SULFIDE COMPLEXES DISSOLVED IN

Dynamic Proccsscs Of Material Transport and Transformation in thc Earth's lnterior,cdited by

F Marumo,pp 97-112 ◎

by Tcrra Scientiic Publishing Company(TERRAPUB),TOkyO,1990

Chapter 1 3 1

SULFIDE COMPLEXES DISSOLVED IN HYDROttHERMAL SOLUT10NS一 SOLUBILI丁 Y STUDIES ON A92S AND ZnS Asahiko SUGAKI,Kenichiro IIAYASHI,and Arashi KITAKAZE rl● β ο′ οgl,α れ′E6ο 4ο Jε G`ο あ.al,Fα ′ `“ “ ″ ScJι 4``, 7ο /2ο たlz 1/71ブ 12′ rslrl,Sθ 71′ αJ 9∂ α J″ α ′ ′げ

ル sr′

Og」 l,P′ 7777θ ″ α′

`夕

f.

`″

ノ rο グタε ′ οれ Iれ ′

Thcrmochcnlical data for dissolved inctal species in aqueous solution at

clcvatcd tempcratures and prcssurcs arc indispensablc for understanding the processcs of transport and deposition of lnctals in hydrothcrmal systems. With thcsc data,、 vc can evaluatc the physico― chcnlical conditions that arc

necessary if orc solutions arc to transport cnough mctals to form orc dcposits. Proccsses of orc deposition such as cooling, pressurc changes, reaction with、 vallrocks,mixing of solutions(including dilution)and bOiling can bc cstilnated quantitativcly froln such thcrmochellnical data.

A recent rcview by BARNES(1979)shoWed that most ore― forming metals dissolve in aqueous solutions as complexes、

vith several ligands.(〕 1 is

an important complexing ligand bccause it is usually the most abundant anion in natural hydrothermal solutions.H2S and HS are also known to bc strong rnctal complcxing agcnts at relatively low tcmperatures.However,thc significancc of sulfidc complexes in an ore― forining fluid has not bcen satisfactorily evaluatcd bccausc thcre is scarcc cxpcrilnental data on sul■ de

complcxing. In thc case of silvcr and zinc, cxperilnental data on sulfidc

complexing undcr hydrothcrmal conditions are fc、 v and fragmcntary, as follows.

The stoichiometry and stabilitics of silver sulidc complcxcs at 25° C were rcported by ANDERSON(1962),CLOKE(1963)and SCHWARTZENBACH and WIDMER(1966), but there was a disagreement among reportcd stoichiometrics.The solubility of Ag2S WaS also lncasured bet、 180°

veen 100°

and

C in H2S Saturatcd neutral to acid solutions by OL'SHANSKH`′

(1959)and MELENT'YEV

α′ .

θ′α′ .(1970), but they could not deterllllinc the

stoichiometry of the silver complexes. Solubility expcrilnents on ZnS in hydrotherlnal sulfide rich solutions wcre carried out by BARNES(1960)and

RoMBERGER and BARNES(1965), but these works were fragmcntary.

98

A Sugaki′ ′αノ

Recently,BouRCIER and BARNES(1987a)alsO Carried out solubility experi― ments on sphalerite in NaHS― I12S aqueous solutions of lilnited pII range, and estilnatcd the equilibriunl constants of zinc sulide complexes.

In our study,solubility experilnents on Ag2S and ZnS in NaC)II― H2S aqueous solutions wcrc pcrformed at temperaturcs bet、 veen 25° and 250° (〕

,

and ne、 v data on stoichiomctry and equilibrium cOnstants for silvcr and zinc

sulfidc cOmplexes、 vere obtained.Thcsc solubility works、 vcre also carricd out ovcr a、 vidc pII range in order to accuratcly deternlinc various kinds Of

stablc silvcr and zinc sulidc complexes and to cstilnate thcir dolllinant rcglons.

2.Eη

,θ

″ θ′ Z`′ 乃οグ α′コ “ “

Thc solubility experiments for Ag2S(argentite or acanthite)and Zns (SphalCritc)wcre performed separately in thc temperature rangc from 25° to 250°

C.Ag2S Or ZnS chips were reacted with NaOH― H2S aquCous solutions

in a Barnes― type

rocking autoclave(BARNES, 1963,1971;BouRCIER and

BARNES,1987b)or in a stirring autoclavc.After a period of mOre than 48 hours of reaction,samplcs of the solution wcrc extracted from thc reaction

vessel and analyzed for total silvcr Or zinc cOntcnts. By varying thc composition of the starting solutiOns,it was possiblc tO cxanline thc Ag2S

and ZnS solubilities as functions Of tcmperaturc, pH and activitics of rcduccd sulfur spccics,H2S(aq)and Hs.Argentitc and sphalcrite uscd in the solubility cxpcrilncnts wcrc synthesizcd by thc cvaculatcd silica― tubc nlcthod,using silvcr or zinc rnctals and sulfur.

glass

Thc proccdurcs in thc solubility expcrilncnts were as follows.Ag2S Or

ZnS and thc rcquircd amount of NaOH wcrc loadcd first in the reaction vessel.After attaching thc valvc assemblage,the vcsscl and Valve asseIIlblagc

were evacuated Then,H2S gaS Was bled into the vesscl froln a small H2S vater was loaded into the vessel using a hydraulic pump Samples of thc solution in the reaction vessel were usually extracted at 25° to 50° C intervals under run conditiOns by lneans of cylinder. Finally,distilled and deionized、

the sample tube method(BARNES,1963,1971;CRERAR and BARNES,1976) or directly by a glass syringc conncctcd by a capillary tubc to the valve

asscmblagc. Sample solutions were treated with cxcess NaC)H solution followcd by H2C)2 in Ordcr tO convcrt reduced sulfur spccics to sulfatc.Thc solution was then acidificd and analyzcd for Ag or Zn by atonlic absorption

spcctrophotomctry.

