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The Canadian M ine ralo gist Vol.34, w. s29-s46(1996)
THEDISTRIBUTION AND MINERALHOSTS OF SILVER IN EASTERNAUSTRALIAN VOLGANOGENIC MASSIVE SULFIDE DEPOSITS
DAVID L. HUSTONI Centrefor OreDeposit a.nd Explnration Sndies, Uniyersity of Tasmania G.P.O.Box 252C, Hoban, Tanmania 7001, Australia
WIESLAW JABLONSKI CentralScience Laboratory, University of Tasmania.G.P.O. Box 252C, Hoban, Tasmania 7001, Australia
SOEYH. SIE CSIRODivision of ExplorationGeoscience, P.O. Box 136, North Ryde, NSW 2113, Australia
ABSTRA T
Silver, an importantby-product in volcanogenicmassive sulfide (VMS) depositsof easternAus[ali4 is emichedmainly in Zn-rich zones;in a few depositsit is enrichedin Cu-rich zones.Minerals that containsignificant a.mountsofAg include galena, tetrahedriteand chalcopyrite.The contribution of Ag sulfosalts,native silver and Ag tellurides to the total Ag budget is generally small. The hosts of Ag in VMS deposis vary spatially as follows: (1) in Cu-rich zones, Ag occurs mainly in chalcopyriteor Birich galena;(2) in overlying Zn-rich zones,Ag occursmainly in galenaand to a lesserextent, tetahedrite; (3) in barite-bearingzones, Ag occws mainly in Ag-rich tetraheddte.The geochemicalfactors that seemto influence the mineralogical distribution of Ag in VMS depositsinclude: (1) temperature,(2) the reLativeabundances of semi-metalsin fte mineralizing fluids, (3) fractional crystallization of tsfahedrite-tennantiteminerals, and (4) redox conditions during ore deposition.Higher t€mperatuesand more reducedconditions favor partitioning ofAg into chalcopyrite,and then galena.Silver partitions into tetrahedriteunder lower temperature,oxidized conditions,assisted by fractional crystallization,hence enriching later-precipitatedtetrahedrite in Ag and Sb.
Keywords:silver, mineral hosts,volcanogenic massive sutfide deposic, eastemAustralia.
SorrnueRe
L'argent, important produit secondairedans I'exploitation de gisementsvolcanog6niques de sulfures massifs,est surtout concenf6 dans les zones enrichies en Zn. D:ns quelquesgisements, il est plutdt associ6aux zones cuprifdres.Parmi les min6raux qui renfermentde quantit6simportantes d'argent figurent galbne,t6tra6drite et chalcopyrite.Les contributionsdes sulfoselsargentifbres, I'argent natif et les tellururesde Ag au bilan global de I'argentsont mineures,en g6n6ral.l,es min6raux hOtesvarient dansI'espace dans ce type de gisementcornme suit (1) dansles zonesenrichies en Cu, on trouve l'axgentsurtout dansla chalcopy'riteou la galdnebismuthifdre; (2) da.* les zonessutrt'rieures enrichies en Zn, l'argent se trouve surtout dans la galbneet i un degr6moindre, danstra tdtra6drite; (3) dansles zonesn barite, I'argent se fouve surtout dansla t6tra6drite argentifdre.Parmi les facteursg6ochimiques qui r6gissentune telle distribution seraienfl(1) temp6rature,(2) abonda:rcerelative des semi-mdtauxdans la phasefluide, (3) cristallisation ftactionn6edes min6raux du groupe de la t€tra6drite-tennantite,et (4) conditionsd'oxydation au coursde la formation du minerai.Aux tempdraturesplus 6lev6eset dansles milieux plus fortement r6ducteurs,I'argent favorise La chalcopyrite, et ensuitela galbne.Aux tempdratuesplus faibles et dansles milieux plus oxydants, I'argentest davantager6parti dansla t6tra6drite;un facteur secondaire,la cristallisationfractionn6e, mbne i un enrichissement de I'argentet de I'antimoinealans la t6taddrite tardive. (Iraduit par la R6daction)
Mots-cl6s:axgen! min&arxr h6tes,gisements volcanog6niques de sulfuresmassifs, Australie orientale.
I Presentaddress: Australian Geological Survey Organisation, G.P.O. Box 378, Canbena ACT 260I, Australia.
