Geo Chemical Environment Japan Veins

7/28/2019 Geo Chemical Environment Japan Veins

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Mineral. Deposita 22, 309-314 (1987) MINERALIUMDEPOSITA Springer -Verlag 1987

The Ag/Au ratio of native gold and electrum and the geochemicalenvironment of gold vein deposits in JapanN . S h i k a z o n o 1 a n d M . S h i m i z u 21 Geological Institute, Faculty o f Science, University of Tokyo, Tok yo 113, Japan2 Department of Petrology and Mineral Deposits, University Museum, University of Tokyo, Tok yo 113, Japan

Abstract . The chemical composit ion of native gold andelectrum from auriferous vein and gold-si lver vein de-posits in Japan has been analyzed and summarized. TheAg/A u rat ios of native gold and electrum from these twotypes of deposits are d istinct, i.e., 10-20 Ag at % (auri-ferous vein) and 3 0-70 Ag at % (gold-silver vein). Thermo-chemical calculat ions suggest that the Ag/Au rat io ofnative gold and electrum should decrease with increasingchlor ide concentrat ion and temperature. This is consistentwith analytical results of native gold and electrum andfluid inclusion studies. Based on the Ag conten t of nativegold and electrum, the Fe con tent of sphaler ite , and theest imated temperatures, i t is deduced that the sulfuractivity for auriferous vein-type systems was lower thanthat o f gold-silver vein-ty pe systems.

The two types of gold vein deposits occurr ing in Jap an are"auriferous veins in sedimentary terrain" and "gold-si lverveins in volcanic terrain" according to the classification byBoyle (1979, 1984). In this paper these deposits are calledauriferous vein and gold-silver vein deposits, respectively.There are distinct differences in the characteristics of thesetwo deposit types. For instance, the gold-silver vein de-posits occur mainly in Tert iary/Quaternary volcanic re-gions while the auriferous vein deposits occur in sedimen-tary terrain associated with Cretaceous felsic plutonicactivity or in regio nally meta mo rph ose d rocks (Fig. 1).Common opaque minerals f rom the gold-si lver veins areelectrum, argenitite, Ag sulfosalts (pyrargyrite, polybasite),sphalerite, pyrite, chalcopyrite, and galena. Native gold,electrum, pyrite, pyrrhotite, chalcopyrite, cubanite, spha-lerite, arsenopyrite, and tellurobismutite occur in theauriferous vein deposits. Sulfide minerals are generallyabundant in gold-si lver veins compared with the aur i-ferous veins. It is noteworthy that silver minerals areabundant in gold-si lver veins, whereas they are poor inauriferous veins. Ag/A u total production rat io o f gold-silver veins is generally greater than 10 (usually 10-20)and that of auriferous veins is less than 10. More detailedinformation on these deposits is available in Urashima(1974) and Shikazono (1986) for gold-silver vein andWatanabe (1936) and Nedachi (1974) for auriferous veindeposits.There is a large amo unt of analytical data available forelectrum from Japanese gold-silver vein deposits (e.g.,

Yam aok a an d Ned achi 1978; Shikazon o 1985 a). Also, thegeochemical environment (activity of 02 and $2, i.e., ao~and ass, pH, total dissolved sulfur concentrat ion, andtemperature) of these deposits has been est imated (Shika-zono 1974, 1977a, 1978; Hattor i 1975; Takeuchi a ndShikazono 1984). In contrast, few analytical data on na tivegold and electrum from auriferous vein deposits areavailable and the geochemical environment has not beenelucidated except for the Koho ku deposit (Nedachi 1974).The objectives of this paper are to (1) present analyti-cal data on native gold and electrum from representat iveauriferous vein deposits in Japan, (2) compare analyticaldata on the native gold and electrum from auriferous veinswith those from gold-silver veins, and (3) discuss thechemical features of ore fluids responsible for the auri-ferous vein deposits.Analytical data on sphalerite coexisting with nativegold and electrum is also reported since the chemicalcomposit ion of this m ineral is a very useful indicator ofthe environm ent of ore deposit ion.In the following sections, Au-Ag 'alloy containing lessthan 20 wt % Ag is called n ative gold and Au-Ag alloycontaining more than 20 wt % Ag is cal led electrum.

