21
Bu11etin of the Geological Survey of Japan,Vol。47(11),p.577-597,1996 G磯e伽㈱翻0歪9醐一窺醜翻 樋y一舳囎gste盟睡醜e聡且置z翻艦 ev董組e聾ee:盤・0㎜.ge⑪且ogy我盟磁m蚤期er誠量za愈量o恥of 蝕e W翻魂e脚s熊,亙恥恥踊,C恥亘鰍 Xiongwei Hu*,Satoshi MuRAo**and Xuchang HuAN Hu Xiongwei,MuRAo Satoshi and HuANG Xuchang(1996)Genetic m -tungsten mineralization:evidence from geology and mine deposit,Hunan,China.B%ll.0601.Sz67∂.ノ吻)ごz7¢,vol.47(11),P.577- tables. Abstract:The Wuxi deposit,situated in the Xuefeng arctuate u China,is a representative of composite gold,antimony and tungst mineralization is of bedding-para11el vein type hosted by Up sericitic slates and controlled by interlayer faults and north-east tr Strike of ore veins is approximately6km with extensions of more Au,SbandWcontent.TheaverageoregradeatWuxiis8.38pp wt。%WO3. The mineralization at the Wuxi deposit took place during the framework of detachment structures and is closely associated with quartz veins in which seven phases showing temporal differen mineralization can be divided into two stages of tungsten minera -antimony mineralization(stage II).The tungsten mineralization substage(Ia)and wolframite substage(Ib)corresponding to tw -antimony mineralization is composed of a pyrite-quartz vein phase phase both of similar origin but distinct from tungsten minerali most likely resulte(i from acidic solutions that underwent neutr precipitation resulted from acidation of solutions.Gold and antim in weakly alkaline fluids and precipitαted by cooling and acidation be categorized as a spatially coupled tungsten mineralization and tion. Our re-examination supports a genetic model for the origin of ore- convection and chemically evolved paleo-ground water.We prop from a deep system into the shallow system is a possible mechanis a process may be initiated and promoted by formation of brittle fault conduits for upward migration of hydrothermal fluid.Periodic Mesozoic age is a significant factor controlling composite quartz ization. 1.臨tro岨賦ct量o醜 *(STA Fellow in1996,Mineral Resources Depart- ment,GSJ) Yichang Institute of Geology and Mineral Resources,Chinese Academy of Geological Sci- ences. **(Mineral Resources Department,GSJ) ***(Geological Division,Xiangxi Gold Mine,Hunan, China.) The Wuxi deposit is a represent posite Au-Sb-W type mineralizat (Huασ1.,1996).Similar deposits Au-Sb or W-Sb mineralization, Keywords:gold,antimony,tungste deposit,composite deposit,spatially- China. 一577一

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Bu11etin of the Geological Survey of Japan,Vol。47(11),p.577-597,1996

G磯e伽㈱翻0歪9醐一窺醜翻 樋y一舳囎gste盟睡醜e聡且置z翻艦        ev董組e聾ee:盤・0㎜.ge⑪且ogy我盟磁m蚤期er誠量za愈量o恥of

                蝕e W翻魂e脚s熊,亙恥恥踊,C恥亘鰍

Xiongwei Hu*,Satoshi MuRAo**and Xuchang HuANG***

Hu Xiongwei,MuRAo Satoshi and HuANG Xuchang(1996)Genetic model of gold-antimony

     -tungsten mineralization:evidence from geology and mineralizat1on of the Wuxi

    deposit,Hunan,China.B%ll.0601.Sz67∂.ノ吻)ごz7¢,vol.47(11),P.577-597,12figs.,3

     tables.

Abstract:The Wuxi deposit,situated in the Xuefeng arctuate upliftedbelt,westem Hunan,

China,is a representative of composite gold,antimony and tungsten deposits in Ch三na.The

mineralization is of bedding-para11el vein type hosted by Upper Proterozoic calcareous

sericitic slates and controlled by interlayer faults and north-east tren(王ing secondary fo1(is.

Strike of ore veins is approximately6km with extensions of more than lkm with significant

Au,SbandWcontent.TheaverageoregradeatWuxiis8.38ppmAu,4.36wt.%SbandO.37wt。%WO3.

 The mineralization at the Wuxi deposit took place during the Mesozoic Era under the

framework of detachment structures and is closely associated with the formation of composite

quartz veins in which seven phases showing temporal differences are distinguished.The

mineralization can be divided into two stages of tungsten mineralization(stage I)and gold

-antimony mineralization(stage II).The tungsten mineralization is comprised of scheelite

substage(Ia)and wolframite substage(Ib)corresponding to two vein phases.The gold

-antimony mineralization is composed of a pyrite-quartz vein phase and a stibnite-quartz vein

phase both of similar origin but distinct from tungsten mineralization.Scheelite deposition

most likely resulte(i from acidic solutions that underwent neutralization,whereas ferberite

precipitation resulted from acidation of solutions.Gold and antimony might be co-transported

in weakly alkaline fluids and precipitαted by cooling and acidation of fluids.The deposit can

be categorized as a spatially coupled tungsten mineralization and gold-antimony mineraliza-

tion.

 Our re-examination supports a genetic model for the origin of ore-forming fluids from deep

convection and chemically evolved paleo-ground water.We propose that fluid discharging

from a deep system into the shallow system is a possible mechanism of mineralization.Such

a process may be initiated and promoted by formation of brittle faults which serve as the main

conduits for upward migration of hydrothermal fluid.Periodic structural mobilization of

Mesozoic age is a significant factor controlling composite quartz vein and composite mineral-

ization.

1.臨tro岨賦ct量o醜

 *(STA Fellow in1996,Mineral Resources Depart-

  ment,GSJ)

  Yichang Institute of Geology and Mineral  Resources,Chinese Academy of Geological Sci-

  ences.

**(Mineral Resources Department,GSJ)

***(Geological Division,Xiangxi Gold Mine,Hunan,

  China.)

  The Wuxi deposit is a representative com-

posite Au-Sb-W type mineralization in China

(Huασ1.,1996).Similar deposits,such as

Au-Sb or W-Sb mineralization,are wide一

Keywords:gold,antimony,tungsten,Wuxi(Woxi)deposit,composite deposit,spatially-coupled deposit,

China.

一577一

Bz6116あ%(ゾ孟h6G60109ぎαzl S%γ∂βy(ヅノ4望りごz%,1茜ol.47,ノ〉o.11

      、載

謡麟、         (ア

     0

↑KK Wu植fa心

一。

   .憲譲窯,餐轍阿

  難舅 許 艶

響羅響朧甜響灘1

、0  1  2㎞

N一

嚢1

      300         〆

     丈          麟議、縄.拶   纏

ヤ27。    300 膿             〆   纒

          >!        撃

                  ぎ

恥轡繊騒露 鵠

Fig.

匡コcon創・merateofC「etaceous

回    metamolphic rocks ofthe    UpPer Proterozoic unit

醗獣盤o躍,濃9雛

〆 faultwi輔P

,’”    axial d廿㏄丘on offold

~    quartz vein andlor ore bo(ly

1Geological map of the Wuxi gold-antimony-tungsten deposit,H:unan,China

 (simplified after Luo6オ召1.,1984).Note diffrent structural features in

 various tectonic layers.Interlayer detachment faults and northeast trending

 folds are limited to the Banxi Group of the Upper Proterozoic unit.

sprea(1in Hunan,China,and also in other

countries,e.g.Bolivia,Spain,Germany,France an(1Turkey(Di116猛1.,19951Gumiel

and Arribas,1987).The gold-antimonydeposit is an important antimony producer in

Bolivia (Di11αα1.,1995).Being(lifferent

from other gold-antimony or gold-antimony

-tungsten d.eposits where it is(1ifficult to

recover gold, (e.g.,Kharma deposit in

Bolivia:Dil16渉α1.,1995),theWuxi deposit is

characterized by economic grades of gold,

antimony and tungsten.At the Wuxi deposit,

go1(i has been mined since1875,while anti-

mony ore was found in1889and tungsten in

1946.Antimony and tungsten were the main

pro(1ucts during the1950’s and19607s.At pres-

ent,theWuximineisamalorgoldproducerofHunan Province,China,where antimony andtungsten are also produce(1.