3.巳⊇

J“ θ ″α′Rω クルs `″

A total of 14 runs for each solubility experimcnt on Ag2S Or ZnS werc successfully completed.Thcsc cxpcrilncntal data have been ininutely reported

Sulfidc ComplcxCs Dissolvcd in Hydrothcrmal Solutions

99

撃 3T割号呪∬ど嗅彗Tλ 鵠勇需e:L∴ li篤 ::譜謝り漁ぅ 1顎五鷺 ‰ 胸れ量urinlぷ l罵 1甜耽:ぶ]:噸:10∬ ul認品」 :も

:∫

Wi鮒難掛麒孤鰐撤締讐 ncutral.

The act

itics of H2S(aq)and Hs as well as the pH ofthe solutions at

淵鸞鱗鸞鮎雌憔 W鋪量灘勤掲擬離揮薫鮒 1鋭員捌∬ま[爵黎鮮11輔ll 脱 Tl驚 [1∬鷲良 generally as,

Ag2S+χ H2S(aq)+ノ HS =Ag2S(H2S)χ (HS)」 (aq) and thc cquilibriun■

(1)

constant is

K=α comメ

・ (α :act ity). (α Hsソ

cx′ (α H2S)χ

tions have been describcd in thC papers by SUGAKI

.(1987)for θ″α′

(2)

Ag2S and

A Sugaki′ ′

`7ノ

︻〇引いヽ︱一¨ ︱

［ｏ引口ヽ︱

口ｏ＝＠¨ ＩＮＯ＝

︻ｏ引﹁い︱

Ｏｏ引﹁∞ １〇〇＝ヽ ∞ ︱Ｏｏ﹁いヽ︱

いい︱

︻ｏ羽

＝

﹁い１０ｏ＝

いヽ︱ぺｏ引

∞ ∞ ︱一‘

Ｏｏｏｏ︼

い寸１〇〇引

ν﹁ｍｏ一

υ ｏｏい︼

Ｏｏい０

￨￨￨￨￨

﹁い ∞１０ｏ引 ”［︱゛ｏ刊ヽい

﹁０い︱寸ｏ引ｏい１０ｏ引﹄寸

Ｏｏ引つ ∞ ︱寸ｏ判﹃ヽ︱

いｏ引 ”ヾ︱゛ｏ引０い︱

︹︶０００一

ＯｏいＮ

一０１〇〇＝ヽつ︱［‘ 引ＯｏＯい︻

ν﹁¨ｏ一

味 ∽〓Ｘ目ＯＩＮ＝あ〓ｏ＋Ｏ二十 ∽●Ｎ︵８︲．田の〓ｘ〓Ｏ γ Ｎ＝ ∽〓＋〇・国＋ ∽●Ｎｅ︶

ｏ︶口Ｎ＝ ∽工ｏ＋ ∽Ｎ〓＋ ∽ｏ国︵ ∽〓︶︲哄︲口Ｎ＝ ∽工＋ ∽ａ〓＋ ∽ｃＮ︵宋∽〓︶つ︶︲ｏＮ＝ ∽倒〓＋ ∽ｃＮ︵ ●︶八∽〓︶

∽ 目●一も ●ｏば

ぁ ●ｏ﹁ ●［Ｏ∽ ∽鰤ｏＯ●。●

¨ ＝＋ ∽ ¨く ∽ ∽べく鳳︵∽目︶︲工︻． ¨＜＝ ∽国０＋ ∽ａ〓＋ ∽品く品︵∽国 ×∽ＮＥ︶∽、︲ ¨＜ ¨く＝ ∽二十 ∽ａ国十 ∽倒只∽〓Ｘ∽︻国︶ ∽Ｎ︲ ∞く ¨＜＝ ∽［二十 ∽［︵ ∽ＮＥ︶∽︻

∽●ｏ﹁ち ●ｏ“

ｏ︶︵

︵０︶

︵ ●︶

︵０︶

ぁ目ｏ〓 ●一ｏ∽

●■ ０一 ∽う００●げ“ 中穴︶● ＺＩ∽ ど出 ●一∽口。〓ｏに。一目ｏ■ ●︼Ｏ∽鷲Ｏｏ一〓 ●ｏ¨﹄● ﹄Ｏ嘔 “ 口●場口ｏｏ日一〓● げ国︼口ａｍくト