An a ly t ic a l p r o c e d u r e a n d r e s u l t sChem ical analysis of native gold, electrum, an d sph aleritefrom the aur iferous vein deposits in Japan was performedwith a Jeol 733 electron microprobe analyser at the OceanResearch I nstitute, University of Tokyo. The acceleratingvoltage was 25 kV, and the s tandar ds fo r analyses werepure gold metal (for Au), pure silver metal (Ag), naturalchalcopyrite with known composit ion (Cu) , synthetic Zn06Fe04 S (Zn, Fe, and S), synthetic CdS (Cd), synthetic MnS(Mn), and synthetic CuInS2 (In). The characteristic X-rayintensities for each point were measured twice for a fixedinterval of five seconds; the averaged values were cor-rected for dead t ime an d b ackground. Quanti ta t ive correc-t ions were made for a tomic number and absorption andfluorescence effects, based on the m eth od o f Swea tmanand Long (1969).Locations of the thir teen aur iferous vein deposits stud-ied are shown in Fig. 1. Auriferous vein deposits can bedivided into two types on the basis of their host rocks, i.e.,deposits occurr ing in sedimentary and volcanic terrainsassociated with felsic plutonic activity of Cretaceous age


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'~ ~ 133 138Fig. l. Map o f Japan showing the locations of auriferous vein de-posits studied. 1, Gree n tu ff and subaerial volcanic regions o f Ter-tiary/Quaternary age; 2, main Paleozoic/Mesozoic sedimentaryterranes; 3, main m etamorphic terranes. TTL , Tanakura TectonicLine; M TL, Median Tectonic Line; ISTL, Itoigawa-Shizuoka Tec-tonic Line

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0 2 4 6 8 10 12 14 16 18F e S m o t e % o f s p h a l e r i te

Fig. 3. Frequency (nu mb er of analysis) histogram o f FeS content(mol %) of sphalerite from auriferous vein dep osits in Japan. Da taare from the present investigation an d N edachi (1974), and Yama-oka (1981) for auriferous vein depo sits and Shikazono (1974,1977b), Sug aki eta l. (1982, 1984), Soeda and Watanabe (1981),and Taguchi and Hirowatari (1981) for gold-silver vein deposits

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GOLD-SILVER VEINaf te r Sh ikazo no (1981)

( type 1 ) and depos i t s o ccur r ing in r ocks a f f ec ted by r e -g i o n a l m e t a m o r p h i s m ( t y p e 2 ). G o l d - s i l v e r v e i n d e p o s i t so c c u r i n r e g i o n s o f T e r t i a r y / Q u a t e r n a r y v o l c a n i c a c t i v it y .T h e A g a t % o f n a t i v e g o l d a n d e l e c t r u m i s s u m -m a r ized in F ig . 2 . Thr e e po in t s a r e ev ide n t ( F ig . 2) , i. e. , ( 1 )t h e A g a t % i n n a t i v e g o l d a n d e l e c t r u m f r o m a u r i f e r o u sv e i n d e p o s i ts is lo w a n d h a s a n a r r o w r a n g e o f 5 - 2 0 , ( 2 )t h e A g a t % fo r t y p e -1 i s l o w e r t h a n t h a t f o r t y p e - 2a u r i f e r o u s v ei n s , a n d ( 3 ) t h e c o m p o s i t i o n o f n a t i v e g o l da n d e l e c t r u m f r o m g o l d - s i l v e r v e i n d e p o s i t s r a n g e s 3 0 - 7 0a t % A g , a n d i s c le a r l y h i g h e r t h a n f o r t h e a u r i f e r o u s v e i ndepos i t s .S p h a l e r it e c o e x i s t in g w i t h n a t i v e g o l d a n d e l e c t r u mw a s s e l e c t e d f o r e l e c t r o n m i c r o p r o b e a n a l y s i s . T h e c o m -m o n m i n e r a l s c o e x i s t i n g w i t h s p h a l e r i t e a r e p y r i t e , p y r -r h o t i te , a r s e n o p y r i t e , a n d g a l e n a . T h e r a n g e s o f F e , C d ,M n , a n d I n c o n t e n t s o f s p h a l e r it e a r e 3 . 7 - 9 . 6 , 0 . 2 - 1 . 2 ,0 . 0 - 0 . 2 , a n d 0 . 0 - 0 . 1 , r e s p e c t i v e l y . F r e q u e n c y h i s t o g r a m so f t h e F e c o n t e n t o f s p h a l e r i t e c o e x i s ti n g w i t h n a t i v e g o l da n d e l e c t r u m f r o m a u r i f e r o u s v e i n d e p o s i t s a n d f r o m g o l d -s i lve r ve in depos i t s in J ap an a r e sum m a r ized in F ig . 3. I t ise v i d e n t t h a t t h e F e c o n t e n t o f sp h a l e r i t e f r o m a u r i f e r o u sv e i n d e p o s i t s is h i g h e r t h a n t h a t f r o m g o l d - s il v e r v e i ndepos i t s .