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TABII 1. MINBRA!)CICAL DTSTRII'LINON OF SILVER IN VOI..CANOCEMC INTRoDUCT'roN M.{SSIVB SULF'IDE DBPOSNS
Of the major metals recoveredfrom volcanogenic Mob@ Quebe PFib:0.614@ ppm (t = 42 Ppn). 1 (VMS) Chal@I,FiE: 1-200 ppm (x = 29 PPn). massivesulfide deposits,Ag is uniquein that it Iaklike,I.IWT GrlcopyriE:-500ppd(T6Toofbbleilvs). occurs mainly as a trace or minor element in ore Galew 0.u74297o (9/' of bhl silver). minerals.As Ag is recoveredas a by-productin Pb or Kidd Cr@k, oatrio 7d-tich (\sl@ptrivn4-25ffi (s = 1047ppq n = Q. Cu concentrates.extensive research has been under- ll6iw PFi@: <7-2! ppn (n = 14; 10 b€Iry deHi@). sdfidc Gal@87-1167ppm(x=638ppun=E). taken on bulk samplesand metallurgical concentrates Sphalerirc:<123{tr (n = U;7 bclowdetecliotr). Othq nimls: uiw silwr, @thitg bEahcdritg sEphmitc, to determineits mineralogical hosts (e.9., Henley & pyfrr8)ldte, p€eie, ois.gtrnc dd dymir. Glrich ClElq}Pydto: x = 1E3 ppa; 59.6Vool Ag, Steveson1978. Jambor & Laflamme 1978. Chen Il6ieo Ild@:
FIc. 1. Cross sectionsshowing the geology of the (A) Ml ChalmersWest l,ode (modified from Large & Both 1980), and @) Roseberynorth-end orebody (modified after Huston & Large 1988).
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Mt Chalmers Silver gradesare highest (50-100 g/t) in the Zn-rich upper portion of the massivesulfide lens. In the Cu- The Mt. Chalmers deposit contains of two Cu- rich lower part of the massivesulfide lens and in the gold-rich massivesulfide lensesthat occur abovewell- stringer zone, Ag grades are 5-20 ppm. The disri- developedpydte - chalcopyritestringer zoneswithin bution of Ag correlatesbest with those of Zn and Pb the Permian Beserker Beds in central Queensland (Large& Both 1980). (Large& Both 1980,Taube & van der Helder 1983; Fig. lA). As Mt. Chalmersis one of the leastdefonned Rosebery VMS deposits in Australia, primary textures are commonly preservedin the ores. Massive sulfide and The Zn-Pb-rich Rosebery deposit consists of a stringer ore are dominatedby pyrite and chalcopyrite. seriesof sheet-likemassive sulfide and barite-bearing Sphalerite, galena and barite occur in the upper lenses within a lens of fine-grained tuffaceous portions of the massive sulfide lenses, along with sandstoneand siltstone above an extensive zone of minor tennantite. Electrum is a trace Ae-bearine pyrite-bearing qtuartiz- sericite t chlorite-bearing mineral. alteredfelsic volcaniclasticrocks in the CambrianMt.
F.€Terrlgenous sandstones and conglomerates [SlArgllllteand chert Ffi Graywacke [i] Felslcvolcaniclastic rocks Fro.2. Crosssections show- ing the geology of the I Massivesulfi de/barite (A) Waterloo and WPyrltlc quartz'serlclteschlst (B) Agincourt deposits t-I N Pyritlc sericiterquartzschist (modified after Berry 50m [i]Andesite et al. 1992).
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ReadVolcanic Suite of westernTasmania (Brathwaite Cu-rich massive sulfide has a much lower average 1974, Grenn et al. L98l). Samplesanalyzed in this grade,lOglt Ag. study were collected from the north end of the mine, which contains a lower lens of Zn-Pb-rich massive Waterlooand Agincourt sulfide and an upper barite-bearinglens. The base of the Zn-Pb-rich lensescontains local zonesof Cu-rich Waterloo (Fig. 2A) and Agincourt Grg. 2B) are fwo massive sulfide (Fig. 1B). Common ore minerals small barite-bearingVMS deposits in the Carnbro- include pyrite, sphalerite, galena and chalcopyrite. Ordovician Seventy Mile Range Group of northern Other Ag-bearing minerals include tetrahedrite, Queensland@erry et al. L992).The Waterloo deposit electrum,pyrargyrite, miargyrite and acanthite. is relatively Zn- and Cu-rich, but Pb-poor, and the The distribution of Ag strongly correlateswith that Agincourt deposit is Zn- and Pb-rich, but Cu-poor of Pb, with the highest grades (>500 g1t) occurring Qluston et al. L995a). Major ore and Ag-bearing most commonly in barite-bearingzones (Brathwaite minerals include sphalerite, pyrite, chalcopyrite, 1969, Huston & Large 1988). On the basis of galena,tennantite and electrum at both deposits,and production assays from the l5-level fan-drilling hessiteand petzite at Waterloo. Waterloo has higher program (conducted in the early 1980s), the Ag grades(20-300 glt) than Agincourt (5-60 g/0. The barite-bearing zone and Zn-ich massive sulfide distribution of Ag correlateswith that of Pb, Cu and have similar average gades at 160 glt, whereas gold at both deposits.