︹︶ｏＯＯ゛

Ｎ‘ 引﹁寸１０ｏ引いい︱

Ｏｏｏい０

﹁﹁ヾ︱ ∞‘ 引﹄ ∞ ＩＮｏ引い ∞

︹︶ｏＯＯＮ

+ +￨ + + +

に一。∽ ﹂ｏ ∽●ｏ● ｏ“ｏ■ 口０● ●一０∽僣０﹄ｏ﹂ ∽︶口に一∽●ｏＯ﹁日 ●■ ０﹁〓●げ四０国コｍくト〓＾０ ●Ｚ ∽ ［〓 ●一ｏ〓ヽｏ︻

￨￨￨￨￨

﹁いい︱ ∞‘ 引 ” い︱゛ｏ＝いい

ＯｏＯ寸［

+￨ +￨ +￨ +￨ +￨

Sulllde COmplcxcs Dissolvcd in Hydrothcrmal SOlutions

101

by HAYASHIθ ′α′.(1990)fOr zns The diss01ution reactiOns of Ag2S Or ZnS and the equilibriun■ cOnstants for the reactiOns are givcn in Tablcs l and 2, respectively.

イ. S′ んθ″αれグ Zブ 46 Cο

イ .I

Sノル

`r sα

J/iグ

θ

ρたχ “

`s

ρルχ

`ο `s Thc conccntrations “ of silvcr sulfide cOmplcxes were calculated at

tcmperatures fron1 25° to 250° C using the equilibriunl cOnstants in Table l.

As an cxample,thc concentration curvcs of the silver sulfide cOmplcxes for the total reduced sulfur concentration ΣS(“ H2s+727 Hs)=1.0 777 at 200° C arc shown in Fig. l as a function Of pH.In the igure,the calculated total Ag concentration is alsO represented、 vith a heavy line.Since the total reduced sulfur concentrations varied in cach run,the solubility data were norlnalizcd to ΣS=1.0“ . leasured solubility values(normaliZed tO Σs=10//1),shOWn

by solid circles in the figure, plot on or very closc tO thc total Ag concentration curve.Silver sulfide complcxcs bccOmc progrcssivcly dOnlinant

in order of Ag2S(H2S),A32S(H2S)(HS),Ag2S(H2S)(HS)3 and Ag2S(HS): with increasing pH as secn in thc figurc.Thc total rcduccd sulfur cOncentra― tiOn (Σ S)alSO

has an effcct On the concentration Of the silver sulide

complcxcs.ThC rClativc cOnccntration ofthe silvcr sulide complcxes fOr pH 4.6,5.6 and 6.6 at 200° C are shOwn in Fig.2 as a functiOn Of

Σs.The pI1 4 6, 5.6 and 6.6 given in the igure shO、 v acidic,neutral and alkaline cOnditions,

一一一

︵ミ〓２一ｏ〓ＥＯｏｃｏｏ︶一３

７，

Ｈ

FIc I COncentration Of silvcr sulflde complexcs at 200° C and ΣS=1 0 777 aS a function of pH Thc mcasured solubilitics Of argentitc,normalizcd to ΣS=10″ 2,arC

comparcd with thc calculatcd total Ag conccntration Error bars represcnt the standard dcviation

A Sugaki′ ′α′

=ノ

ＯｏＯｏＮ

一● ︵ｏ口〓お鍼［“︶つつ０口“ ︵［“︼一● ｏ● ︶０．一パｏ一０一ｏ● ︶０ヾ︼円０や● ︵∽ 因︶口ｏ ■ ●ヽ゛● ｏＯ口００﹄●鮨︼● ∽ 一ｏｏ● 一ｏ﹁

●やｏ一ｏ工０﹂ｏ口ｏ﹁ｏ● ●鮨 ● ∽● ∽ｏ× ｏ一。日００〇一綱︻● ∽ ﹄ｏ＞〓 ∽ ｏ〓゛︺ｏ ∽ｏ﹁﹁口 “ ● げｏ●３ ●︻９ “ ．一０［﹄︼

︵ミ・Ｌ３一´ ０﹁一●ョ０●﹂ “●一０卜︶一一″ ，

“ ょｔａこ３〓ち ‘ ｏｏＯＯＮ

Sulfldc Complcxcs Dissolvcd in Hydrothcrmal Solutions

103

cly,at 200° C.Silvcr sulfide complexes Ag2S(H2S),Ag2S(H2S)(HS) and Ag2S(H2S)(HS): become predominantin this order with increasing ΣS. Thc predolninant ΣS rangc for each sulfide complcx moves toward the low

rcspect

ΣS sidc with thc increasc in pH.

A tempcrature cffect on silvcr sulfidc complexcs has been rcported by

SUGAKI

θ′α′ .(1987).According to thcm,the prcdorninant rcgion for each complex shifts toward high ΣS and pH sides whcn thc tcmpcrature riscs ― 1静蹴〕∬ き l脳鐵も翼 l量 ξ ふ肌贈 ∬ 智服賀よI::∫ 『『 predolninant at the alkaline state,and its dollninant field rnovcs to the higher

pH side with increasing temperature.According to OHMoTO(1972)and BARNES(1979), the tOtal reduced sulfur concentration(Σ S)in an Ore solution is generally between O.l and O.0005 η2. In such a ΣS range, Ag2S(H2S)iS thC mOst important silver complex around neutral pH in the abscncc of chloride.