  The Wuxi deposit is located about200km

west of Changsha,the capital of the Hunan

Province,China (Fig.1).The deposit is

tectonically situated in the Xuefeng arcuate

uplifted belt which is a significant Au,Sb and

WmineralizedbeltinwhichmanyAu,Au-Sb,W-Sb,Au-Sb-Wdeposits occur.TheWuxi is

the northwestemmost(ieposit in the belt,

where the north-northeast strike bends to east

一west.Within the belt other representatives

are Xichong Au-Sb,Zhazixi W-Sb,Fuzhuxi

Au-Sb,MobinAuandXi’anWdeposits,etc.(Li,1987).

 During the past decade,the Wuxi deposit

(alsorefereedastoWoxi)hasbeengeologica1-

1y described(Hunan Metallurgic Geological

Party No.237,1978,unpublishe(i paperl Li,

1989)and genetically discusse(1with respect to

tmgsten,gold mineralization and antimony

mineralization(Liu6!召1,19831Bao,1988;Wu

6砲1.,19891Shi6短1.,1993).Mineralogicalwork on ore,geochemical studies of alteration

and fluid inclusions as well as sulfur,oxygen,

and hydrogen isotopic studies have been done

since1984 (Luoαα1.,19841Bao an(i He,

19911Wang,19931Zhang,19851Liang andZhang,1986).Three kinds of genetic models

have arisen from these earlier studies:1)Epith-

ermal deposit originating from magmatic fluid

(Hunan Metallurgical Geological Party No.

237,1978,an unpublished paperl Yang,1986).

Yang proposed that mineralization might take

place during the Later Yanshanian acidic

magmatism.2) Sedimentary-metamorphicmodels.These consist of a number of widely

accepted hypotheses which assume metamor-

phic solutions are the main component of ore

一578一

G6藺♂6解0461げgol4一伽渉吻0矧一伽9S伽吻難醐1伽あ0%(伽6地1。)

forming fluids(Liu6渉α1.,19831LuQαα1.,

1984).}Iowever,no conclusive model hasbeen

proposed because of divergences on the ore

forming Process. Sedimentation an(1/or

metamorphism are consi(1ered to be significant

in the ore forming process.Therefore,the

deposit is referred to as either sedimentary

-metamorphose(i or sedimentary-metamor-phic deposit and metamorphic deposit(Xiao

and Li,19841Zhang,19851Bao,19881Li,19891

Bao an(1He,1991).3)Hydrothermal model

for fluid originating from circulating paleo

-ground-water(Shiα召1.,19931Shi and Hu,

1994)which proposed that antimony mineral-

ization took place within a“paleo-hydrother-

mal field”during the Mesozoic Era.

  The principle objectives of this paper are to

re-examine the geology and mineralization of

the Wuxi deposit on the basis of field and

microscopic observations of ores as well as

careful review of previous data,to discuss

relationships of gold,antimony and tungsten

mineralization and to develop a comprehensive

geologic-genetic model for the deposit.

  The specimens in this report were taken

from representative ore bodies of the deposit,

No.3and No.4veins,veinlets an(i ceiling or

floor veins of the No.3vein.Three galleries

were sampled(150m level above sea leve1,

一185m level below sea level and-210m leve1).

Underground examinations were also taken on

No.1and No.3veins at the-60m levelgallery.

2. Geo亘09童c就sett睡9

  The Xuefeng arcuate tectonic belt,tectonically being a part of Jiangnan Uplift of

Yangzi Platform,Southem China,is an anti-

clinorium consisting of Middle to Upper

Proterozoic metamorphic rocks (Lenjiaxi

group and Banxi group)mainly,UpPer Proter-

ozoic to Lower Paleozoic marine detrital and

carbonate rocks and Mesozoic continental

clastic rocks locally.The pre-Sinian rocks

were deposited in an active synclinal environ-

ment characterized by volcano-sedimentary

formation&nd were subsequently met&mor-phoSed to subgreenschist facies and deformed

during Xuefengian orogenesis (1ater Proter-

ozoic in age).The Sinian and Lower Paleozoic

sediments formed in a platform environment

and were deformed during the Caledonianperiod(earlier Paleozoic)(Hunan Bureau of

Geology and Mineral Resources,1988).This

belt was uplifted after the Ordovician Period

and underwent strong stmctura1(1eformation

during the Indosinian and Yanshanian periods

(Mesozoic)whichwerecharacterizedbyblockfaulting an(l thrusting and are responsible for

the development of Mesozoic continentalbasins.

  Composite interlayer-slip stmctures are a

significant tectonic feature of the Xuefeng

belt.Sun6渉σ1.(1993)proposed that there are

several kin(is of interlayer-slip and/or dipPing

-sliding structure systems within the Banxi

Group characterized by four-D deformation,

i.e.,decoupling一,detachment一,disharmonic

-and(16collement-stmctures.It was revealed

that development of regional fractures foll-

owed a sequence of interlayer-sliding faults

through cross-cutting sliding faults to overth-

rust faults,and fina11y to napPe structures.In

the eastem part of Xuefeng belt,overthrust

structures and/or napPe structures have been

examined in the UpPer Proterozoic mit and

timing of structural development to Jurassic or

Cretaceous periods is in(1icate(i by Proterozoic

rocks that structurally overly Mesozoic sedi-

ments of Jurassic and/or Cretaceous age(Liu,

1992). Interlayer-slip  structures in  the

Xuefeng belt formed during the Mesozic Era

served as the tectonic framework for the

mineralization in the study area.

3.Loc&亙G・eo亙ogya翻m亘醜era盈亘z罐o聖a意the

             W腿x且:D脚os亘t

 The mining area is composed of Proterozoic

metamorphic rocks and Cretaceous continen-

tal conglomerate(red sediments)(Fig.1).

The country rock is thick purplishred sericitic

slate of the Upper Proterozoic unit with a wide

distribution (Madiyi Formation of Banxi

Group).The Cretaceous red sediments occur

tothenorthoftheEWtrendingWuxifaultandare limited to Cretaceous basins.The Wuxi

一579一

B錫JJ6‘∫%(ゾ孟hεG¢olo9ゼ6αJ S%7∂召ly(ゾノゆα%,▽10♂.47,ハみo.』Zヱ

fault dips towards north at an angle of30。and

cuts the country rocks at a low angle(Fig.2).

There are two other faults occurring in the

eastem part of the district with dip directions

concordant with strata(Fig.2)and both these

and the Wuxi fault are interlayer faults.The

Wuxifaultwasconsideredtobeareversefault(Li,198gl Liu,1993),a positive fault (Shiε渉

6zl., 1993) and/or a strike-slip fault (Peng,

1991).The fracture zone of the Wuxi fault is

20to100meters wide(Liu,1993)and several

structure planes can be identified within the

fracture zone comprised of fault gouge,

m

400

200

 0

-200

一400

・600

v?で

     ヨ

\℃//v4

F

ムK

%洩娩

0  100m

一匡コs㎝dst・ne・fCretace・us

回        じ     quartz veln

黙slate・ftheUpPerPr・ter・z・icunit[至]面11

囚fault

Fig.

回     exploi樋ng opening

2No.4geological section of the Wuxi deposit,

 Hunan,China,showing the,bedding-parallel

 qua「tz veins occurring in interlayer faults as.

 sociated with the Wuxi fault.

granulate(1slate,mylonite,and breccia which

indicate a long lived-history.At least,two

stages of fault、activity with differentrheological behaviors can be diStinguished,

ductile fracture stage an(i brittle fracture

stage.The ductile stage may be earlier than

the brittle stage and represents a dip-slip:fault

movement while the brittle stage may be as・

sociated with strike-slip movement.The dex・

tral strike-slip nature of the Wuxi fault is

clearly indicated by the occurrence of a set of

small northeast striking folds adjacent to the

fault which formed by fault drag.

 The ore controlling structure in the area is

interlayer faults/(1etachment faults contacting

theWuxiatalowangle.Sixfaultsareindicat・ed by the filling of quartz veins(Figs.1an(i

2).These faults belong to secondary faults of

the Wuxi fault and are gently dipping and

generally concordant with the horizontal bed・

ding of slate.