lgmmE 23oonE DRy RTVERSOUTH 5300mN
ll llcranlte *porphyryhl OuarE-feldspar Undlfterentlated masslvssulfldo
sulfidelPbgossan
sultlde Quartz-chlorlte schlst quartz. BALCOOMA _-,..ff-r"es-- 9000mN Pyrltlc muscovlteschlsi, showlngst|ingers :ffi Quartz-muscovlte- + + + btotlte schlst + + + + + * + + + + + F^arlFelslcvolcanlc + + + + rocKs f;:DA + + + Pelltlcschlst + + + + Metagraywacko {s:3: +
Ftc. 3. Cross sections showing the geology of the (A) Dry River South o lo "rjlQq0ooo0oo ooo0000 and (B) Balcooma METEFS gooooo roooo deposits(modified after Htsstonet al. L992.a).
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Dry River Southand Balcooma lenses,the distribution of Ag correlatesonly with Pb. Silver gradesin Cu-rich zonesare 5-50 g/t. In Cu-rich Dry River South and Balcoomaoccur in the lower- lenses,the distribution of Ag has a strong correlation to middle-amphibolite-faciesCambro-Ordovician with that of Cu. and a weakercorrelation with that of Balcoomametamorphic suite of northern Queensland gold andPb. (Huston et al. I992a). The Dry River South deposit (Fig. 3.{) consi$tsof a sheet-likepolymetallic massive Hosrs or SnvsR sulfide lens along the contact between an extensive zoneof pyrite-bearingquartz-muscovite schist (altered Minerals in which Ag is an essentialcomponent are volcanic rocks) and hanging-wall turbiditic metasedi- not major hosts of silver in most VMS deposits.As mentaryrocks. Major ore mineralsat Dry River South tetrahedrite-groupminerals, galena and chalcopyrite include pyrite, sphalerite, chalcopyrite and galena. are tle most common major hosts of Ag (Table 1), Silver-bearing minerals include tetrahedrite, Sb this study emphasizedAg contents in these three sulfosaltsand elecffum (hrstonet al. 1992a).Both Dry phases. River Southand Balcoomalack barite. At Dry River South,silver is enrichedin Zn-Pb-rich Analytical techniques zonesthat typically grade 70-140 Clt AC. In Cu-rich zones,Ag gradesare typically 20-50 glt. The distribu- The CSIRO proton microprobe was used to tion of Ag correlateswith that of Pb and gold through- determine levels of Ag in pyrite, sphalerite and out the deposit,but with that of Cu in Cu-rich zones. chalcopyrite.Relative to the electron microprobe,the Balcoomaconsists of five massivesulfide bodies proton microprobehas the advantageof lower limits of in a pelitic lens within a metaturbiditic sequence detection,but the disadvantageof a larger sampling (Frg. 3B). Sulfide lensesoccur at three stratigraphic volume. In the minerals analyzed,the effective depth positions, with Cu-rich lenses occupying the inter- of analysisis 30-40 pm, and the spot size is typicaly mediate position. Copper-rich zones contain pyrite, 20 pm. For Ag, typical detection-limits (3o) are chalcopyrite, magnetite and pyrrhotite as major ore 5-15 ppm for all three minerals.Detailed descriptions minslals; Ag-bearing minerals include sphalerite, of analytical methodsare summarizedby Sie & Ryan galena,tetrahedrite and electrum (Huston et al. L992a). (1986) and Huston et al. (L995b). The results of Silver gradesat Balcooma are highest in Zn-Pb-rich proton-microprobeanalyses are summarizedin Huston massive sulfide lenses (50-150 g/t). Within these et al. (1995b).