イ .2 Z滋

`s“

J/id`ε ο ″

ルχθ s

Solubility experilncnts for sphalerite showed that zinc forms stablc sulfide complcxes in sulfidc― rich aqueous solutions. Rcccntly,BouRCIER ギ轟躍

朧

:鑑 llr鰤

::1鼎期 ,裁吉 :1滸

騒嵩

ツ犠

∫:

lilnitcd pII range.Howcver,the sphalerite solubility data obtaincd in thc

prcsent study could not be explained solely by the combinations of thc

器:;岬 1lFl遮謬:燻認鷺

l肌瑞町群箭

complexes above.The stoichiometry of zinc sulide complexes and equilibrium

'∬

０︱・

C＼

｀92S● 29“ 9ち

200°

C ＼

＼A92S輌 29“ 98

A02S(H2S)(HS) A92S(llS場

マ A12S(H2S)

A12S(HaS)

３・

︵ミ．﹂Ｌコのつ００コつ０﹂一０一０卜︶ぃ０一，

100°

Neulrot,H

5

Neuirlt p"

6 pH

FIG 3. pH― ΣS conditions for prcdominant rcgions of silvcr sulflde complcxcs Ag2S(H2S),Ag2S(H2S)(HS),Ag2S(H2S)(HS): and Ag2S(HS): at looO C and 200° Ncutral pH at cach tcmperaturc is also indicatcd.

C


104

constants of sphalcrite dissolution rcactions obtaincd by our solubility work arc prcscntcd in Table 2 (〕 and ΣS=1.0 Thc conccntrations of cach zinc sulfide cottplCX at 200° ηっare shown as a function of pII in Fig. 4. NIlcasured solubility values,

plottcd as solid circlcs in the figure, agrec with the calculated total Zn concentration curve(heavy line)The figure also indicates that zinc sulfide

complexes become・ dominant in the order of Zn(HS)2,Zn(HS)5,Zn(HS): e concentrations of zinc and Zn(OH)(HS)5,With increasing pH The relat sulfide complexes at 100° , 150° , 200° and 240° C under neutral pH conditions are shown in Fig.5 as a function of ΣS.With increasing ΣS,the

sulide complexes also become progressively predolninant in order of

Zn(HS)2,Zn(HS)5 and Zn(HS):.ZnS(OH)(HS): iS not important at neutral pII,but becomes donlinant at high pH. ′ 乃わrノグθ イ.3 Cο ραrおOκ ″′ ρたχ `ο `s `乃 ln ordcr to discuss inctal transport and dcposition as sulfide lninerals “ “

from ore solutions,thc relative importance of llnetal sulfidc and chloride

complexes in orc solutions should be compared,becausc Cl is the l■ ost illnportant anlon in natural fluids.Ag2S Or ZnS solubilitics(tOtal Ag or Zn concentrations)at 250° C are also shown in Fig.6 as a function of the total

reduced sulfur and Cl contents.In the figure,the Ag concentrations were calculated using the solubility data from this study,stability constants for

silver chloride complexes(SEWARD,1976)and s01ubility products for Ag2S

ヽゝ０

９

７Ｈｐ

FIG 4

Conccntration of the zinc sulidc complcxes at 200°

C and ΣS=10“

function of pH Thc measurcd solubilitics of sphaleritc,normalizcd to

vith the calculated total Zn conccntration Error bars represent the compared 、 standard dcviation

as a

ΣS=10″ っ ,arc


100 100° C

Zn(Hs):

80

60

1500C

Zn〈

。２

HS)3

8

Zn(HS):

Zn(OHXHS): zぼ Ontts壻

―

。。０︲ Zn(HS)2

∞

Ｎ一ｃ Φ Ｏ﹂Φ Ｌ ∽ ＯＸＯ ″α Ｅ０００ｃ一

“

pH=5。

Zn(HS)2

。６

200° C

240° C

6

,H=5。

PH=5.5

40 20

-4

-3

-1

-2

0

1 -4

-3

-2

Log(lolct「 educed sutfur,′ η ) FIG 5

Rclativc amounts of zinc sulfldc complexes as a function of total rcduced

sulfur conccntration at 1000,150° ,200° and 240° C undcr ncutral pH

(IIELGESON, 1969).For calculation of Zn concentrations, equilibrium constants for zinc sulfidc cё

mplexes(thiS Study),stability constants of zinc

chloridc complexcs and a solubility prOduct for ZnS (BOURCIER and

BARNES,1987→ wcre alSO Cmploycd.The pH valucsin Fig.6 were calculatcd on the assumption that thc pH was buffercd by reaction betwccn thc hydrothermalfluid and thc alumino―

silicate asscmblage,K― feldspar,scricite

and quartz,as follows:3KAlSi308+2H+=KA13Si3010(OH)2+6Si02+2K+. The equilibriunl constant for this rcaction referred to ARN6RSSON θ′α′ .