  IntheWuxi district,theMiddleProterozoic

mit appears as E-W trending folds,and the

UpPer Proterozoic unit shows a“sigmoidal

type”deformation which resulted from over・

printing of northeast trending folds on the E

-W trending folds.The northeast trending

fo1(1s are obviously limite(1to the Upper

Proterozoic unit an(i are apParently relate(1to

the detachment faults includingboththeWuxifault and eastern faults in the district(Fig.1).

Small sliding faUlts adjacent to quartz veins

Table l Timing constraints on geological evolution at the Wuxi deposit,China

T Cons柱aint Evidence

E-W fblds

Wuxi fault

O山er reve蔦c fau1お

N-E fblds

Qua霞zveins

N-S fb豆ds

』e oldest structuτe in the mining aτea

Predate or synchronous with the Ca置edoni{m period

Predates the Cretaceous,pos芝dates the metamo【phism which

took place during Xuefengian period(Later Proterozoic)

PostdatctheE-W fold andpostdaしe orsynchronous widl

the Wuxi fault

Postda重eE-Wfbld

Synchronous with the Wuxi fauk

Postdate orsynchronous with the Wuxi fault

Postdate or synchronous wi山the NE folding

Foロned dudng the Jurassic PedQd

Multiplestagesoffo㎜atlon

Postdatc山eCre訟ceouspedod

In metamorphic f㏄ks

Concordant with the regional st皿cture line ofbasemcnt r㏄ks

Does not cut through the Cretaceous basin,cuts the metamorphic rocks

CutE-Wfolding dlrection,does notcutthe Wuxl fauk

Limited wkh量n Upper Proterozo正c Unit

Overprlnt on theE-W爺01ds

Limited to within the Uppcr Pro監erozoic Unit

Occurimhe footwall oftheWuxi faultand mled along interlayer

fauks ass㏄iated the Wuxi fauk

The quartz veins were de∬o㎜ed Imd appear in NE-trending folds

            *Rb-Sr isochron age ls144.8±12Ma

Composite vein in which seven phases can be distingu玉shed

Occur both in Cret且ceous and UpPer Proterozoic units of

the han in wall ofthe Wuxi fault

*Shi ct al.,1993

一580一

06剛」6解0461げgol4一伽オ初0ηy一擁9S伽禰%徽1如オ加(翫6地」.)

and small sliding folds inclu(1ing folded thin

veins(pre-ore phase)can also be examined

underground.The Cretaceous strata apPearswithin small NS-trending foldsi

  The timing of events within the geological

evolution of the Wuxi district are shown in

Table1.In spite of the long geological evolu-

tion from middle Mesozoic to Cenozoic Era,

the Mesozoic Era is specifically emphasized

because ore-controlling faults formed in the

Jurassic perio(i as indicated by Rb-Sr dating of

quartz samples from ores(Shiα召1.,1993).

Although it is not clear when the Wuxi fault

was initiated,spatial relations within the

Cretaceous basin implies that the Mesozoic

Era isthesignificantactiveperiodoftheWuxifault.Two stages of the Wuxi fault maycorrespond to regional tectonic backgroun(i of

foreland nappe structure environment in the

In(10sinian (Triassic period) an(1 interior

extension environment during the Yanshanian

period(from Jurassic to Cretaceous in age)

(Sun6!α1,1991).

  Quartz veins at the Wuxi deposit structur-

ally occur in interlayer faults and are bedding

-parallel and concordant with host slates.Six

quartz veins with variable extension of100m

to5300m have been found at the surface and

more concealed quartz veins were revealed

during undergroun(1exploitation.These veins

are sub-parallel to each other with a spacing of

5-80m.Thus,they are also termed“beddedveins”by local geologists.

 The bedding-parallel veins at Wuxi are a

kind of composite vein system with a mean

thickness of O.4to1.4m comprising several

vein phases.Seven phases can be distinguished

in the composite quartz veinsbased onrecogni-

tion of structural features,tempora1-spatial

relationships,vein fabric,mineral associa-

tions and mineralizing character(Table2).

The pre-ore phase (phase 1)and post-ore

phase(phase7)are characterized by transpar-

ety quartz which is different from milky or

light gray quartz from vein phases2to4.

Quartz from vein phases5and6is character-

ized by transparent euhedral crystals.The

post-ore phase is also characterize(1by the

occurrence of aragonite (Luo αα1.,1984).

Later vein phases often overprint earlier ones.

The main vein phase(phase4)erased most of

previous phases and was overprinted by later

Table2 Schematic table showing various phases of composite quartz vein through time,

Wuxi deposit,Hman,China

vein hase 丘nle    n並nerahzadon Evidence

1

2

3

4

5

6

7

P爬一〇爬qua畝carbonate vein

scheelite-beadng quar惚vein

wolfどamite-bearing qualtz vein

main quarしz vein phase

pyrite(一quar切?)vein

stibnite-qu韻zvein

ost-ore  ua血一carbonatc vein

㎝ly

1ater

none

WW?

Au

Au.Sb

none

Calcite-quartz vein with small s1呈ding folds、

Scheelite occurs as re旦icしsin quartz vein andcutby laτerphases・

Occurs as wolframite qualtz veins and fissure f重11ing of woif㎞埴te crystals,

Host lkhology of gold and antimony mineralization.

Banded ore fo㎝edby pydte(一qua鴬z)行11ing行ssures in ve五ns.

Stibnite fiUs in fissures ofquartz veins showing banded smlcmre or ponded ores。

Calcite-ara onite- uar吃vein.

Table3 A summary of ore bodies at the Wuxi deposit

(simplified after Wu et a1,1989and an unpublished exploration

report,1978)

vein  length of

No.vein/alteration

      (m)

      economlc ore body

numbers striking dipPing

       (m)  (m)

        averagegrade

thic㎞ess Au Sb W(m)   m (wt.%)(wt.%)

1234

5300

650

1500

100

13

264

100-320

40-70

30-290

50-350

180-2010

360-720

550-1975

500-2280

1.35

0.42

0.79

1.01

5,67

6.41

10.33

10.13

1.34

4.26

3.91

5.55

0.22

0.55

0.24

0.76

一581一

Z3z6116渉」%(ゾ渉h6G60109ぎαzl S%7∂6ツ(~プノ4ゆα%,yiol.47,〈b.1ヱ

phases characterize(i by fissure filling o:f su1-

fi(ies in quartz veins.Among thesevenphases,

only:four phases are found to be responsible

for mineralization corresponding to tmgsten

and gold-antimony mineralization.The occur-

rence of multiple phases implies that the for-

mation of quartz veins is closely related to

periodic structural dilations。

  Economic orebodies have been fomd in four

quartz veins.Table3gives a summary ofeconomic orebodies.No.1vein has the lon-

9est interrupted exposure of5300m at the

surface and No. 4 vein only exhibits an

approximately100m long alteration zone/belt

at the surface.However,the No.4vein is the

leading vein of the deposit.The economic ore

bodies usually appear as bedded column shapes

which are about50to350m long in strike,0.l

to3m thick and300to1700m long in dip,and

are strongly structurally controlled.Such bed-

ded ore bo(iies occur in the axial parts of

northeast-trending  anticlines or synclines.

Twenty three economic portions have beendiscovered within the four mineralized quartz

veins.Along the strike of interlayer faults,

1enticular orebodies and alteration zones are

observe(1alternatively.

  Three kinds of ores can be distinguishe(1at

the Wuxi deposit.The first is so called“bed-

ded vein ore”which is mineralized interlayer

quartz vein concordant with the host strata

and is predominant in the mine.The average

grade is8.38ppm Au,4.36wt.%Sb and O.37

wt.%WO3.Thesecondisveinlet and/orjointvein ore which is composed of a set of high

angle (68-70。) thread veins adjacent to the

bedded vein.The veinlet usually is1-4cm

thick and20-30m long(Wang and Yang,1990).The grade of veinlet ore varies from

O.03to2.43wt.%WandfromO.67to15.OppmAu.No antimony mineralization has beenreported in the veinlet ore.The third,is stock-

work ore which is associated with the main

vein and shows variable ore body shapes in-

cluding bed,1enticle,wedge,and po(1.The

mean grade is4ppm Au,1wt.%Sb and O.16

wt.%W.The veinlet and stockwork ores

were considered to be synchronous and have

the same genesis with the bedded vein type

ores(Huang and Cheng,pers.commun.).It

is proposed that the(1evelopment of bedded

veins could be closely related to the develoP-

ment of interformational detachments andveinlets as well as veinlets may be related to

interlayer shear.