TABLB 2. ABUNDANCE OF SILVER INIMPORTANT SILVER.BEARING MINERAI-S TetEheddte Ctalcryydlel Sphaleritel Raoge Mm n Raoge Mm tr Rmge Mean o Rmgg l\4ean (E) (v") (r'| (r'") (ppn) (ppn) GPn) (!en) Mt Clalmsrs 18 <0.07- 2.49 0.22 44 0.03- 0.69 0.x 22 <10 - 80 1',7 10 <10 - 30 13
Aghoun IO 2.01- 76.734.17 t6 0.03- 0.08 0.05 30- 110 60 'Wal€dm ?a <0.01- 0.25 0.07 0.04- 0.24 0.08 s <8-30 10 4 <15- 30 n Rosebery Cu-rich sningen 5 0.85- 1.07 0.96 5 150- 200 170 Zo-rich maslve suliide2 61, 1.00- 3356 8.56 5v 0,09- 0.67 4.24 5 mo-470 ar9
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Electon-microprobeanalysis was usedto determine the concentration of Ag and related elements in Rosebery Ronob*zed electrum,tetahedrite and galena.Electron-microprobe analysisof galena was used in preferenceto proton- a o.a microprobeanalysis because it offers better detection 9? limits for Bi. Bismuth M lines are difficult to resolve ctt from Pb M lines on energy-dispersionspectr4 hence < 0.2 E'I the detectionlimit of the proton microprobeis of the order ofseveral thousandppm. Conversely,the use of 0.t wavelengtl-dispersionspechometers on the elecfion microprobeallowed detection limits of severalhundred ppm. Sb/(Sb+As+Bl) Elecfron-microprobeanalyses were done using the University of Tasmania'sCameca SX50 microprobe. Software supplied by Cameca @ouchou & Pichoir Agincourt 1984)was used for analysisand datareduction. Prior to o,4 analysis,potential interferences involving the lines of = interest were determined,and potential problemswith o.s I? interferenceswere avoided by adjusting background ctl offsets or using background-slopecorrections. Y 0.2 C't Tetrahedrite 0.1 Minerals of the tetahedrite-tennantiteseries occur oB oS 0.0 in all depositsexcept Balcooma.Table 2 summarizes 0.0 o.2 0.4 0.6 0.8 l.o the abundanceof Ag in tetrahedrite-tennantitefrom Sb/(Sb+As+Bl) Rosebery, Mt. Chalmers, Agincourt and Waterloo. 4. showing the relationship between (i.e., Frc. Scattergrams Tetrahedritefrom Cu-poor Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/3/529/3435587/529.pdf by guest on 26 September 2021 DISTRIBUTIONOF SILYER. AUSTRALIAN YMS DEPOSITS 535 X Badtlc masslvo aullldo +Zn{ich maoolvg 8ulfldg O Cudch maslvs rulida Mt Chalmers Rosebery Agincourt Waterloo Zn-rlch Dry RiverSouth Balcooma FIc. 5. Triangulardiagrams showing the relationshipsamong silveq antimonyand bismuth (in relative atornicproportions) in galenafrom the Rosebery,Mt. Chalmers,Dry River Soutl, Balcooma"Agincourt and Waterloodeposits. All dataare from this study. sphalerite- galena- pynte and barite-bearingzones. compositionsplot closer to the Ag apex along the At Balcooma" galena from Cu-rich massive sulfide Ag-Sb join, and at Dry River Soutb and Balcoomq lensesand sfinger zonescontains higher levels ofAg where many of tle data points (mainly from Cu-rich than galena from Zn-rich massive sulfide zones. zones)plot closerto the Bi apexalong the Ag-Bi join. Although lesswell defined,a similar relationshipholds Galena from Cu-poor deposits (Agincourt and at Dry River South. Rosebery)generally is Sb-rich,relative to Bi, whereas Figure 5 shows triangular diagrams relating the galena from Cu-rich depositstends to be Bi-rich. A atomic abundancesof Ag, Sb and Bi in galena.In all similar relationship is present in other deposits. At six deposits,most dataplot suchthat Ag approximates Rosebery,Dry River Southand Balcooma, galena from the proportion of Sb + Bi. Deviations from this Cu-rich zones is Bi-rich. Conversely, galena from ggneralization occur at Agincourt, where some Zn-ich zonesis more Sb-rich"although Birich galena Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/3/529/3435587/529.pdf by guest on 26 September 2021 536 TITE CANADIAN MINERAI,OGIST Galena Chalcopyrite Ag (wt %) Sb+Bl (mole 7") Sb/(Sb+Bi) Ag (ppm) *T.'----T0.0 0.4 0.8 1.20.0 0.4 0.8 1.2 0.OO.2 0.4 0.6 0.8 1.0 10 t OU.