(1982).The K+content was estimated by the rccent empirical calibration θ′α′ .,1983),assuming thatthc total Cl content cquals the sum of Nダ and K十 The boundary between the silver sulfide and chloride complex dolninant

(ARNORSSON

.

fields is indicated as a dotted linc in the figure.The boundary inoves toward

A Sugaki′ ′αノ

pH

7.5 7.0

6.5

6.0

。。一︵Ｅｃａ．﹂っ卜一っいＯΦＯっＯΦ﹂ ″０一０卜︶

４３２

―ヽ____ δ .δ 労 _ Ag concentratlon ‐Zn● OnoOntr● ――― ―

llol

tog(lo十 ct Ct , ppm) FIG 6

Ag and Zn concentrations at 250° C as a function of Cl and ΣS pH buffcrcd

by thc asscmblagc K―feldspar+inuscovitc+quartz is indicated Thc boundary of chloride and sulide complex dominant regions is shown by a dotted linc

the lo、 v ΣCl

sidc with increasing tcmperature to reduce the dominant ficld of

the sulfide complexes; i.e., thc silver sulfide colnplexes become more ilnportant as stable species at lower temperatures. Figure 6 suggests that

sulide complexes become more dolninant than chloride complexes in the hydrothcrmal solution with high ΣS,lo、 v ΣCl and high pH.In the sulfide complcxes donlinant region,the concentrations of silver and zinc in the hydrothermal solution are on the same ordcr ofllnagnitude as shown in the figurc,but thc zinc conccntration immediately exceeds that of silver with increasing Cr contcnt in the chloride complexes donlinant region.

5.Gθ οわgた α′いゃ′たα′われ BARNES and CzAMANSKE(1967)and BARNES(1979)suggCSted that colninon base― metal conccntrations in orc― forlning solutions havc to bc at

least 10 ppm for the formatibn of ore deposits oh an economic scale.A zinc

S∼ 1.O z4 at 250° with only sulfide complexes as the dissolved metal spccies. However, concentration of 10 ppm can be attaincd in a solution ofΣ

C


::

視

i緊嶋

棚

塩雪 ;瀾漁 ‖

:1‖潔 :ξ 器 :備

ゝξ当

F晩

簿 :

thought to be unablc to reach 10 ppln as show,in Fig.6.Accordingly,zinc sulfide complexcs are not significant in ore solutions responsible for zinc deposits. As rnentioned abovc,metals are apt to dissolve in larger amounts in ore

solutions as chloride complexes rathcr than sulfide complexes,so chloride complexes are considered to play an important role in lnctaltranSpOrtin ore

solutions.However,sulfidc complexes become thc predominant metal(Ag Or Zn)Species in hydrotherllnal solutions of high ΣS,19w ΣCl,high pH and rclatively low tcmperature(Fig. 6). TheSe cOnditions arc expected for hydrothermal solutions forllling epithermal Au― Ag deposits and in some geothcrmal wclls. Thus, the Ag and Zn contcnts in ore― forlning fluids responsiblc for epithcrmal Au― Ag dcposits and in gcothcrnlal fluids wcrc calculatcd to confirm whethcr or not sulfide complcxcs arc indeed predonll― nant in such fluids.

A pH-logル 2 diagram indicating ore― forming conditions of epithcrmal Au― Ag deposits in Japan is shown in Fig.7. This diagranl is constructed undcr conditions of ΣS=0.01“ ,Σ Cl=0.2 7η and ΣC=0.01 7η at 200° C. These ore― forlning conditions and fluid compositions、 vere estilnated from data such as temperature(SHIKAZONO,1985),the Salinity of■ uid inclusion θ′α′ 。 ,1981;SuGAKIθ ″α′.,1984)and sphalerite (TAKENOUCHI,1970;IZAWA composition(SuGAKI,1976;SuGAKIθ ′α′.,1982,1984;SHIKAZONO,1985). The relativc amounts of various cations in the fluids have not been Ktt ratio of the orc solution can be dctcrmined dircctly.Now the Nダ ′ K+ratiO rel激 ionship g en by approximatcd using the tcmperature― Nダ ′

ARN6RSSON

θ′α′ 。(1983).Adularia occurs in many cpithermal Au―

Ag

deposits(SHOJI,1985,1986)assoCiated with lnuscovite(seriCite)and quartz, whilc kao nitc sometimcs appears with inuscovitc and quartz.Thcrcforc,thc asscmblages K― fcldspar+muscovitc+quartz and muscovitc+kaolinitc can be used to furthcr consider thc probable pH rangc ofthc fluids.Pyrite is the

principal iron mineral in epithermal Au―Ag deposits.Sulfur fugacitics(ヌ 2) of ore forlnation were estimated from the compositions of sphalerite、 vith

pyrite(ScoTT and BARNES,1971). From the data above,the pH-log/02(OXygen fugacity)conditiOns ofthe ore solution which formed the Au― Ag deposits are estilnated as the stippled area in Fig. 7. In the igure,the calculated silver and zinc concentration contours with solid and dottcd lines,respectively,and the dolninant regions for chloride and sulfidc complexes of silvcr and zinc are also shown.The Ore¨ forlning(pHザ Ъ2)COnditions

indicatcd as the stippled area in Fig.7 arc in

the sulidC Complcxes dolninant rcgion.The calculatcd inctal contcnts in the ore― forlning solution at 200° C is very low,probably 10 to l ppb for Ag or Zn.Thc metal contcnts calculated arc farlcss than 10 ppnl,corrcsponding to