  The ore is composed of native gold,

scheelite,stibnite,pyrite,wolframite mainly,

with subordinate arsenopyrite, sphalerite,

galena and chalcopyrite.Quartz is pre(iomi-

nant as a gangue mineral but calcite,ferroan・

dolomite,chlorite,illite,pyrophyllite,andkaolinite are also present.The signature alter・

ation of the deposit is decolourization of pur・

pli$h-red slate into gray yellow or light yellow

slate in which pyritization,muscovitization,

sericitic alteration, an(1 silicification can be

observed. Chloritization apPears as chlorite

veinlets in slates which forecasts thiming out

of ore bodies.Alteration zoning from quartz

veins adjacent to the ore body to those distant

from the ore and barren systematically pro-

gresses from the muscovite-pyrite zonethrough the silicification and sericitic altertion

zones to sericitic and/or chloritization zones

and finally to fresh slate in which iron or

titanium oxides remains(Shao6渉α1.,1989).

4.Re亘罐0薦踊夢soft懸gste聾,90且岨鍛面繭・          mo醜ym醜r誰Z&t童0期

 Figure3is a geological sketch of quartz vein

No.1showing the typical banding texture of a

composite quartz vein consisting of scheelite

relicts enveloped in quartz,pyrite bands filling

horizontal fissures in the vein and stibnite

bands occurring at the margin of the quartz

vein.Phases2,4,5an(i6can be observed in

Figure3.Mineralization at Wuxi deposit is

spatially composite and temporally separated.

Here we describe tungsten,go1(1and antimony

mineralization respectively on field evidence

and mineral paragenesis.

  4.l T眠囎gste聾m量聾era亙izatio魍

  The tungsten mineralization at the Wuxi

deposit comprises two phasesl scheelite quartz

veins an(1wolframite quartz veins an(1both are

mainly concentrated in the shallow part of the

deposit with ore grade decreasing(10wnwards.

一582一

G召%α乞(7窺04ε1(ゾgol4一αηあ彫0%y一孟%ηgS哲ε%伽ぎ%(3名α」澹α孟乞0% (π%εご(Z♂.)

Fig.3Underground geologic sketch showing banding texture of com-

 posite quartz vein consisting of

 scheelite relicts (sch, phase 2),

 main quartz vein phase(Q,phase

 4), pyrite-bands (py, phase 5)

 and stibnite-bands(stib,phase6)

  (No。1vein,一60m level,westem

 gallery,scale bar二1cm).

Along the strike of the quartz veinfrom eastto

west,scheelite concentration decreases and

wolframite increases.

  The scheehte mineralization(phase2)isobviously earlier than the wolframite-bearing

quartz veins (vein phase3).Details of the

mineral assemblage of the scheelite-quartz

phase is not assessable because scheelite gener・

ally occurs as relict islands within the veins.

Coarse grained scheelite appears white and

several centimeters in size exhibiting brecciat-

ed or rounded shapes (Figs. 3 and 4a).

Scheelite grains are enveloped in quartz aggre-

gate with later fissure filling of wolframite

(Figs.4a and4b),or cut by pyrite veins

(Fig.4b)and overp血ted by stibnite minera1-

ization(Fig.5).Corro(ie(i scheelite is included

in quartz with fissure filling of stibnite(Fig.6

a)or is cemented by stibnite(Fig.6b).The

scheelite quartz vein phase may represent the

earliest mineralization at the Wuxi deposit.

  Wolframite mineralization is well recorded

by wolframite-bearing quartz veins(phase3)

occurring in both composite veins(Fig.4c)

Qa

難 灘羅.磯

Fig.

W

y

’  f

 WQ 、

   /、

、〆

4Sketches of the tungsten mineralization at the Wuxi deposit(scale bar=1

 cm).a,tiny wolframite crystals(black)filled in fissures of a scheelite grain

  (sch)included in quartz(Q)(No.1vein,60m leve1);b,disseminated

 wolframite crystals(wo1)in scheelite grain(sch)replace(i by quartz(Q)and

 cut by pyrite veins(py)(No.1vein,No.9steep draft of60m level)l d,

 wolframite-bearing quartz veins(Wq,phase3)in slate cut by pyrlte veins

  (py,phase5)(WQ)inbrecciatedslate(No.3vein,一60mleve1)lf,plate(pw,

 4-8mm),needle(0.5-3mm)wolframite crystals and disseminated pyrite in

 quartz vein(Q)(No.3vein,west ga11ery of-60m leve1).

一583一

Bz6〃6あ7z(ゾ渉h6G召olo9歪αzl Sz6矧のノ(ゾノ4望りα%,Vlol.47,No.1ヱ

and host rocks(Fig.4d).Wolframite orenests can be foun(i in brecciate(i slate(Fig.4

e).Wolframite quartz veins are laterthanthe

scheelite phase (Fig.4b)and earlier than the

formation of pyrite veins(Figs.4b,4d).Two

generations of wolframite are indicatedby two

kinds ofwolframite crystals,plate-shaped and

needle-shaped,in the No.3vein(Fig.4f).

  Wolframite at the Wuxi deposit is nearly

pure ferberitewith MnO/FeOratio of O.05and

total Nb205十Ta205content of O.18wt.%(Luoαα1.,1984)).This composition is quite

different from the wolframite from the Daji-

shan Mine,Jiangxi,that has MnO/FeO ratio

ranging from1.2to3.2,total Nb2055十Ta205

content ranging from O.47to1.37wt.%,and

which likely originated from magmatichydrothermal solution(Shi and Hu,1988).

  4.2 A聾t量mo蟹ly m量聖er&且izat董o聾

  Stibnite is the only primary antimony min-

eral at the Wuxi deposit,which differs from

the Kharma gold-antimony deposit where aur-

ostibnite and Au-Sb oxides were reported(Di11

αα1,1995).Antimony mineralization at the

Wuxi deposit is marked by the formation of

Fig.

  .灘聾

麟翼懸、轡   鎌灘獺

羅       3cm

鍵誕一5Photograph of handspecimen showing texture

 ofcompositeorefromtheWuxideposit.Note antimony mineralization shows banding in the

 vein and scheelite is cut by stibnite-quartz

 veinnet.There are two quartz phases:one is

 main vein quartz(phase4)which replaced scheelite (phase2)and appears as residua in

 stibnite bandl the second is quartz veinnet

  (phase6)associated with stibnite which cuts

 both scheelite and main vein quartz(phase4)。

stibnite banding in composite quartz veins

(Fig.7a)and veinlet ore in slate (Fig.7b).

Massive stibnite orepods arenotmcommonincomposite quartz veins.Banded ore formedby

stibnite filling in bedding fractures of quartz

veins an(1massive ore/ore pods formed by

open-space filling by stibnite in brecciated

quartz veins suggests that brittle deformation

occurred after formation of the main quartz

veins and was possibly associated with anti-

mony mi耳eralization.Antimony mineraliza-tion is apparently later than the formation of

the main quartz vein(phase4).Quartz that is

paragenetic with stibnite forms small euhedral

crystals included in stibnite crystals or occurs

as micro-veins which cut previous phasequartz and scheelite (Fig. 5). It is thus

proposed that antimony mineralization at the

Wuxi is strongly controlled by fractures or

brecciation of quartz veins and is best char-

acterized as a fracture filling type mineraliza一

麟箋

鰻 響

Fig. 6Photomicrographs  showing  etched scheelite(sch)in quartz aggregation(Q)

  (a)and cemented by stibnite(stib)(b).

 Scale bar=100 micrometer,reflected

 light.

一584一

G6紹づ0吻0461げgoJ4一伽オ加0御一伽9S伽痂%醐π1乞鰯加(伽召地」.)

Fig. 7a,Photograph showing ban(1ed stibnite

 ore formed by stibnite(black)(phase6)

 filling in fissured quartz (white)vein

 (phase 4)l b, Stibnite veinlet filling in

 fissures of slate (adjacent No. 3 vein,

 west gallery of-60m leve1).

tion.