- tr, r+o--t o E = 3 flrocks EBarhlc massivesulflde/ sphalsrlte- pyrite- tootwall ffiHost -rilassivo ffiMassive -chalcopyritelrrlasstue XlAltered barit€ *galena-pyrite -volcanic rocks, showlngslringers Ftc. 6. Variations in silver, totd Sb + Bi and Sbi(Sb + Bi) in galena and silver in chalcoplrite in drill hole R3397, which passesthrough the Roseberynorth-end massive sulfide lens and barite zone. also is present, especially at Dry River South. At (Sb + Bi) whereSb/(Sb + Bi) exceeds0.2, a significant Rosebery, galena from barite-bearing zones is positive correlation (r = 0.66; n = 32) is presentin generally more Bi-rich than galena from massive barite-bearingsamples from Rosebery. sphalerite- galena- pyrite zones. hevious analytical data on Ag contentsof galena In core from drill hole R3397 at the Rosebery exist only for Rosebery.Henley & Steveson(1978) deposit (Fig. 6), galena from stinger zones and the reported that galena from Roseberyaverages 0.107o basal part of the massivesphalerite - galena- pynte Ag, which differs from the averageof 0.2LVofor all zoneis extremelyBi-rich relative to Sb; the relative Sb galena compositions determined in this study. contentincreases into the massivesphalerite - galena- However, our samples were not collected in a pyrite zone, and then decreasessignificantly in the representativemanner; hence the results are biasd barite-bearingzone. The differencesin Sb(Sb + Bi) particularly by the collection of extremelyhigh-grade occur within an overall trend of decreasingSb + Bi, samFles.Samples of massive sphalerite - galena - which closely mimics a trend o1 4ssls6sing Ag pynte in which Ag grades exceed 350 gt contain contents. Ag-rich galena(0.16-O.23Vo). If thesesamples, and Figure 7 illustratesthe relationshipbetween Ag and samplesfrom Cu-rich zones,which are not mined, are Sb(Sb + Bi) in more detail. In all deposits except excluded,the averageAg concentrationof galena is Agincourt (which lacks Bi-rich galena;not shown),all 0.l6Vo, closer to the value determinedby Henley & data points plot along a curvilinear trend in which the Steveson(1978). Ag contentis low (<0.2 mol.Vo)and relatively constant Huston et al. (1992a) estimated that galena in for Sb/(Sb+ Bi) above0.2. Below 0.2,the Ag content Zn-rich zonesat Balcoomawould contain0.l2%o Agrf. increasesrapidly to values that corffnonly exceed galenahosted all Ag. This value is closeto the average 0.5 moLVowhere Sb(Sb + Bi) is less than 0.05. 0.l5%ocontent for galenafrom Zn-rich massivesulfide Althougl Ag is generally not correlated with Sb/ zonesat Balcooma. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/3/529/3435587/529.pdf by guest on 26 September 2021 DISTRIBUTION OF SILVER, AUSTRALIAN VMS DEPOSITS 537 Mt Ghalmers 1.2 7.2- A6rllo hasrlvs sslfd6 i Zn.rlch naoslv. oolfid! I 1.0 t.o S o.e b{ o.o + () g + = 0.6 9 o.e E E clt 0.4 ED 0.4 fr, .g:s+ 9.2 o.2 o.o L o.o L 0.0 0.0 Sb/(Sb+Bi) Balcooma 1.2 1,2 1.0 1.0 !R o,s bRo.a g g 9 o.s 9 o.e E E cD 0.4 ED0.4 o,2 o.2 o,o L 0.0 o.2 0.4 0,6 0.8 1.0 o,4 0.6 Sb/(sb+Bl) sb/(Sb+Bi) FIc. 7. Scattergramsshowing the relationships between silver and Sb/(Sb + Bi) (atomic proportions) in galena from the Rosebery,Mt. Chalmers,Dry River Southand Balcoomadeposits. All data are from this study. Chalcopyite chalcopyrite (<10-120 ppm). A small number of determinations in material from Dry River South Table 2 summarizes the results of proton- suggeststhat chalcopyritefrom Cu-rich zonesis more microprobeanalyses of chalcopyritefrom all deposits Ag-rich (55-120 ppm) than that from Zn-rich zones exceptAgincourt. The maximum concentrationof Ag (<15 ppm). Chalcopyrite from the Cu-rich Mt. in chalcopyrite grains was found to be 470 ppm, Chalmers and Waterloo deposits has relatively low although most analysesindicated less than 100 ppm. levels of Ag (mainly <30 ppm). Chalcopyritefrom the Theselevels are similar to valuesreported at Mobrun stringerzone at Mt. Chalmershas a higher averageAg (Larocque et al. 1995) and Kidd Creek (183 ppm; content(20 ppm; 'hot detected"values taken as 5 ppm; Cabri 1988), but lower than values reporled at Izok MDL = 1G-15ppm) than chalcopyritein the massive Lake (500 ppm; Harris et al. 1984), and