A Sugaki``α ′

-34 -36

一一一

ｏＥ一０．．ヽぃ０一

-48 -50

pH

FIG 7

Thc pHヵ

2 diagram showing stability flclds of Cu―

Fc― S_O mincrals,calcitc,

ΣS=o01 ″,,Σ Cl =0 2 771 and ΣC==001 ″, at 200° C Cation cOnccntrations in thc Orc solution arc assumcd as baritc and aqucous sulfur spccics undcr conditions of

follows:Na+=0189,K+=0011,Ca)=00001,Mg2+=0 00001 and B♂ +=o ooool Ag and Zn conccntrations are indicatcd by solid and dotted lincs, respectivcly The chloridc and sulide cOmplcxcs donlinant rcgions for Ag and Zn conccntration arc also indicated.Stability flclds of K― fcldspar,rnuscovitc and kaolinitc arc given Thc pHり

62

conditions of the ore solution forming thc cpithemal Au― Ag deposits in Japan arc sho、 vn as stipplcd arca bn:bornitc,cp:chalcopyritc,henl:hcmatite,int:magnctite, py:pyritc,po:pyrrhotitc,sp:sphalcritc

the minimum contcnt rcsponsiblc for basc― Inctal dcposits(BARNES and

CZAMANSKE,1967).HoweVer,such very low metal contents on the ppb order inay not be unreasonable for fluids which formed epithermal Au―

Ag

veins consisting all■ ost exclusivcly of quartz. Indeed,precious metals are

precipitatcd from lluids with very low metal contents(several ppb)at a geotherlnal well,as described below. Estilnation of the precious and base metal contents and other physico―

chemical data of thc hydrothermal solutiOns fOrlniご g Ore deposits is gencrally difficult.However,we can sometilnes obtain such data directly,as above,from the fluids at gcothcrmal wclls.There are lnany geothermal wens θ′α′ .,1979;HENLEY and BROWN, 1985).At well No.BR-2 in the area,precipitates containing

in the Broadlands area,Ncw Zcaland(WEISSBERG

precious lnetals arc found at thc surfacc.Data on compositions and othcr

Sullldc Complcxcs Dissolvcd in Hydrothcrmal Solutions

109

physico― chemical prOpertics ofthe fluids from the well are obtained by direct

measuremcnt thcrc.Temperature,pH and compositional data of the fluid werc 260° C,pH 6.2,107 ppm H2S,121 ppm C02,1181 ppm Cl.Thc gcothcrmal fluids at Broadlands are a good tcxtual cxamplc for cxallnining our solubility data in comparison tO natural data.

Thc pH-logル 2 diagram explaining the physico― chemical condition of thc fluid from thc well BR-2 at Broadlandsis shown in Fig 8.The ngure was principally constructed using thc data on thc compositions of thc fluids VEISSBERG`′ α′.,1979).CompOsitions(BROWNE and LovERING,1973)of sphalcritc prccipitatcd were also used for estimation ofス 2.Calculated (ヽ

conccntration curves for silvcr and zinc arc also shown in Fig 8 Thc pHlプ Ъ condition ofthe■ uid from wcll BR-2 is prcscntcd as a sman stipplCd arca in 2

Fig 8 Under thc physico― chelnical condition of the■ uid from the BR-2 、 vell,the calculatcd silver and zinc contents of the nuid are l.4 ppb Ag and

2.8 ppb Zn,and both the inetals dissolve as sulide complexes These inetal contents agrce reasonably、 vell、 vith the observed concentrations of O.7 ppb

Ag and 1 0 ppb Zn(WEISSBERG

θ′α′ .,1979).

一一

一一一

８ “ “ ３

。Ｅ一０．．ヽぃ０一

-42

-46 pH

FIc 8 Thc pH「ん,diagram

illustrating thc physicO― chcmical condition of thc

geothermal fluid flom thc BR-2 wcll at Broadlands,Ncw Zcaland Tcmpcraturc,pH and compositions measurcd wcre 260° C,pH 62,107 ppm H2S,121 ppm C02,1181 ppm Cl(WEISSBERG`′ ′′,1979)Stabihty flclds of Cu― Fc― S-O orc mincrals,calcitc and aqucous sulfur spccics arc shown Ag and Zn conccntration arc also indicatcd by solid and dottcd lincs,rcspcctivcly Abbrcviations arc thc samc as Fig 7


σ. S夕 777″ 2α り′

Thc cxperimental studics on Ag2S and ZnS solubilities in NaOH― H2S

and 250° C aqucous solution、 vcrc carried out at tcmpcratures between 25° using a Barncs― typc rocking autoclave and a stirring autoclave. Fronl the results of thc cxpcrilnents and thcrmochelnical calculations,silvcr sulfidc

complcxcs,Ag2S(H2S),Ag2S(H2S)(HS),Ag2S(H2S)(HS): and Ag2S(HS):, and zinc sulfide complcxcs,Zn(HS)2,Zn(HS)],Zn(HS):,Zn(oH)(HS)]and Zn(OH)(HS):WCrc found to cxist as stablc spccics dissolved in thc solution. The equations and equilibriun■ constants of thc rcactions producing thcsc complcxcs were deterlllincd as given in Tablcs l and 2 Thc conccntrations of cach silver and zinc sulfide complcx in the solution varies、 vith temperature,

pII and total reduccd sulfur conccntration(Figs l,2,3,4 and 5)