0 20 40 60 80 100Ag a tomic %

Fig. 2. Frequency (nu mb er of analysis) histogram of Ag content(at %) of auriferous vein and gold-silver vein deposits in Japan.Frequency means numbers of analyses. Data are from the presentinvestigation, Nedachi (1974), Abe (1981), and Yamaoka (1981)for auriferous vein deposits and Shikazono (1981, 1985a, 1986) forgold-silver vein d eposits

Fact ors cont ro l ling t he A g/A u rat io of nat ive go ldand electrumI t i s e s s e n ti a l t o k n o w t h e m o d e o f t r a n s p o r t o f A u a n d A gin o r e f lu ids to cons ide r the f ac to r s which con t r o l theA g / A u r a t i o o f n a t i v e g o l d a n d e l e c t r u m . M a n y s t u d i e s o nA u a n d A g c o m p l e x e s i n o r e f l u i d s h a v e b e e n c o n d u c t e d


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a n d r e v i ew e d b y s e v e r a l w o r k e r s ( B a r n e s a n d C z a m a n s k e1967; Barn es 1978; S eward 1981; S hen ber ge r 1986) .A c c o r d i n g t o t h e s e p r e v i o u s s t u d i e s , t h e m o s t d o m i n a n td i s s o lv e d s t a t es o f A u a n d A g i n o r e f l u i d s a re b i s u l f i d ea n d c h l o r id e c o m p l e x e s, d e p e n d i n g o n t h e c h e m i s t ry o fthe f lu id (pH, s a l in i ty , redox s ta te , e t c . ) . However , expe r i -m e n t a l s t u d ie s o f A u s o l u b i l it y d u e t o c h l o r i d e c o m p l e x e sa n d A g s o l u b i l i t y d u e t o b i s u l f i d e c o m p l e x e s u n d e r h y -d r o t h e r m a l c o n d i ti o n s o f i n te r e st h e r e h a v e n o t b e e np u b l i s h e d . U n f o r t u n a t e l y , t h e e f f e ct s o f th e s e i m p o r t a n ts p e c ie s o n t h e A g / A u o f n a t i v e g o l d a n d e l e c t r u m t h u sc a n n o t b e e v a l u a t ed . O t h e r A u a n d A g c o m p l e x e s w i t ht e l l u r i u m , s e l e n i u m , b i s m u t h , a n t i m o n y , a n d a r s e n i c m a yb e s t a b l e i n o r e f l u i d s b u t a r e n o t t a k e n i n t o a c c o u n t h e r ed u e t o l a c k o f d a t a .A s s u m i n g t h a t A u i s t r a n s p o r t e d d o m i n a n t l y a s b i -s u l f i d e o r c h l o r i d e c o m p l e x e s , t h e f o l l o w i n g r e a c t i o n c a nb e u s e d t o d e t e r m i n e w h i c h s p e c i e s a r e d o m i n a n t u n d e rn e a r n e u t r a l c o n d i t i o n s :AuCI~- + 2H2S = Au(HS)~- + 2C I- + 2H +. (1)

