. The rocks

c o n s is t of a rk o se , san d s to n e , lim esto n e , sh a le , m eta -a rk o se , quartz­

i te , h o m fe ls , and a rg illi te . They form a conform able sequence 4000 to

5000 fee t th ick extending a c ro ss the a re a . As a g en era liza tio n , the

stra tig rap h ic sequence changes from predom inantly arkose near the

bottom to predom inantly lim estone near the to p , although thin beds of

a lm ost any sedim entary com position can be found sca tte red throughout

the sec tio n . The sequence probably rep resen ts n ear-sho re deposition

in a tran sg re ss iv e s e a .

All sedim ents have undergone some degree of m etam orphism .

M etam orphism in the arkose and sandstone is m anifested m ainly by re ­

c ry s ta lliz a tio n of q u a rtz , forming m eta-arkose and quartz ite resp ec tiv e

ly . A rgillaceous m aterial in the m atrix of th e se rocks may have a lso

y ie lded se r ic i te . H o m fe lses are the dom inant metamorphic product of

lim estones and s h a le s . Common m inerals in unaltered h o m fe lse s are

d io p sid e , q u artz , p la g io c la se , s e r ic i te , and c a lc i te . S tra tig raph ically

76

77

se lec tiv e metamorphism w as contro lled by the m igration of flu ids along

sheared and b recc ia ted zones and p referen tia lly along bedding ra ther

than ac ro ss bedding . Except for lo s s of CO g, metamorphism appears to

have been iso ch em ica l.

Prior to m etam orphism , n o rth east-so u th w est com pression dur­

ing Laramide deform ation folded the sedim ents in to a large syncline w ith

a northw est-trend ing ax is and a so u th eas t p lu n g e . The syncline e ith er

has an undeformed sou thw est limb w ith subsid iary folds on i ts no rth east

lim b, or has developed a com plex se rie s of subsid ia ry fo lds in i ts cen­

tra l portion , probably a s a re su lt of tig h ten in g . Although the syncline

probably does not reach to grea t dep ths in the c ru s t, i t may w ell have

formed a shallow contro l for the em placem ent of magmas moving upward

along an underlying sh ear zone.

W eak and sparse p o ta s s iu m -s ilic a te a lte ra tio n and variab le

q u a rtz -se r ic ite a lte ra tio n are recognized in the s to c k s . In the p o ta s s ic

a lte ra tio n , which w as not strong enough to form new b io tite , o rthoclase

p artia lly rep laced p la g io c la se . In q u a rtz -se r ic ite a lte ra tio n , se ric ite

norm ally rep laced p lag io c la se and ch lo rite a fte r b io tite ; where a lte ra tio n

has been in te n se , o rthoc lase is a ls o rep laced by s e r ic ite .

S e ric itiza tio n in th e sed im ents is observed throughout m eta-

arkose and q u a rtz ite . S e ric itic a lte ra tio n in the h o m fe lses v a rie s from

nonex isten t to in te n se , p o ss ib ly varying w ith chem ical su sce p tib ility of

the sedim ents to the a lte rin g f lu id s . Porosity in the h o m fe lse s , caused

by m etam orphism , is genera lly le s s w here a lte ra tio n is more in te n se .

Both d issem in a ted and v e in m ineraliza tion occur in a ll rock

ty p e s . D issem inated su lfid es are p y rite , py rrh o tite , and c h a lco p y rite .

78

Pyrite i s d issem inated in a ll rock ty p e s , pyrrhotite is found only in the

h o m fe lse s , and chalcopyrite is found in the h o m fe lses and porphyritic

quartz m onzonite . Q uartz -carbonate ve ins contain p y rite , g a le n a , spha l­

erite , ch a lco p y rite , b o rn ite , te tra h e d r ite - te n n a n tite , and native go ld .

Gangue is norm ally quartz and c a lc i te , but in and near the porphyritic

in tru s ions barite is a lso a gangue m ineral. Veins f ill b reccia ted and

sheared zones in sedim ents and m etased im en ts , arid move outward from

th ese zones p a ra lle l to bedding . In the igneous ro ck s , v e in s f ill jo in ts

and minor f a u l ts , both of w hich dip toward the cen ters of the in trusive

ro ck s . Q uartz -carbonate v e in s in a ll rock ty p es con ta in the same su l­

fide m ineralogy.

Sheared and b recc ia ted zones w hich abut the s tocks seem to

have p layed a m ajor p a rt in con tro lling m etam orphism , a lte ra tio n , and

m ineraliza tion in the m etased im en ts. W here th ese zones abut the s to c k s ,

a ll three phenomena generally occur. All three phenomena penetra te the

sedim ents and m etased im ents on e ith e r side of th ese s tru c tu re s , and a ll

th ree are a lso most ex ten siv e and in tensive along them .