 4.3  Go亘dl m豊醜era1童zatio1董

  Native go1(1is the only gold mineral at the

Wuxi deposit with pyrite and stibnite serving

as the main host minerals of gold(Li,19891

Bao and He,1991).Native gold grains in our

sPecimens range from l to300micrometers.Several kinds o:f gold occurrence are distin-

guished on the basis of relations with its host

minera1.Such occurrence records parts of the

history of gold precipitation. Native gol(i

occurrences are summarized as follows.

  (1)Gold grains disseminated in quartz

aggregates:such gold grains are small with

irregular shapes,usually1-5micrometers.The gold may be synchronous or post(1ate the

precipitation of(luartz(Fig.8a).

  (2) Gold grains closely related to pyrite:

three types of pyrite can be distinguishe(1.The

first is coarse cubic pyrite showing apparent

etch pits.The second is fine cubic and octahe-

dral pyrite showing ban(1ed distribution in

quartz veins.Both can occur within one speci-

men from quartz veins.However,the finegrained pyrite is obviously later than coarse

grained pyrite.The formation of fine pyrite

may result from hydrothermal solution filling

because both banded pyrite fille(1fissures in

quartz veins (phase5) and pure pyrite veins/

veinlet in slate are present(Figs.9a,9b).The

former formed by the overprinting of pyrite

-quartz phase on brecciated or fissured quartz

veins and the latter formed by pyrite filling in

brecciated or fissured slate.Pyrite veinswhich

cut wolframite-bearing quartz veins indicate

that they post(iate the tungsten mineraliza-

tion.

  Gold grains show close relations with both

coarse and fine pyrites.In the case of coarse

pyrite,gold grains fill in etch pits of pyrite

and/or are enveloped by stibnite(Figs.8c and

8e),which indicates that gold postdates the

pyrite and predates or is synchronous with

stibnite.Withinfinepyritebandsofthequartzvein,fine irregular gold grains (less than 5

micrometers)are fomd to be paragenetic with

pyrite.In this case,gold apParently precipitat-

ed from hydrothermal solution synchronously

with the pyrite(Fig.8d).

  The third type of pyrite is paragenetic with

stibnite.We observe that gold precipitated on

the surface of pyrite(Fig.8f)an(1both gold

and pyrite are included in stibnite crystals,

which indicates that gold postdates the pyrite.

Therefore,gold mineralization related to

pyriteattheWuxi depositismainlyassociatedwith fine grained pyrite which may be a

hydrothermal product and later than the main

quartz vein phase.Coarse pyrite serves as an

important goldhost mineral but goldprecipita-

tion is later than formation of the pyrite.

  (3)Gold grains associated with stibnite:

stibnite is another significant gold-bearing

mineral in which native gold occurs.Three

kinds of gold occurrences can be(iistinguished

on the basis of relations with stibnite:pre

-stibnite,syn-stibnite an(i post-stibnite gold

gralns.

  The pre-stibnite gold grains are wholly or

一585一

βz6〃6ガ盟0ブ孟h6060Jo9づ041S%7∂の26ゾノめり4銘,y∂1.47,ノ▽o。11

Fig.8Photomicrographs  showing    native gold occurrences at the

    Wuxi deposit.Scale bar=25    micrometers,reflected light.a,

    gold grain in quartz aggrega-

    tionl b,a euhedral gold crystal

    (P) occurring in quartz crystall

    c,gold filled in etched pits of

    coarse pyrite;d,gold grain oc-

    curring in fine graine(l pyrite

    band;e,gold filling in etche(i

    pits of pyrite and enveloped by

    stibnite; d, gold  grain

    precipitated on pyrite surface

    included in a stibnite crystal;9,

    round gol(i grain enveloped in

    stibnitel h, round gold grain

    partly  enveloped  in  stibnite

    crystall i, go1(i occurring in

    crystal boundary of stibnite;j,

    go1(i grain included in stibnite

    crystal an(i fille(i in fissure of

    the crystal。

。膨

一586一

G召襯露窺0461げgol4一雌渉加0%y一齢9S擁勉枷観1如云加(伽αα1。)

partly  enveloped  in  stibnite crystals an(i

exhibit rounded shapes ranging from several to

about100micrometers(Figs.8g an(18h).The

gold filled etch pits of pyrite also predates

stibnite because both pyrite and gold are in-

cluded by stibnite(Fig.8e).The syn-stibnite

gold is indicated by micro-veins consisting of

stibnite and nαtive gold filled fissures in quartz

(Figs.9a,9b and10).The dendritic native

gold occurs in the open space of the fissure.

Gold.grains filling in fissures are apparently

larger than those included in pyrite,stibnite

CryStalS Or(iiSSeminated in quartz aggregateS.

Native gold fissures from the Wuxi deposit

which can be observed by unaided eyes were

found during the mining.Post-stibnite gold

precipitation is indicated by intercrystal gold

grains an(i/or gold grains filling in fissures

within stibnite crystal(Figs.8i and8j).

  In a euhedral quartz crystal of phase 6

(stibnite-quartz vein phase), a native gold

grain exhibiting a hexoctahedral crossection

typical of euhedral crystals with a dia血eter of

about12micrometers(Fig.8b)was observed.

  Natlve gold occurrences mentioned above

indicates that gold mineralization attheWuxi

deposit started at vein phase5and lasted until

the end of the phase6 and is later than the

main vein phase(phase4)consistent withgeological evidence indicating that the gold ore

bo(1y is control16d by secon(iary folds・super-

impose(i on composite quartz veins.In addi-

tion to a Ionger lasting Period of gold precipita-

tion than that of stibnite,gold and stibnite

belong to products of the same mineralization

stage.

  Therefore,mineralization at the Wuxi(ieposit can be divided into two stages oftung-

sten mineralization stage(1)and gold-anti-

mony mineralization stage(II).The former

can be divided into two substages correspond-

ing the formation of scheelite-quartz veins

(substage Ia)and wolframite quartz veins

(substage Ib).Gold and antimony mineraliza-

tion are later than tungsten mineralization.

Stage I corresponds to vein phases2and3,

stage II to vein phases5and6.

5. 1)亘se1腿ssio髄

Fig.9 a,Sketch of pyrite veinnet (py) in

 slate (S) (in host rock adjacent to

 No.1vein,west gallery of-60m level), scale bar=1cm; b, Photo-

 graph of banding quartz vein formed

 by pyrite filling in fissures of quartz

 vein.

 5.1De脚s篤tyPle The recognition of (1eposit type highly

depends on classification criteria adopted.For

the purpose of exploration and prospecting,

the Wuxi deposit was categorized as bedded

type hosted in detrital rocks based on the

classification of occurrencemode andcompari-

son of various antimony mineralizations in

China(Hu6齢1.,1996).The Wuxi depositwas also referre(1to as a stratabound gold,

tmgsten or antimony(1eposit respectively(Liu

6渉α1.,19831Bao,1988,1990).However,

what“stratabound deposit”means here isdifferent from its original concept which is

widely accepted. For example, tungstenmineralizationsinAustriaandWestGreenland

一587一

B%IJ(訪勿z(~プ云h6G60109づαzl Sz67%ly(~プノ砂召銘,1茜ol.47,ハb.ヱヱ

奄齢

Fig.10Photomicrographs(a and b)show-      ing native gold (light gray) and

      stibnite (gray) filled in fissures of

      quartz (black an(i dark gray)vein.

      Scale=50micrometers.

are hosted in calc-silicate rocks which under-

went amphibolite facies metamorphism,andscheelite mineralization formed prior to the

main metamorphism (Raith,19911Appe1,1994).AtWuxi,thecountryrocksurroundingthe mineralization is lithologically homogene-

ous calcareous-slate,mineralization is strong-

1y structurally controlle(1 rather than lith-

010gically controlled.Geological characteris-

tics of the Wuxi deposit described earlier sug-

gestitshouldcategorizedasacompositeW-Au-Sb concordant bedding-parallel vein type

mineralization.The vein type tungsten miner-

alization(stage Ia and Ib)and composite gold

-antimony mineralization in fractured or brec-

ciated bedding-parallel veins and country

rocks are spatially coupled.A summary of the

detailed deposit features is as follows:

  1)Mineralization occurs in interlayer

faults under a regional background of detach-

ment StmCtUreSl  2)Quartz veins are concordant with bed.

ding horizons of the country rock,occurring

as interlayer-fault filling and consisting multi・

Ple phaseSl

  3)Tmgsten mineralization is of vein type

consisting of scheelite-bearing quartz Fveins

and wolframite quartz veins;

  4)Gold and antimony mineralizations are

characterized by filling of fractured or brec・

ciated bedded veins as well as slate and are

later than tungsten mineralization and post・

date the formation of the main quartz veins.