Arnong thc metal sulidc and chloride complcxcs, the former is nOt always necessarily dorninant in natural ore solutions.In general it inay bc thought that silver and zinc chloridc complcxes are more collllinOn than

sulide complcxes as donlinant species in ore solutions. However, in thc hydrothcrmal solution― formcd epithermal Au― Ag deposits in Japan, thc silvcr and zinc sulidc complcxes arc thought to have been doIIlinant spccies,

as shown in Fig.7.In this casc,Inctal concentrations dissolvcd as sulide complexcs in thc solution arc vcry low,probably 10 to l ppb for2へ g or Zn (Fig.7).ThcrCfOre it is cxpcctcd that thc cpithcrmal Au― Ag deposits wcrc produccd from such hydrothermal solutions oflow lnetal contcnt as above.

As an examplc of lo、 v mctal― containing gcothcrmal iuid precipitating precious mctals,there isthc No.BR-2 wen at Broadlands,New Zealand Thc

pH andル 2 conditiOns ofthc geothermal■ uid

frOm the well are shown in Fig

8.Silver and zinc concentrations of the fluid,estilllllated from the conditions,

are l.4 ppb Ag and 2.8 ppb Zn These values agree reasonably wcll with analytical data(0.7 ppb Ag and l.O ppb Zn)for the fluids.Ag and Znin the fluid from the wcll are thought to dissolve as sulidc complcxes Valuablc advicc on solubility expcrilncnts by Prof.H L Barncs of Pcnnsylvania nivcrsity and by Prof.S.D Scott ofthe Univcrsity of Toronto are gratcfully apprcciatcd A part ofthc cxpcnsc for this study、 vas defrayed by a Grant― in― Aid for Scicntiflc Research from thc Ministry of Education,Scicncc and Culture of Japan_ Statc l」

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to 370° C, C`ο ε/1ブ 777

Sullldc Complcxcs Dissolvcd in Hydrothcrmal Solutions

Hl

ARNORSSON,S,GuNNLAUSSON,E and SvAVARSSON,H (1983):Thc ChCnlistry of gcothcrmal invcstigations,G`ο σ力J“ Cο s7770ε 77j777 ∠r′ α,47,567-577 g″ f77S′ bο οた,59,137-141 ′ aS力 y′ α″

BARNES,H L(1960):OrC SOlutions,Car″

BARNES,H L (1963):Orc S。 lution chcmistry I「Expcrimcntal dctcrmination Of orc mincral solubilitics,Ecο 4 G′ οノ,58,1054‐ 1060 BARNES,H L (1971):InVCStigations in hydrothermal sulidc solutions,in R′ Ss17″ J● ″〃な力 P″ ′

′″ α′ ク″ ′,Ed ′α″″ Ht癬ル列り

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α″

rθ

ε/1れ J9tr′ s

`力

G G Ulmcr,Springcr― Vcrlag,Berlin,pp

317-335 ″αノ0″ ′ 力′″ BARNES,H L(1979)Solubilitics of orc mincrals,in G′ οc力 ′ ブげ ffl″ ο′ "げ Intcrscicnce,Ncw“York,pp 404-460 Deρ οSrrs 2′ ノ ,Ed H L Barncs,Wilcy― `グ BARNES,H L and CzAMANSKE,G K (1967):Solubilitics and transport of orc nnincrals,in G′ οc力

`“

777α ′ 0″ ′ 力′″ ブ s′ げぽ Jfl″ ο

Winston,Ncw York,pp 334-381

`Dψ

οsル s,Ed

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BOURCIER,W L and BARNES,H L (1987a):Ore SOlution chclllistry VII Stabilitics of chloridc and bisulfldc complexcs of zinc to 350° ,Ec071 G′ ο′,82,1839-1863 BOURCIER,W L and BARNES,H L (1987b): Rocking autoclavcs for hydrothcrmal cxpcri― ′ル C力 ″ 9″ as,Eds G た″ 771α ′ExParJ777`″ mcnts I F cd volume systcms,in lfl″ ο′ `α Ulmcr and H L Barncs,Wiley lntcrscicnce,Ncw York,pp 189 215 BROWN,P R L and LovERINC,J F(1973):CompOSition of sphalcritcs from thc Broadlands (〕

G

gcothcrmal flcld and thcir signiflcancc to sphalcritc gcothcrmctry and gcobaromctry,Ecο

72

G′ ο′,68,381-387

CLOKF,P L(1963):Thc gCO10gic rolc of polysulidcs― Part II,Thc solubility of acanthite and 4c′ α,27,1299-1319 covclitc in sodium polysulfldc solutions,C′ ο 7777 COS7720σ /7J777 ン `力 CRERAR,D A and BARNES,HL(1976):Orc S01ution chcmistry V Solubility of chalcopyritc to 350° C,Ecο 72 G′ 0ノ ,71, and chalcocitc assclllblagcs in hydrothcrmal solution at 200° 772-794 HAYASHl,K,SuGAKl,A and KITAKAZE,A (1990):SOlubility of sphalcritc in aqtlcous sulfidc and 240° C,G′ ο 712 Cο s“ οεカプ И ′,54,(in solutions at tcmpcraturcs bctwccn 25° “