Zoned a lte ra tio n in the in tru s io n s and sim ilar vein m ineraliza­

tion in a ll rock ty p es to g e th er offer strong ev idence for a hydrotherm al

origin of a lte ra tio n and m inera liza tion . The cau se of metamorphism is

le s s c lear; perhaps a com bination of hea t from the magmas or early hy­

drotherm al w a te rs in te rac tin g w ith connate or m eteoric w aters in itia ted

m etam orphism .

In co n c lu sio n , a predom inantly hydrotherm al origin for the ob­

served phenomena is supported by major fie ld and laboratory evidence:

1) p rog ressive q u a lita tiv e change from p o ta s s ic a lte ra tio n in the cen ters

79

of the in trusive stocks to s e r ic itic a lte ra tio n tow ard th e ir m arg ins, 2)

increasing in te n s ity and ex ten t of s e r ic itic a lte ra tio n from the cen ters

of the s to ck s tow ard th e ir m arg in s, 3) sim ilar ty p es of a lte ra tio n in both

stocks and ho st ro c k s , 4) v e in s in the porphyritic quartz m onzonite

which dip toward the cen te rs of the s to c k s , 5) s im ila ritie s of vein su l­

fide m ineralogy in a ll rock ty p e s , and 6) s tru c tu ra l control of metamor­

phism , a lte ra tio n , and m ineraliza tion in sedim ents and m etasedim ents

around the s to c k s .

Economic p ro sp ec ts are d ifficu lt to e v a lu a te . Exploration of

the stocks m ight show econom ic amounts of copper a t dep th , and explor­

a tion e a s t of G ranite M ountain might loca te a lim estone replacem ent

type d e p o sit, e ith e r le a d -z in c or le a d -z in c -c o p p e r . If p resen t, the

lim estone rep lacem ent d e p o sit is probably c lo se ly a sso c ia te d w ith a

sheared and b recc ia ted zone along the c re s t or trough of a fo ld .

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Lindgren, W aldem ar, 1933, M ineral D e p o s its , 4th E d.: M cG raw -H ill, New York, 930 p .

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82

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R 15 E R 16 E

EXPLANATION

Potassium-silicate alterationWeak , sparsely scattered.Partially replaced plagioclase crystals. No apparent new biotite.

Quartz -sericite alterationPervasive; weak where associated with potassic alteration.Increasing intensity and abundance toward margins of intrusions.

Hydrothermal (?) quortz-sericite alterationDifferentiating sedimentary from metamorphic from hydrothermal sericite is difficult.Within 'interbedded metasediments, part of the sericite seems hydrothermal in origin, although some beds of hornfels are unaltered.

Limit of metamorphismWest of this line, metamorphic effects are extensive; to the east, only slight recrystallization has occurred.

Sampled locations

330 330 660 990 1320 Feet

Scale -1= 393 0 or l " = 330 '

T o p o g r a p h i c b a s e from U. S. Ge o l og i c a l Su r ve y

S o h u o r i f a and Mt. Wr i gh t s on Q u a d r a n g l e

Con t our i n t e r v a l 4 0 feet

Figure 12. ALTERATION MAP OF THE GREATERVILLE AREA

GEOLOGY BY M. P. GROSS, 1969

E W \

iy

T 19 S

EXPLANATION

Oal

TKpqm

LK(?)

PCgr

Quaternary alluviumConsists of interbedded sand and gravel

Laramide porphyritic quartz monzonite

Breccia

Lower Cretaceous (?) sediments and metasedimentsConsists of arkose, sub-arkose, quartzite, feldspathic quartzite, shale, limestone, sandstone and hornfels

Precam brian granodiorite

Fault showing trend and plunge of slickensides

----------- Inferred fault

— $- Anticline showing plunge of axis

— f - Syncline showing plunge of axis

y^\c Strike and dip of beds

a Shaft

>— Tunnel or adit

X Pospect pit

Contours interpolated and plotted from aerial photographs

House

Lithologic contacts

Lithologic contacts approximately located

Inferred lithologic contacts

A

N

330 165 330 660—S e

990 1320 1650 Feet

Scale |: 3930 or I = 330'

Topographic base from U S. Geological Survey

Sahuarita and Mt. Wnghtson Quadrangle

Contour interval 40 feet

R 15 E R I S E

Figure 3. GEOLOGY OF THE GRANITE MOUNTAIN AREA

GREATERVILLE DISTRICT, PIMA COUNTY, ARIZONA

GEOLOGY BY M P GROSS, 1969