F r o m t h e e q u i l i b r i u m r e l a t i o n f o r E q . ( 1 ) w e o b t a i naAuClJaAu(HS); = mA uciJmau(HS) ~= (a~l- at~+)/(K1 a2H,S), (2)wh ere by a i s ac t iv i ty , m i s m o la l i ty , and K 1 i s the equ i -l ib r ium cons tan t fo r Eq . (1 ) . I t i s a s sum ed tha t theact ivity co eff icie nt rat io, YAuCl;/YA~(HS)~, s o ne .S e v e r a l i n v e s t i g a t i o n s t o e l u c i d a t e t h e g e o c h e m i c a le n v i r o n m e n t o f J a p a n e s e e p i t h e r m a l g o l d - s i lv e r v e in - t y p ed e p o s i t s h a v e b e e n c o n d u c t e d ( S h i k a z o n o 1 9 7 4 , 1 9 7 7 a ,1978 , 1985a , b ) . Based on these s tud ies , the va lues o fm A u C lJ m A u( H S) ; a s a f u n c t i o n o f t e m p e r a t u r e m a y b eca lcu la te d (F ig . 4 ) , by t ak in g the av e rage va lues fo r ac l - ,a l l+, aH~S, an d tem pe rat ur e . Th e ra t io of mAuCl; /mAu(HS)~i n c r e as e s w i t h i n c r e a s i n g t e m p e r a t u r e a n d a c F ( F ig . 4 ),w h i l e a n i n c r e a s e i n p H c a u s e s a d e c r e a s e i n m a u c i ; /m au(HS h. F rom F ig . 4 it i s a p p a r e n t t h a t g o l d b i s u l f i d es p e c i e s a r e m o r e a b u n d a n t t h a n g o l d c h l o r i d e s p e c i e su n d e r t h e g e o c h e m i c a l c o n d i t i o n s c o m m o n f o r o r e f l u i d sr e s p o n s i b l e f o r J a p a n e s e g o l d - s i l v e r v e i n s , a s a l r e a d yp o i n t e d o u t b y s e v e r a l w o r k e r s ( S h i k a z o n o 1 9 7 4 ; I c h i k u n i1 9 8 1 ) . H o w e v e r , g o l d c h l o r i d e s p e c i e s m a y d o m i n a t e g o l db i su l f ide spec ie s in o re f lu ids re spons ib le fo r the aur i fe rousve in dep os i t s , as show n in F ig . 4 .R e l a t i o n s h i p b e t w e e n t h e A g / A u r a t i o o f n a ti v e g o l do r e l ec t r u m a n d p h y s i c o c h e m i c a l v a r ia b l e s i n A g C I ~-a n d A u ( H S ) f - d o m i n a n t f l u id sA s s u m i n g t h a t A g C 1 j a n d A u ( H S ) ~ a r e t h e p r e d o m i n a n tA g a n d A u s p e c i e s , t h e f o l l o w i n g r e a c t i o n m a y b e u s e d t od e r i v e t h e r e l a t i o n s h i p b e t w e e n t h e A g / A u r a t i o o f n a t i v eg o l d o r e l e c t r u m a n d t e m p e r a t u r e o r o t h e r v a ri a b l e s:( A u ) + A g C 1 2 + 2 H 2 S = (A g ) + A u ( H S ) 2 + 2 C 1 - + 2 H + ( 3 )w h e r e b y ( A u ) a n d ( A g ) ar e t h e A u a n d A g c o m p o n e n t s o fn a t i v e g o l d a n d e l e c t r u m , r e s p e c t i v e l y . F r o m t h e e q u i -l i b r i u m r e l a t i o n f o r E q . ( 3 ) w e o b t a i naAg/aAu = (a2H~S' mAg c1;' K3)/mA u(HSh a ~ l - ' a i 2 t + ) , (4 )w h e r e b y K 3 i s t h e e q u i l i b r i u m c o n s t a n t f o r E q . ( 3) .E q u a t i o n ( 4 ) i m p l i e s t h a t t h e a A g /a A u o f n a t i v e g o l da n d e l e c t r u m i s c o n t r o l l e d b y t e m p e r a t u r e , a c b , a r ~ s , p H ,a n d mAgCll-/mAu(HS);.

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T T i i i i T f i , p i200 250 300 T ( C )Fig. 4. The relationship between mAucl;/mAu(HS);nd temperature.Hatched and d otted areas represent the probable geo chemical en-vironment fo r typical Japanese gold-silver vein an d auriferousvein d eposi ts , respectively. A, mcj---10, m w =2 , ar~s= t0 -3,K-feldspar/K-mica/quartz equilibrium; B, ma - = 1, mK+ =0.2,aH~s= 10 3-5, K-f eldspa r/K-m ica/qu artz equilibrium; C, m cl-= 1,mK+=0.2, an~s= 10 2, K-feldspar/K-mica/quartz equilibrium; D,m & = 0 .2 , m K +=0.04, aH~s=10 2, K-feldspar/K-m ica/quartzequilibrium; E, m cl =0.2, mK+ =0.04, aH2s=10 3, K -feldspar/K-mica/quartz equilibrium; F, ma -= 0.2, mK+= 0.04, aH,s= tO z,K-feldspar/K-mica/quartz equilibrium. Thermochemical data forthe calculations were taken from Helgeson (1969), Seward (1973),Dru mm ond (1981), and Henley et al. (1984)