  The Wuxi deposit is a representative spa・

tially-coupled vein type tungsten mineraliza・

tion,and fracture filling composite gold-anti・

mony mineralization.Both of these are signifi-

cant deposit types in the Xuefeng belt.

 5。2 H圃ro出erma置history&t the W麗      岨e夢osit

  The hydrothermal history at the Wuxideposit was well recorded by composite quartz

veins which resultedfrom periodic discharge of

deep hydrothermal flui(1s.Hydrothermal dis・

charge at Wuxi might have been structurally

initiated and stimulated.Detachment faults(interlayer faults)as we11as later brittle frac・

tures in quartz veins in the Wuxi area served

as dilation structures delivering open space for

hydrothermal mineral deposition.Periodicstructural mobilizations of interlayer(1etach・

ment faults is indicated by the occurrence of

multi-phase veins.

  Various mineralizations at Wuxi indicates

the chemica1ρhanges of discharging fluids

through ore mineral paragenesis.Two miner-

alization stages responsible for tungstenminer.

alization(1)and gold-antimony mineraliza-

tion(II)may have resulted from two kinds of

hydrothermal fluids because transportationand(ieposition conditions of tungsten is differ-

ent from that of gold and antimony.

  Experimental studies in chloride bearing

solutions(Forst,19771Wood and Vlas-sopoulos,1989;Wood,1992)indicate thechemical species H2WO4(aq)is responsible

forWtransportinhydrothermalsolutionsaccording to:

   WO3十H20(1)=H2WO4(aq)    (1)

一588一

G6吻ぎ0彫0461げgol4一観渉吻0%γ伽9S吻吻盟醐」如オ加(磁6勧1.)

a

Solubility of tungsten mineral as a simple

oxyacid species can attain values between10

to100ppm under geologically reasonable con-

ditions without any complexation with a(1di-

tional ligands(Wood,1992).Scheelite deposi.

tion can be expresse(i by the reaction:

   H2WO4(aq)十Ca2+

     二CaWO4(scheelite)十H+     (2)

These reactions(1emonstrate that scheelite

dissolutionandWtransporttakeplaceinacidic solution and scheelite precipitation is

caused by increasing of pH value (neutraliza-

tion of solution).Experimental data(Wood,

1992)(Fig.11a)showadramaticdropofWconcentration with increases in pH value from

3to5.Such neutralization of fluids can arise

from interaction with CaCO3in host rocks.

  Tungsten solubility is also strongly affected

by temperature.Scheelite solubility according

to reaction(2)is prograde from300to400。C,

but is thereafter retrograde from400to6000C

(Wood,1992).According to Wood(1992),

scheelite precipitation takes place at a block・

ing temperature between300to400。C duringcooling of hydrothermal solutions.It is there-

fore proposed that hydrothermal solutionsresponsible for scheelite mineralization(phase

2)attheWuxi depositmayhaveresultedfromacidic  aqueous fluids containing  tungsten

oxyacid and scheelite deposition most likely

took place through cooling and interaction

with calcareous slate。

  The formation of ferberite-quartz veins indi-

cates the existence of Fe2+cation in solution

because Mn/Fe ratio of“source material”is

the most important factor controlling wo1-

framite composition (Hsu,1976).Chlo血ecomplexes may be the possible species of iron

in solution.Theoretical calculation demon-

strates that wolframite can precipitate in

aci(iic reduced solutions rich in calcium and

iron(Zhang,1989).According to the replace-

ment reaction between wolframite andscheelite=

   FeWO4(wolframite)十CaCO3     =CaWO4(scheelite)十CO2十Fe2+  (3)

o溜窪(

『冒目

8鼠琴)リ

120

100

80

60

40

20

O

一{》一300。C

一醤一400。C

+500。C+600。C

2 3 4pH

5 6

ハ>ロ調国

0。5

0

一〇.5

一1.0

    お  駕30\。    ゆ         \

Fe2+    \         Fe203◆\

£erberite難\。    9・

※.

\。 頃

寺薗。

      ♂

b

\。

scheelite

’\   Fe304

0 5pH

10 14

一4

£一80bDo一

一12

liquid sulfur C

pyrite

pynrhotite 栖拶o直℃o

Fig.

一44 一40 一36   -32

10g ao2

一28

11 a,Scheelite solubility vs.pH for reac-

  tion of CaWO4十2H+=Ca2+十H2WO4  (aq)l b,Eh-pH diagram showing stabi1-

  ity fields of scheelite and ferberite in

  aqueous solution at300。C (simplified

  after Schrδcke at a1.,1984);c,fs2vs.

  fo2diagram showing the possible condis-

  ions of mineralization stage I and the fs2

  『increase from stage I to stage IL

一589一

Bz6116蕗%ρプ汝h6(}60109づαzl Sz67∂6夕(ゾノのごzn,Vlol.47,〈b.1ヱ

  Ferberite is stable un(1er acidic and a high

Pco2conditions which differs from the alkaline

environment responsible for scheelite deposi-

tion.Accor(1ing to Schr6cke6!o:1.(1984),

stability fields of scheelite and ferberite in Eh

-pH diagrams of Fe-W-0-OH and Fe-Ca-W

-0-OH systems are distinguished largely by

pH value(Fig.11b).Hsu少s(1ata(1976,1977)

demonstrate that scheelite and ferberite have

similar wide stability fields of oxygen and

sulfur fugacities,which range from adjacent

magnetite/pyrrhotite-hematite/pyrite bound-

ary in fo2-fs2diagram to high fo2conditions.

The paragenetic pyrite and lack of hematite at

Wuxi place constraints on a narrow field

below the hematite/pyrite bomdary in fo2-fs2

diagram for formation of ferberite quartzveins (Fig.11c).

  Therefore,ferberite precipitation(phase3)

most likely resulted from decrease in pH(aci(iation of solution)in contrast to the neu-

tralization process responsible for scheelite

deposition.Such acidation seems mlikely to

be the result of in situ fluid-rock interaction

because the host rock is calcareous slate and

ferberite-quartz veins appear as a later phase

overprintingthescheelite-quartzphase.Aprobable explanation for acidation of ore-for-

ming fluid is the chemical changes of the dis-

charging solution originating from a deeper

variety of parent solution,such as the increase

of fluid fs2which is indicatedby the occurrence

of paragenetic pyrite.

  Gold mineralization at the deposit started at

pyrite-quartz phase(phase5)which predates

antimony mineralization.Within fine grained

pyrite ban(1s, rare stibnite can be observed

whic虹implies that the flui(1responsible for this

phase was too low in antimony concentration

to precipitate abundant stibnite.However,

gold an(1antimony mineralization are closely

related to each other in the phase6which is

clearly indicated by the paragenetic native

gold and stibnite.The correlation coefficient

between gold and antimony grades of No.4

vein is O.79,that of go1(i and antimony con-

tents in altered slates is O.67 (Liu α σ1.,

1983).Experimental studies of Wood6地1.(1987),KrupP (1988),and Hu (1994)provide

a comprehensive understanding of go1(1and

stibnite solubility.According to these studies,

thioantimonite complexes (H2Sb2S40,HSb2S4一, Sb2S42一),hydroxothioantimonite

complex(Sb2S2(OH)20and/or hydroxy com-

plex are the dominant chemical species of

antimony in solutions.Gold and antimonyhave higher solubility in alkaline solutions than

in acidic solution.High fs2solution is required

for antimony transport.Gold can be co-tran-

sported as bisulfide complexes with hydroxy

antimony complexes in chloride-free aqueous

solution.Kolpakova and Manucharyants(1990)also proposed the existence of H2

AuSb30and HAuSbS3-in solutions.Tempera-ture and pH value are two significant factors

controlling antimony and gold solubility by

affecting chemical species in solution.Aci(1a-

tion and cooling are two important mecha・

nisms causing stibnite and gold deposition.