much lower sulfide lens (13 ppm). than values reported for the Hilton sediment-hosted massivesulfide deposit(2000 ppm, Haris et al. 1984). Sphnkrite The most Ag-rich chalcopyriteoccurs at Rosebery, where chalcopyritefrom pynte - chalcopyritestringer Owing to the gleat abundanceof sphaleritein many and massivesphalerite - galena- pydte ores contains VMS deposits,even low levels of Ag in sphalerite 150-470 ppm Ag. In contrast,chalcopyrite from one could accountfor a large portion of the silver budgets. barite-bearingsamFle contains less than 20 ppm Ag However,half of atl determinationsof Ag in sphalerite (Frg. 6). The averageof all determinationsat Rosebery, were below the detectionlimit of 15-10 ppm, and a 220 ppm, compareswell with an averageestimated significantly higherproportionof the determinationsin for Roseberymill feedby Henley& Steveson(1978), material from Balcooma,Mt Chalmersand Rosebery 180ppm. were below this limit. Most determinationsfrom Dry Balcooma shows the next highest levels of Ag in River South, Agincourt and Waterloo exceededthe Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/3/529/3435587/529.pdf by guest on 26 September 2021 538 TIIE CANADIAN MINERALOGIST detectionlimit. Even in depositswith detectableAg in observedin oneinstance. At Rosebery,trace quantities sphalerite, the values ari mainly below 30 ppm. of pyrargyrite occur in Ag-rich samplesin association Agincourt hasthe highestlevels of Ag in sphalerite;the with Ag-rich tetrahedriteand miargl'rite, and acanthite maximumconcentration was found to be 110ppm, and occurs as a rare trace mineral in massivesphaleriie - ttre average is 60 ppm. These results suggest that galena- pyrite ore at Rosebery. sphaleriteis not a major host of Ag, exceptpossibly at Agincourt. QUALnATTVEMtr{ERALocIcAL BaraNcB oF STLvER Other major ore minerals To illustrate the importanceof various hosts of Ag amongand within dspcils, Table 3 showsthe results As pyrite is the most abundantore mineral in most of qualitative mineral-balance calculations. The VMS deposits,it also could host a significant portion calculationsused compositional data derivedfrom this of Ag. Huston et al. (1995a) analyzedpyrite grains study (Table 2), and grade data either from the from all six deposits.They recorded Ag contentsto literature, or calculated for different ore-types (cfl 460 ppm, but suggestedthat this results from Rosebery and Balcooma) using diamond drill-core inclusionsoftetrahedrite or galena,as the level ofAg assaydata. The resultsare inherently qualitative owing commonly is correlatedwith concentrationsof Sb and to the nature of data collection. If 'hnusual" samples Pb. The concentrationof Ag in the pyrite structurewas are excluded, the calculations sugge$t several inferredto be lessthan 5 ppm, the detectionlimit of the interestingdifferences both amongand within deposits. proton microprobe. Ofthe deposir and oretypes considered, in only one @oseberybarite-bearing samples) does the total Ag Silver minerals accountedfor by significant Ag-bearing minerals amountto the estimatedAg grade(*l0%o). Two others In addition to electrum,which was observedin all (Mt. Chalmersand Balcooma7ll.-ncQ arewithtn25Vo deposits,two of the depositscontain small amountsof of the Ag grade.The calculatedmineral-balances for Ag minerals. At Waterloo, hessite occurs as a Ag of these three deposits or ore types probably moderatelywidespread trace mineral, and petzite was approximatethe true mineral-balances,given the large TABI,E 3. QUAIJI.{TIVE ESTMATES OF TIIB MINERALOGICALBAIANCE OF SILVER IN SOMEEASTERN AI]STRALIAN VMS DEPOSTN Dspsit lem@ Galma3 Oalmpydtea Sphaledtd Electrqm6 Ag Sb1 Irdoq{ AS l\.fneral AC Mioeral AC Minqal Ag Mi!sEl A€AgAs lo orc in ote abudle budgpt ab@dm brdget abqda€ butget abudre budget abqrdre lerel bodge! boder (gt) (ppe) (E") Glt gd Gh) (E4 Gh) (E") (zl\) (pFm) g") GD (v"), Mt oralEes 44 <50 <0.086 <1.9 r.2 29 4.9 0.8 o.7 2.7 14 0.4 74:79 Walerlm 94 0.14-2 0.7-1-4 11.0 1.1 5.9 2.4 17 0.4 Roebo4f Tajjcb 1,62 242 0.13 47 9.7 247 1.2 2.6 34.8 3.8 43 33 1.4 186 oasdw sllide 162 282 0.11 42 9.7 109 I.2 2.6 34.8 3.8 33 t.4 98 Badb+earhg 150 3.0 <0.1 3,4 1.0 106 06$ire sllide Dry Rlver So[t! 3.9 152 2,0 14.0 J5 13 L6r-n6 Bql@ma9 Zt-rlch t' 43 61 2.7 1., L2,8 13 0.79 n 0.2 1A Q-rich 21 - 0.092 3.7 10.6 4,9 0J9 <0.1 059 n o,2 43 I Tte Mt. CbalDeE dimate is a composlte aalysis oI all oa q?s (H!