T h e c u r v e s r e p r e s e n t i n g t h e r e l a t i o n s h i p b e t w e e n t e m -p e r a t u r e a n d a A o/ aA u ( aA ,, a c t i v i ty o f A g c o m p o n e n t i n. ~ 5 E , . .n a t i v e g o l d o r e l e c t r u m : a A u, a c t i v it y o f A u c o m p o n e n t i nna t ive go ld o r e lec t rum ) a re shown in F ig . 5 . I t i s a s sum edt h a t a K + /a H + i s c o n t r o l l e d b y t h e K - f e l d s p a r / K - m i c a /q u a r t z a s s e m b l a g e w h i c h c o m m o n l y o c c u r s i n g o l d - s i l v e rv e i n d e p o s i t s i n J a p a n ( S h i k a z o n o 1 9 7 4 ) . T h e v a l u e s o fH2S ac t iv i ty in o re f lu ids re sp ons ib le fo r go ld-s i lve r ve insa r e a s s u m e d t o b e 1 0 - 2 - 1 0 -~ , w h i c h a r e e s t i m a t e d f r o ms u l f i d e m i n e r a l a s s e m b l a g e a n d c h e m i c a l c o m p o s i t i o n s o fm ine ra l s (e. g. , F e con te n t o f spha le r i t e ; e . g ., S h ikaz ono1974).R e l a t i o n s h i p b e t w e e n t h e A g / A u r a t i o o f n a t i v e g o ldo r e le c t r u m a n d p h y s i c o c h e m i c a l v a r ia b l e s in A u C l f -a n d A g C l ~ - d o m i n a n t f l u id sT h e t r a n s p o r t o f A u i n t h e o r e f l u i d s r e s p o n s i b l e f o r th ea u r i f e r o u s v e i n s p r o b a b l y t a k e s p l a c e a s g o l d c h l o r i d ec o m p l e x e s r a t h e r t h a n a s g o l d b i s u l f id e c o m p l e x e s ( F i g. 2) .T h i s s u g g e s t i o n i s b a s e d o n t h e f a c t t h a t t h e t e m p e r a t u r eo f f o r m a t i o n o f a u r i f e r o u s v e i n d e p o s i t s is h i g h ( p r o b a b l y2 5 0 - 3 5 0 C ) a n d t h e c h lo r i d e c o n c e n t r a t i o n o f t h e o r ef l u id s is h i g h ( p r o b a b l y m o r e t h a n 1 m o l e a n d l es s t h a n10m ole ) based on the f lu id inc lus ion s tud ies (e . g . , Ne-d a c h i 1 9 74 ; S h i k a z o n o , u n p u b l i s h e d ) . B a s e d o n p r e l i m i -n a r y e x p e r i m e n t a l d a t a o n t h e s o l u b i l i t y o f A u d u e t oc h l o r i d e c o m p l e x e s ( H e n l e y 1 9 7 3 ) A u i s h i g h l y s o l u b l e a t


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2 0 250 300Temp. ( C)F i g . 5. T he relationship between aAg/aAuof native gold and elec-trum and temperatures. A, aH2S=10-% m cr= 0.2 , mK+=0.04,aAgCl;/aAu(HS)= 102", K-feldspar/K -mica/quartz equlibrium; B,a n ~ s = 1 0 - 2 , m cr= 0 .2 , mK+=0.04, a A g c i J a A u ( n S ) 7 = 1015;C,as, = 10 5,_ _ _ 2.0. _ 10 -5mc l--0.2, mK+--0.04,a A g C l T / a A u ( H S )~ - 10 , D , aH~ - , m c l -_ _ _ _ _ _ 1 .5 . _ _ 2 .0 .- - 0 . 2 , i n K + - - 0 . 0 4 , a A g C l ;/ a A u ( H S ) - - 1 0 , E , a A g C I J a A u ( H S )~ - 1 0 ,F, a A g C I ~ / a A u ( H S ) ; = 1015. Calculations we re made assum ing K-feld-spar/K-mica/quartz equilibrium for A, B, C, and D. Thermo-chemical data for the calculations we re taken from Helgeson(1969), Seward (1973, 1976), Dru mm ond (1981), and Hen ley et al.(1984)

h i g h t e m p e r a t u r e s a n d / o r i n f l u id s o f h i g h c h l o r id e c o n -cen t ra t ion .I n f l u id s w h e r e p r e c i o u s m e t a l t r a n sp o r t is d o m i n a t e db y A g C I ~ a n d A u C 1 L t h e f o l l o w i n g r e a c t io n m a y b ewr i t t en to cons ide r t he com posi t iona l va r i a t ion of na t ivegold or e l ec t rum:(Au) + AgC l~ = (Ag) + AuC 1L (5)