The aci(1ation of solution which most likely

resulted from decomplexation of antimony一,

gold-bisulfide complexes was clearly recorded

by etched scheelite cemented by later phase

quartz and stibnite(Figs.6a and6b)。

  There were obvious changes of pH and fS2

values of ore forming flui(1s between the

scheelite and stibnite phases of mineralization.

Based on the homogenization temperatures(Luo6れzl.,19841Liuαα1.,19911Shiαごzl.,

1993)and mineral assemblege in various vein

phases,we summarize a brief hydrothermalhistory of the Wuxi deposit in Figure l2.

Hydrothermal temperature decreases from the

earlier to later phases(Fig.12a).Cooling was

a fmdamental hydrothermal process inWuxi.

The pH and log as2values of solutions increase

from tungsten mineralization(stage I)to gold

-antimony mineralization(stage II)(Figs.11

c,12a and12b)assuming no significant change

in fo2.

  The chemical changes of the ore-forming

fluids at Wuxi are also indicated by trace

element contents of the ore-minerals.For

example,the auriferous pyrite and stibnite are

characterize(i by high arsenic concentrations.

The As content of pyrite increases obviously

from fresh slate(20ppm)to altered slate(2770

ppm)and to quartz veins(4600ppm)which is

一590一

0ω26あ6窺0461ρプgolご一召%あ〃30%ダー渉%ngS孟6n窺∫%6勉1勿αあon(撫6オα1.)

consistent with the increase of gold content of

pyrite from2.5to81.2and73。7ppm(Bao and

He,1991).This demonstrates that ore-form-

ing fluids responsible for Au-Sb mineraliza-

tions were enriched in arsenic concentrations.

The geochemical association of arsenic and

antimony in many hydrothermal deposits and

geothermal waters is well known.Such associ-

ation can be well understood based on the

similar stable conditions of arsenic sulfide

complexes and antimony sulfide complexeswithin the pH range of typical geochemical

systems(Spycher and Reed,1989).The pref-

erential accumulation ofvisible goldon arsenic

enriche(1pyrite was explained to be an electro-

chemically dominated process(M611er,19931Mδ11er and Kersten,1994).According to their

studies,arsenic is the most important element

Qoの

弩おお儀目o卜

350

250

150

50

         働磁

           。鳥

       剰ヤ隣 stageII

           ,   /

           蓼

           嗣

           ’ 匹      Cc・s・Lやc・l

             l l

a

         1

         /

        /

       /

      /

      /

     /

HCO3一’CO32一

     ‘

     亀

     、

     ¥

2 4  6

pH8 10

Fig. 12a,A brief hydrothermal history respon-

  sible for various phases of composite

  quartz vein (from pre-ore phase (1)

  through scheelite phase(2),wolframite

  phase (3), pyrite phase (5), st至bnite

  phase(6)to post-ore phase (7).The

  arrows represent chemical processes  (aci(iation and neutralization)resulting

  in ore mineral precipitation.The curves

  represent equal activities of aqueous and

  separate their regions of predominance,

  Cc=calcite,(After Bames,1979,Fig.

  8.5)l b, possible pH increase of ore

  -forming fluid from tmgsten mineraliza-

  tion stage(1)to gold-antimony mineral-

  ization stage(II)shown in the fo2vs,

  pH diagram at250。C(After Bames,  1979).

一28

一32

8bD-36

2

一40

一44

       ロ       l     b       コ

 HSO4榊i        量  SO42一、、、、 讐%

  、、        1

     、、   1      、、 1       、、

pyrite

H2S

 、  ¥  、、

    i

    i Hpy曲・tlte

    l    き     1

2  4  6  8 10       pH

一591一

B%∬6海%ρ∫渉hθGεo♂09∫oαl S%甥εツ(ゾノ碑)ごz7z,Vbl.47,No.■Z

in establishing p-type conductivity o:f Pyrite

and arsenopyrite,and Au accumulates at p

-type conductors in n-p junctions.

 5.3Mo翻董醜gofA腿,Sba醜dWmi髄e聡且一      融tio聾a窟W麟re亘ate嶺o geo亘ogica亘      evO1朋tio鵡

 Geological constraints on the timing of geo-

logical evolution and tungsten,gold and anti-

mony mineralization are given in Table l and

demonstrate that the Mesozoic Era is an

important period for development of ore

-controlling structures.Mineralization may

have taken place in the Jurassic period,un(1er

a detachment stmcture background corre-sponding to the regional extensional environ-

ment of the Yanshanian period(Sm6オα1.,

1991).Periodic development of bedding-para1-

lel fractures (interlayer faults)as well as

overprinting of sliding folds are important

elements of the structural model for the forma・

tion of composite bedding-parallel quartz

veins and composite mineralization.

  Timing of mineralization does not support

metamorphic models(レiu6齢1.,19831Luo6渉α1.,1984)which assume that mineralization

took place (1uring the Xuefengian orCale(10nian periods.Lack of spatial relation-

ships between mineralization and magmaticintrusion in the mining area,even in a larger

scale area,makes it difficult to develop the

hypothesis of magma devolatilization.Compared with tmgsten(1eposits and anti-

mony-tungsten deposits related to magmatic

activity (Chen 6∫ 6zl., 19851Chen and Sun,

19871Shi and Hu,1988),the composition of

wolframite and antimony mineral association

at the Wuxi deposit do not support the model

of fluid origin from magmatic intrusion.

  The results of our research suggest a model

for the origin of hydrothermal fluids involving

(1eep convection and chemical evolution of

paleo-ground water(Shi6オα1.,19931Shi and

Hu,1994).The penetration of water through

rock units in the brittle regime of the continen-

tal crust can extend to12-15km estimated on

the basis of the brittle-ductile transition at

temperatures of350to450。C (Nesbitt and

Muehlenbachs,1988).Fluid inclusion studies

(Luo 6!6zl.,1984;Shi6渉ごzl.,1993) indicate

that flui(1s responsible for the composite quartz

veins in Wuxi are low salinity aqueous solu-

tions.In contrast to the genetic model for Hg,

Sb and mesothermal Au (1eposits of the

Canadian Cordillera(Nesbitt and Muehlen-bachs,1988),we emphasize that paleo-ground

water may play a more important role than

meteoric water during the circulation of fluids

based on considerations o:f paleohy(irology and

mineralization timing,even though the fluid

was mostly likely a mixture of many kinds of

free water in rock units inclu(ling Paleo

-ground water(fossil water)and meteoric

water as well as others.The fluid chemistry

evolves through fluid-water interactions and

varies from different circulation systems.

  The Middle to Upper Proterozoic unit is

characterized by high go1(i,tungsten and anti-

mony concentrations and has been emphasized

to be the source of gold,tungsten and anti-

mony(Liu6渉σ1.,19831Luo,1990).Theaverage gold concentration of pre-Cambrain

systems varies from6.5to14.Oppb(Liuαα1.,1983)or from l.8to9.Oppb(Luo,1990).

Tungsten and antimony exhibit a similar pre-

liminary enrichment in the Proterozoic rocks,

especially in the volcanic rocks.It is possible

that the basement rock in Hunan serves as the

ultimate source of mineralizations and is of no

importance as a protore.An antimony enri-

ched geochemical block mainly composed of

the Middle to Upper Proterozoic unit and the

Lower Paleozoic group has been proposed to

be responsible for the high antimony minera1-

ization in Hunan(Hu,1995).The convecting

fluidsaquireSi,W,Au,SbandSwhichwereprecipiated later at Wuxi through interaction

with the Proterozoic rocks.

  The deep convection system of fluids is like-

1y initiated or promoted by structural deforma-

tion,especially the development of brittle

faults or the change of fault property from

ductile to brittle.The brittle fracture zone will

COmeCt VariOuS UndergrOUnd water SyStemSand allows for water discharge from one sys・

tem to another.Such discharge is mainlycontrolled by fluid pressure difference between

two systems.

  The downward-migrating water will be

一592一

Gε泌才0勉046ZげgoJ4一朋孟魏0%ダー伽8:3伽吻%名切」伽渉加(磁6地1.)

exposed to a larger rock surface area produc-

ing lower water/rock ratio and the rocks will

impose their chemical characteristics on the

fluids that flow through them by interactions.