$m et al., 1992b). The Roebery simat* re calcolated froo avenge Ag Nys using the stlug @reldim betuq sb md Ag (ct SEfih ed Sqstm, 199a. Ihe Dry Rirer Soqth stiEde ls a wighred average of Sb melyss of dlill @ ffiive sulde wpler (Hu$m, 198$. 2 Te;aledrie Eioeral abudns wrc stinated ftoe rsidml Sb levels (i.e. toral Sb le$ Sb itr galaa) ud avefge ualyss of brahedrite. Fo! the Watqloo deposi! tbe tFnlndre abud@F w6 esdnared lom ftio wdm. 3 Galma abudams wen delemioed ftoo avqage tead mys. Two budgets fpr galena werc estlmated for Rosbert msire sphalorltlgal@a-pydb oF. Io botb stlmatq! @alyss fron highly Ag-dch mpl€s verc exclud€d. The finc sdmate usd all ofiq da&, wbeM tie s@d e$imate also excluded Bldcb galoa + Cbalopyrtb ab@d&e wrc deE@ited &om @age Co Nys. ) Sphaledb abudEs w debmhed from areEge Za Nys. o Elm abudme ed Ag mqmdoG io electrum w deremhed ftom megs Aq Nys, elecum finas data nd cyadde lachiog dara (Hsto ot a1.,1992b). / Tolrl Ag bldget expB$ed m a pemage of rhe remge Ag Ny. 6 Asy val@s us,d b mibale Dheml abud@s fm Rosebery ole typs 8e avemge valB of eys ftoD ddll boles €ll{ed @ 15 lwel in Oe north€nd orebodlB. Tho Nys we Nigled to !h* @ type bas€d m @mpuy &ill logs md m lqgging by tie mior author ' Asy valres used b e6doe niqqal abudes for Bal@ @ typs e mEge valG of w)6 ft@ddl bols driled betwm 1978 md 1986. Tbe Nys w asdgoed m tytrs bed @ @Epey dri[ log$ md 6e loggiryby t[e miu aitlo. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/3/529/3435587/529.pdf by guest on 26 September 2021 DISTRIBUTION OF SILVER. AUSTRALIAN YMS DEPOSITS 539 uncertaintyof the calculations. sphalerite- galena- pyrite. The proportion estimated For massive sphalerite - galena - pyrite ore at by Henley & Steveson(1978;65Vo) lies betweenthe Rosebery and at Dry River South, the Ag budget proportions estimatedfor the two ore types in this calculatedfor major Ag-bearingminerals exceedsthe study. true gradeby more than 50Vo.Tlis resultsmainly from At Balcooma chalcopyriteis the predominantAg the inclusion of anomalouslyAg-rich galena in the host in Cu-rich massivesulfide, but it is insignificantin calculations.As discussedabove and summarizedin 7n-rich massive sulfide ore. In contrast, galena is a Table L, the Ag content of galena from these two significantAg hostin both typesof massivesulfide ore, depositsis highly variable.A few "atypically''highAg but evetrmore importantnZn-ich massivesulfide. concentrations significantly bias the final average value. When analyical results for Ag-Bi-rich galen4 Other deposits which is resticted to the lower portion of the massive sulfide lens. are excluded from the data set for Silver is enrichedin the upperand lateral portions of Roseberymassive sphalerite - galena- pynte ore, the most deposits,mainly in associationwith Zn and Pb averageAg contentdecreases from 0.27Voto 0.1.IVo, [e.g., Mobrun: Larocque et al. (7993), Hellyer: and the total Ag budget decreasesfrom l86Vo to 98Vo McArthur & Dronseika (1990), Hokuroku disfrict: (Iable 3). Shimazaki (1974)1. An imFortant exception is the In the Waterloo deposit and Balcooma Cu-rich Millenbach deposit(Knuckey et al. L982),in which Ag lenses, major Ag-bearing minerals account for less occursmainly with Cu in the pyrrhotite-richcore of the than 50Voof the total Ag, suggestingthe presenceof massive sulfide lens. Perhapsthe most complicated other major hosts of the Ag. At Waterloo,hessite and distribution occursat Kidd Creek,where Ag-rich zones petzitecould accountfor the 6SVo Ag deficit. The siting occur in both the Zn-ich tops and Cu-rich basesof of the unaccountedAg at Balcooma is problematic. massivesulfide lenses,and also in the stringer zones