F rom the equi l ib r ium re l a t ion for Eq . (5 ) we ob ta inaAg/aAu = (aAgCl~"Ks)/aAuc1~, (6)wh ereby K5 is t he equi l ib r ium cons tan t fo r Eq . (5 ) .C u r v e s E a n d F i n F i g . 5 w e r e d r a w n a s su m i n g t h a t t h ev a l u e o f Z A g / Z A u ( Z A g , t o ta l d i s so lv e d A g c o n c e n t r a t io n ;ZAu, to t a l d i sso lved Au concent ra t ion) i s approxima te ly102-10 l s , which cor respond s to the ra t io for c rus t a l rocks .These curves ind ica t e tha t t he aA ./ aAu ra t io o f na t ive ~oldo r e l e c t r u m d o e s n o t c h a n g e w i t h t e m p e r a t u r e a t ac o n s t a n t ZAu/ZAg ra t io . I t is no tewo r thy tha t t he t emp er -a ture d epen den cy of t he aAg/aau ra t io o f na t ive go ld ande lec t rum in AuCl~-dominant o re f lu ids i s qu i t e d i f fe ren tf rom tha t i n A u(HS )~-dom inant o re f lu ids (F ig . 5 ). Thi sd i f fe r e n c e m a y a c c o u n t f o r th e r a n g e o f v a r i a t io n a n d t h eA g / A u r a ti o s o f n a t i v e g o l d a n d e l e c tr u m f r o m a u r i fe r o u sve in and go ld-s i lve r ve in depos i t s . However , i t has to ben o t e d t h a t u n c e r t a in t i e s o f t h e r m o c h e m i c a l d a t a o n g o l dchlor ide spec ie s mig ht be l a rge .R e l a t i o n s h i p b e t w e e n t h e A g / A u r a t io o f n a t i ve g o l do r e l e c tr u m a n d p h y s i e o e h e m i c a l v a r ia b l e s in A u ( H S ) ~ -a n d A g ( H S ) ~ - d o m i n a n t f lu i d sAs suggested by Henley e t a l . (1984) and Brown (1986), i ti s poss ib l e tha t Ag(HS)K i s dominant i n low-sa l in i ty and

n e a r n e u t r a l o r e f l u i d s . U n d e r t h i s c o n d i t i o n t h e A g / A ura t ios o f e lec t rum an d n a t ive go ld a re cont ro l l ed by thefo l lowing reac t ion :(Au) + Ag(HS )~ = (Ag) + Au (HS ); . (7)

I t i s expec ted f rom the equi l ib r ium ra t ion for Eq . (7 )t h a t t h e A g / A u r a t i o s o f n a t i v e g o l d a n d e l e c tr u m a r ec o n t r o l l e d b y t e m p e r a t u r e a n d 2 A g / Z A u i n o r e f l u i d s ,though the equi l ib r ium cons tan t fo r Eq . (6 ) has no te x p e r i m e n t a l ly b e e n d e t e r m i n e d .T h e ZAu/ZAg a t i o o f o r e f l u i d sT h e Z A u / Z A g r a t i o i n o r e f l ui d s r e sp o n s ib l e f o r g o l d -s i lve r ve ins could be ca l cu la t ed f rom Eq. (4 ) ; t he t em pera -tu re o f fo rm a t ion i s t aken f rom f lu id inc lus ion s tud ies(Enjo j i and Tak enou chi 1976; Sh ikazon o 1985 b) an d thee lec t rum-sph a le r i t e -a rgen t i t e -pyr i t e a ssemblage (Shikazo no1985 a ). The NaC1 equ iva len t c oncen t ra t ion o f o re f lu ids isapproxima ted f rom f reez ing da ta on inc lus ion f lu ids (En-j o j i a n d T a k e n o u c h i 1 9 7 6 ) , t h o u g h t h e f i n a l m e l t i n gtemp era ture of f lu id inc lus ion ice i s a l so a f fec t ed by CO2c o n c e n t r a t i o n i n e p i t h e r m a l o r e f l u i d s ( H e d e n q u i s t a n dHenley 1985) . The pH va lues a re e s t ima ted a ssuming thee q u i l ib r i u m a m o n g K - f e l d sp a r , K - m i c a , a n d q u a r t z w h i c hoccur com mo nly in these depos i t s ; t h i s i n tu rn a l lows aca l cu la t ion of t he ac t iv i ty o f K + . The ac t iv ity o f H2S i se s t i m a t e d b a se d o n t h e e q u a t i o n sh o w i n g t h e r e l a t i o nbe tween the pa r t i a l p ressure of H2S gas and t em pera tu refor ac t ive geo the rm a l wa te r s (G iggenb ach 1980; Arno rsson1985) . Using the typ ica l va lues of t hese va r i ab le s andXAg= 0 .5 , which i s a t yp ica l va lue for e l ec t rum f rom gold-si lver veins, ZAu/GAg i s calcu la ted to b e ab ou t 10 -1,w h i c h i s s i m i la r t o t h a t o f B r o a d l a n d s ' g e o t h e r m a l w a t e r( see T able I ). Th e above ca l cu la t ion i s based on theassum pt ion tha t AgCIK i s the pre dom inan t Ag spec ie s inore f lu ids . However , Brown (1986) and Henley (1985)recen t ly sugges t ed tha t s i lve r b i su l f ide comp lex (Ag(HS)K)could cont r ibu te s ign i f i can tly to the t rans por t a t ion of Agin low-sa l in i ty geo th e rm a l wa te r s (e.g. , Broad lands , NewZea land) . Thus , s i l ve r b i su l f ide complex i s a l so probableas a dominant Ag spec ie s in o re f lu ids re spons ib l e fo rgo ld-s i lve r ve ins in Jap an , t hou gh the sa l in i ty o f o re f lu ids(0 .1-0 .3 M) i s gene ra l ly h ighe r t han tha t o f Broadlands 'geo the rma l wa te r . I f go ld and s i lve r b i su l f ide complexesa r e t h e d o m i n a n t g o l d a n d s i l v e r sp e c i e s i n B r o a d l a n d s 'geo the rma l wa te r and ore f lu ids re spons ib l e fo r Japanesegold s i lve r ve ins , t he Ag /A u ra t io o f o re f lu ids re spons ib l efor Japanese go ld s i l ve r ve in may be s imi l a r t o tha t o fBroadlands ' geo the rma l wa te r which i s about 0 .1 (Ta -ble 1).I f A gC I~ a n d A u C 1K a r e t h e p r e d o m i n a n t A g a n d A uspecies, we can calcula te ZAu/ZAg i n o re f lu ids by us ingt h e r m o c h e m i c a l d a t a o n g o l d c h l o r i d e c o m p l e x e s b yD r u m m o n d ( 19 8 1) , H e l g e so n (1 96 9 ), a n d t h e r m o c h e m i c a lmix ing prop e r t i e s o f Au-Ag a l loy by W hi t e e t a l . (1957).The ra t io o f XAg/XAu (X , a tom ic f rac t ion) fo r na t ive g o ldor e l ec t rum f rom aur i fe rous ve in depos i t s i s t aken to be0 .25 . The ca l cu la t ed va lue of mAucl;/mAgCl;bases ont h e r m o c h e m i c a l d a t a o n A u C 1K b y D r u m m o n d ( 19 81 ) i sabo ut 10 -2. Using the rm och emic a l da t a fo r go ld ch lor idecomplexes compi l ed by He lgeson (1969) , we ob ta inmAuc12/mAgCl; wh ich is very d i fferen t f rom the A u/ A g