In shallow circulation systems characterized

by high water/rock ratios,the fluids retain

geochimical characteristics that most closely

resemble those of the parent fluids.With

respect to low water/rock ratio circulation

systems at deeper levels, the fluids acquire

geochemical characteristics that resemble

thoseofthecontrollinglithologies.Afluidthat closely resembles a flui(1produced by

metamorphic devolatilization will be the end

result of downward-migrating process(Nes-

bitt and Muehlenbachs,1988).The water/

rock ratio estimated on oxygen isotope(1ata

also implies the possible depth of circulation

system and the geochemical characteristics of

the fluids.

  Mineralization may mainly relate to upward

discharge processes of deeper fluids which

accompany multiple physica1-chemicalchanges resulting in precipitation of hydrother-

mal minerals,e.g.,cooling,depressuriza-

tion,mixture of different fluids as well as

change in redox state.Hydrothermal minerals

record more evidence of discharge process

rather than pre-influx history.The ore-form-

ing fluids in the Wuxi deposit were character-

ized by high and variableδ180values.Various

vein phases exhibit an obvious decrease ofδ18

0value from stage1(13.6%)through phase2十3 (8.0-9.8%),phase 5十6 (4.3-7.2%) to

phase7(2.0%)(Luo6∫α1.,1984).However,

the mixture model of metamorphic fluid with

meterioc water proposed by Luo6緬1.(1984)

cannot account for the remarkable chemical

changes of ore-foming fluids which we discus-

sed earlier.According to our mode1,fluids

which circulated in different environments

with variable water/rock ratios must have

been involved in the formation of composite

quartz veins.At the begiming of hydrother-

mal activity,deeper solutions with higher tem-

peratures and geochemical characteristics con-

sistent with flow in low water/rock ratio sys.

tems were discharged at the deposit site with

little mixture with shallow water.Then sha11一

ower hydrothermal fluids with lower tempera-

tures were allowe(i to discharge or shallower

fluids were added to the deeper discharging

fluids.In the final stage,the discharging flui(1s

were mainly composed of solutions character-

ized by low temperatures and characteristics

t如ical of flow through high water/rock ratio

systems.Tungsten-bearing fluids may haveoriginated from deeper systems and gold-anti-

mony bearing fluids represent relatively shal1-

ower hydrothermal systems.

6. Co醜c亘腿s量o亙盈s

  IthasbeenshownthattheWuxi depositcanbe categorize(1as a bedding-parallel vein type

composite tungsten,gold-antimony minera1-

ization.Mineral associations of scheelite,

wolframite,native gold an(i stibnite arehosted

by Proterozoic slate and controlled by detach-

ment stmctures.Based on our re-examination

of the geology and mineralization,the follow-

ing can be concluded (propose(i)=

  1)Mineralization took place during the

Jurassic un(1er a structure backgroun(i of

detachment faultingl  2) Periodic dilation  of bed(1ing-parallel

fractures is responsible for formation of com-

posite quartz veins and composite mineraliza-

tions.Quartz veins can be divide(1into seven

phases an(1mineralization is(1ivided into two

stages;

  3)Tungsten mineralization and gold-anti-

mony mineralization resulte(1from two kinds

of fluids which are low salinity aqueous solu・

tions with variable chemical properties and

temperatures.Neutralization and acidation of

solutions are possible mechanismsfor scheelite

deposition and ferberite precipitation,respec-

tively.Gold and antimony share a similar

origin and were probably transported by the

same reduced solution characterized by high

fs2.Cooling and acidation might be the signifi-

cant mechanisms for stibnite and gold precipi-

tation.There are obvious changes in pH and

fs2values of the ore-forming fluids between

scheelite deposition and gold-antimony miner-

alization.The(1eposit is thus categorized as a

spatially coupled tungsten mineralization and

一593一

B%ll6あ7z(ゾ云h6G60109ぎ66zl Sz67”6夕6ゾノ吻戯z銘,レリ1.47,ハゐo.11

gold-antimony mineralizationl

  4)The Wuxi deposit can be genetic&11y

interpreted according to a model for the origin

of fluid involving deep convection,chemical

evolution  and  (1ischarge  of paleo-ground

water.Brittle faulting woul(1connect a(1eeper

hot fluid system and allow for discharge to a

shallower system.The mineralization mostlikely takes place at the discharge location.

Periodic stmctural activity allowed spatial

coupling of tungsten and gold-antimony miner-

alization at the Wuxi deposit.

Ack亙亙ow亘eδgme薦s:This paper was finished

duringX.H.’sstayattheGeologicalSurveyof Japan which was financially supported by

the Science and Technology Agency(STA)of

Japan Fellowship.The authors would like to

expresstheir sincere thanksto Prof.M.K.Shi at the Yichang Institute of Geology and

Mineral Resources,Chinese Academy of Geo-

10gical Science,for beneficial discussion,to

Mrsses Z.L.Zou,L.J.Luo and Y.H.Gao at

the Xiangxi Gold Mine,Hman,China,fortheirassistanceinthefield.Wearegratefulto

Dr.C.W.Mandeville for helpful commentsand English corrections to an earlier version of

this mamscript.

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    ment.〃ガn6鵤1∠)勿osJお,8,no.1,65-69(in

    Chinese with English abstract).

A謄Sb-W鉱化作用の成因モデル=中国湖南省沃渓鉱床の地質と

           鉱化作用に基づく考察

胡 雄偉・村尾 智・黄 緒蝉

要  旨

湖南省西部の雪峰弧形隆起帯内に位置する沃漢鉱床は中国を代表するAu-Sb-W鉱床である.沃漢は

欧州や南米のAu-Sb-W鉱床とは異なり地層面にほぽ平行な鉱脈群からなる.鉱脈の規漢は走行方向に

6km,傾斜方向に1kmで平均品位はAu8.38ppm,Sb4.36%,WO30.37%である.

鉱床の母岩はセリサイトを含み,紫がかった赤色を特徴とする石灰質頁岩で,板漢層群(中国での表

記は板漢群)に属する.鉱体の位置は地層面にほぽ平行な断層と東北方向に伸びる2次摺曲に規制され

る.断層はデコルマンの特徴を示す沃漢断層に低角で接する.2次摺曲は中生代に形成されたものであ

る.全体として鉱床はデタッチメント断層に強く規制されている.

 鉱化作用は中生代に起きており,断層の周期的な広がりによる熱水の周期的供給を示す石英の複合脈

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L.

G6%孟露窺0461げgol4一α%渉魏0%夕一!観8S吻吻銘徽」伽渉づ0%(∫動6地1.)

を特徴とする.複合脈のマクロストラクチャーから断層の運動は7回起こったと判断される.鉱化作用

は断層運動の2,3,5,6期に起きており,タングステンの時期は2,3期,金とアンチモニーは,

5,6期に対応する.ここでは鉱化作用に注目する視点から,タングステンの沈殿した時期をステージ

1と呼び,更に灰重石のla期と鉄マンガン重石のlb期に細分する.金一アンチモニー鉱化作用の方は5期

の黄鉄鉱一石英層と6期の輝安鉱一石英層からなるがこれをステージIIと呼ぶ.タングステンは恐らく酸

性溶液に起因し,中和によって灰重石が,pH減少によってフェルベライトが沈殿したと考えられる.金

とアンチモニーはややアルカリ性の溶液によって同時に運搬され冷却とpH低下で沈殿したと思われる.

すなわち本鉱床はタングステン鉱化作用と金一アンチモニー鉱化作用という質的に異なるもの同士が空

間的に接した結果と考えられる.

 筆者らの研究によると沃漢の流体は地下深部で循環していた古地下水が化学的に進化したものであ

る.鉱化作用の重要なメカニズムは深部から浅所への熱水の排出である.本地域に形成された断裂がそ

の通路である.湖南においては中生代の断裂は周期的に活動しているが,その周期性が沃漢地区の複合

脈形成の主因である.

(受付11996年7月23日;受理:1996年10月28日)

中英対照表

Banxi Group

Changsha

Fuzhuxi

HunanMayidi FormationMobin

WuxiXi’an

Xichong

Xuefeng arcuate uplifte(1belt

Zhazixi

板漢群

長沙

符竹漢

湖南

馬底蜂組

漠演

沃漢

西安

西沖

雪峰弧形隆起帯

渣津漢

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