`/2′

prcss)

`′

HELGESON,H C(1969):ThCrmOdynamics of hydrothcrmal systcms at clcvatcd tcmpcraturcs and prcssurcs, 4″ 7 工 Sでブ,267,729-804 HENLEY,R ヽV and BROWN,K L(1985):A practical guidc to thc chcmistry and epithcrmal α′sys`′ ″S,Eds B R Bcrgcrand P M οgック′グ G′ οご力′777お ′ systems,in G′ ο′ ッ 9/巧 ?ルカ `″ “ Bcthkc,R′ νj`"sノ ″Ecο 720/21r`G`0ノ οgyッ ο′2,Socicty of Econonlic Gcologists,EI Paso,pp

25-44 1ZAWA,E,YosHIDA,T and SAKAI,T (1981):Fluid inclusion studies on thc gold― silvcr quartz s'4 ,"α 72,Ed Y ′ S77ν ′″ Or`D`ρ οs′ ′ vcins at Kushikino,Kagoshima,Japan,in Cο ′ Jα ′ おStra JVο fθ ,25-34 Urashima,ソ Иf12jれ gC`ο ム `ε

MELENT'YEV,B N,IvANENKO,V `レ V and PAMIFILOVA,L A (1970):Solubility of somc orc― ′,7,416-460 forming sulfldcs undcr hydrothcrmal conditions,G′ οで力 `711 r71``r77α 0HMoTo,H (1972):SystCmatics of sulfur and carbon isotopcs in hydrothcrmal orc dcposits, Ecο κ G′ οノ,67,551-578

0L'SHANSKH,Ya I,IVANENKO,V V and KHROMOV,A V (1959):ThC S01ubility of silvcr sulfldc in aqueous solutions saturatcd with hydrogcn sulfidc,19ο

た′ ∠たαグ Nα λ SSSR, “

124,410-413 RoMBERGER,S B and BARNES,H L (1965):SulfldC Solubilitics in synthctic orc solutions (abStr),ル α″S/“ G′ 9ρ ″ s1/7770″ ,44,116

SCHWARZENBACH,G and WIDMER, I(1966):I)iC ν C″ 771 Иαα,19,111123 (I),〃 ′′

lё Slichkcit von

lctallsulfldcn II Silbcrsullld

ScoTT,S D and BARNES,H L (1971):Sphalcritc geothcrmctry and gcobaromctry,Ec・ 0″ G′ ο′,66,653-669

A Sugaki′

112

SEWARD,T M

′α/

(1976):Thc Stability of chloridc complcxcs of silvcr in hydrothcrmal solutions C,G′ οσ力′ Cο s7770ε カプ777 И ,40,1329-1341

up to 350°

``α

“

SHIKAZOヽ 0,N(1985):A comparison of thC tcmpcraturcs CStimatcd from clcctrum sphalcritc― pyritc― argcntitc asscmblagc and t11ling tcmpcraturcs Of fluid inclusions flonl cpithcrmal Au― Ag vcin― typc dcpositS in Japan,Ecο ′ G′ ο′,80,1415-1424 ー

SHoJl,T (1985):Gold― silver vein and adularia,in σο′グ s′ ′ν′′ 0″ ′s Urashima,Mining Mctall lnst,TOkyo,pp l13-132(in Japancsc)

j72 J￠ ρar2,Pα

SHoJI,T (1986): CloSC rclationship bctwCCn adularia and ruby silVcr in vcin― dcposits Of Japan,ノ И j72J4g G`ο ′,36,439-443

r′

3,Ed Y

typc gold― silvcr

SUGAKI, A (1976): A revicw of application in cconomic gcology of thc clCCtron probc microanalysis,工

Mi77′ rα i Sο ε Jttα 72,12,Spccial

lssuc,158-183(in Japancsc)

SUGAKl,A,ISOBE,K and KlTAKAZE,A (1982):SilVer mincrals from the Sanru rnine, Hokkaido,Japan,工 Jα ρα″ И SSο ι iイア71 PO′ Ec・ ο″ G′ ο′,77,65-77(in Japancse) SUGAKl,A,KITAKAZE,A and ISoBE,K(1984):On thC g01d― silvcr dcposits ofthc Koryu minc, Ⅱ okkaido,Japan,′

J《 ρα″

E`ο 72 G′ 0′

И И SSο ι ソ '77 P′

,79,405-423(in Japancsc)

`r′

SUGAKI,A,SCOTT,S D,HAYASHI,K and KITAKAZE,A(1987):Ag2S S01ubility in sulfidc solutions up to 250°

777 J,21,291-305

C,C′ ο `/7ο

TAKENOUCHl, S (1970): Fluid inclusion study by mCans Of hCating― microscopc,Mi77j77g G′ 0ノ ,20,345-354(in Japancse)

stagc and frcczing― stagc

WEISSBERG, B C,BROWNE,R P L and SEWARD,T M (1979)i Ore mctals in actiVC οs″ s24グ力′″ α′0″ 771ブ s′ 71a/‐ ″ レ″ο′ gcothcrmal systcms,in G′ ο ι `Dψ `力

Barncs,Wilcy-lntcrscicncc,Ncw York,pp 738-780

“

`グ

,Ed H L