5/6

313Table 1. I; Au/Z Ag in o re f lu ids es t ima ted from chem ica l compos i t ion o f na t ive go ld and e lec trum in ac t ive geo the rm al wa ters and incrustal rocks. XAg=0.2 for auriferous veins and Xag=0.5 for gold-silver veins are assumed. Assumed values include: (1) 200C,an2s = 10 -35, mcl -= 0.1, pH = 5.6; (2) 250 C, aH2S----10 2, mcl-----0.1,pH = 5.4; (3) 300 C, all2 = 10 -1, mcl- = 0.1, pH = 5.4; (4) 300 C;(5) 300 C

Au /A g log (Au/A g) References(a tomic ra t io )Crustal rocks (average)SeawaterAct ive geo the rm al wa te r

Broad lands (BR 22)Imperial ValleyM a g m a m a x 1Beppu

Ore fluidsGold-s i lve r ve in

Auriferous vein

( 1 )( 2 )( 3 )( 4 )( 5 )( 6 )

0.03-0.01 - 1.6- -2.00.013-0.004 - 1.1 --2 .20.10 -0.990.11 -1.000.03 -1.60.09 -1.00.09 -1.00.08 -1.10.007 -2.21 . 4 8 x 1 0 - 7 - 6 . 8

0.007 --2.2

W ehdepohl (1978)Holland (1978)Brown (1986)Henle y et a l. (1984)Ko ga (1957, 1961)Thermoch emica l da ta f ro m Helgeson (1969) ,Seward (1973, 1976), Henley et al. (1984)Thermochem ica l da ta f rom Helgeson (1969),Seward (1973, 1976), He nle y et al. (1984)Thermoch emica l da ta f rom Helgeson (1969),Seward (1973, 1976), Henley et al. (1984)T h e rmo c h e mic a l d a t a f ro m D ru m mo n d (19 81 ),Seward (1976)Thermoch emica l da ta f rom Helgeson (1969),Seward (1976)T h e rmo c h e mic a l d a t a f ro m D ru mm o n d (19 81 ),Seward (1976)

r a ti o s o f c ru s t a l r o c k a n d a c t iv e g e o t h e r m a l w a t e r s ( B r o a d -l a n ds , N e w Z e a l a n d ; I m p e r i a l V a l l e y M a g m a m a x 1 , C a l i -f o r n ia ; B e p p u , J a p a n ) , t h o u g h t h e A u / A g v a l u e s fo r t h eI m p e r i al V a l le y M a g m a m a x 1 a n d B e p p u g e o t h e rm a lw a t e r s d o n o t r e f l e ct t h o s e i n t h e d e e p f l u i d .

S u l fu r a c t iv i ty ( a s 2 )U s i n g t h e F e c o n t e n t o f s p h a l e r i t e c o e x i s t i n g w i t h p y r i t ea n d t e m p e r a t u r e s e s t i m a t e d f r o m f l u i d i n c l u s i o n s t u d i e sa n d t h e c h e m i c a l c o m p o s i t i o n s o f e l e c tr u m a n d s p h a le r i tec o e x i s t in g w i t h a r g e n t i t e a n d p y r i t e , t h e p r o b a b l e as~ a n dt e m p e r a t u r e r a n g e s f o r a u r i f e r o u s v e i n a n d g o l d - s i l v e r v e i nd e p o s i t s a r e e s t i m a t e d a s s h o w n i n F i g . 6 . T h e s e r a n g e s a r ed i s t in c t f r o m e a c h o t h e r . A t a g i v e n t e m p e r a t u r e t h e a s2 f o ra u r i f e r o u s v e i n s i s l o w e r t h a n t h a t f o r g o l d - s i lv e r v e i n s.

A s a l r e a d y d i s c u s se d , t h e A g / A u r a t i o s o f n a t i v e g o l da n d e l e c t r u m m i g h t b e c o n t r o l l e d b y s e v e r a l f a c t o r s s u c ha s t e m p e r a t u r e , p H , a s 2 , m H 2 S , a n d ZAg /ZAu , a l t h o u g ht h e s e v a r i a b l e s a r e n o t i n d e p e n d e n t . I n t h i s p a p e r t h er e l a t i o n s h i p b e t w e e n t h e s e v a r i a b l e s w a s d e r i v e d , a n d i tw a s s h o w n t h a t t h e A g / A u v a l u e o f e le c t r u m a n d n a t i v eg o l d i s u s e f u l i n e s t i m a t i n g t h e g e o c h e m i c a l v a r i a b l e s ( a s 2ZAu /ZAg , t e m p e r a t u r e , e t c. ) o f o r e f l u i ds r e s p o n s i b l e f o rA u - A g d e p o s i t i o n . H o w e v e r , i n o r d e r t o c o n s i d e r m o r er i g o r o u s ly t h e m o d e o f tr a n s p o r t o f A u a n d A g i n or ef l u i ds a n d t h e c a u s e s f o r t h e o b s e r v e d c o m p o s i t i o n a lv a r i a t i o n s i n n a t i v e g o l d a n d e l e c t r u m , b e t t e r e s t i m a t e s o fth e s e v a r i a b le s fo r t h e o re d e p o s i t s s tu d ie d a re n e c e s s a ry . Ina d d i t io n , f u r t h e r t h e r m o c h e m i c a l d a t a o n g o l d a n d s i l ve rc o m p l e x e s ( e . g . , A u C l f , A u ( H S ) C 1 - , A g ( H S ) f , e t c . ) a te l e v a t e d t e m p e r a t u r e s a r e e s s e n t i a l .

-5 ~ 0 .005

-1 5

150 200 250 300 350 Temp.(C)Fig. 6. Typical sulfur activity and temperatur ranges for Japaneseauriferous vein (dotted) and gold-silver vein (hatched) deposits .Iso-FeS content curves for sphalerite were draw n based on theequa t ion o f Barton and Sk inner (1979) . py , pyr i te ; po , pyrrho t i te

Acknowledgements. We express our s incere thank s to the s ta ff andgradua te s tuden ts o f the Econom ic Geology Sec tion o f theGeological Institute , University o f Tokyo, for their valuablecomments and discussion. Dr. J . W. Hedenquist reviewed theoriginal manuscript and improved the English. Most samples usedfor chemical analyses we re prov ided fro m the collection o f theDepartm ent o f Pe tro logy and Minera l Depos its, Univers i ty Mu-seum, University of Tokyo. This wo rk was supported in pa rt byfunds f rom the Coopera t ive Program (No. 85136) p rov ided by theOcean R esearch Ins ti tu te, Univers i ty o f Tokyo , and from a G ran tin A id fo r Scientific Researc h N o. 60540518 and No. 61740469from the M in is try o f Educa t ion o f Japan .


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Received: July 10, 1986Accepted: May 12, 1987

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