Hustonet al. (1992a)suggested that chalcopyritehosts (M. Hanningtonand E. Koopman,pers. comm. 1994). Ag at a level of 140 ppm owing to an excellent The highestAg gradesoccur in a bomite-rich zonettrat correlationbetween concentrations of Ag and Cu, but has locally replacedchalcopyrite-rich ore. this study suggeststhat chalcopyrite containsoon In most VMS deposits,galena and tetrahedriteare average,only 46 ppm Ag. Hence, either the chalco- the main hosts of Ag (Iable 1), althoughchalcopyrite pyrite analyzedin this sfudy is unusuallyAg-poor, or is a significant,if not dominant,host of Ag in Cu-rich some other mineral accountsfor the Ag. A possible zoneswithin the Izok Lake (Ilarris et al. 1984),Kidd candidate is pyrrhotite, which can contain up to Creek (Cabri 1988), and Pyhasalmi(Helovouri 1979) 100ppm Ag (Fleischer1955, Cabri et al. L985),andis deposits. Deposits in which tetrahedrite is the common in Cu-rich zones. Alternatively, Ag could predominant host of Ag outnumber those in whicb occur in Bi sulfosaltsin Cu-rich zones. galena is the predominanthost. In particular, tetra- hedrite dominates in the Mt. Read and Bathurst Comparisonsamong and. within deposits disricts, in which the deposi* are rather Cu-poor. Conversely,galena is tle domin4l host in the more Mineral-balance calculations suggest that the Cu-rich deposits.Deposits in which Ag mineralshost a mineralogical distribution of Ag differs among and significantportion of the total Ag arenot that common. within deposits. Except for the Waterloo deposit, Native Ag accommodatesa significant portion of the Roseberybarite-bearing ores, and Balcooma Cu-rich Ag at Kidd Creek (Cabri 1988); Ag sulfosalts are massivesulfide ores,galena is the main host of Ag. In significant hosts at Heath-Steele (Chen & Petruk BalcoomaCu-rich zones,chalcopyrite is the dominant 1980). host, whereasin Roseberybarite-bearing ore, teta- hedrite is the main host. Tetrahedriteis a significant Gnoloclcar- em Grocnmnclr Conrnors host of Ag only in Cu-poor deposits. In contrast, ON TIIE DISTRIBUTION OF SILYER tetrahedrite-tennantiteaccounts for at most 4Voof. the tr{ VoIcANocEI\[cMessnu Suflle DEPosrrs total Ag in Cu-rich deposits at Balcooma, Mt. Chalmersand Waterloo.Except for BalcoomaCu-rich Although in most VMS deposits, Ag undergoes lenses,chalcopyrite, sphaleriteand electrum account zone refinementlike Pb and 7no the detailed mecha- for small fractions (mainly 4Vo) of the Ag. The nisms of Ag enrichmentare complicated,owing to the Waterloo deposit is unique in that hessite is the large variability 1t 16e mineralogical hosts of Ag. probablemajor host of Ag. Factors that may affect its spatial and mineralogical Mineral-balance calculations at Rosebery and distribution include: (1) the temperature,pH andredox Balcooma also suggestintradeposit variations in the state of the hydrothermalfluid, (2) the speciationof distribution of Ag. At Rosebery,tetrahedrite accounts Ag, Sb and Bi in the hydrothermalflui4 and (3) the for more than 907o of Ag in barite-bearingmassive solubility of Ag in sulfide mineralsand the effect of Sb sulfide ore, but for only 25Voin barite-free massive and Bi on that solubility. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/3/529/3435587/529.pdf by guest on 26 September 2021 540 Pltysicochemicalparameters of volcanogenicJluids than AgSbS2, These solubility relationships led Amcoff (1984) to proposethat coupledsubstitution of Owing to detailed paragenetic,isotopic and fluid- Ag with (mainly) Sb or Bi in galena controls tle inclusion studiesof ancientVMS depositsover the past distribution of Ag in VMS deposits. The overall twenty years,the physicochemicalparameters of ore- relationshipin galenaof Ag = Sb + Bi (in atoms per forming fluids have beenwell constrained(e.g.,I-,arge forrnula unit) observedin this study is consistentwith L977, Green et al. L987, Pisutha-Amond& Ohmoto the model of Amcoff (1984),but the overall control on 1983, Ohmoto et al. 1983). The most important Ag is more complicated,as tefrahedriteis also a major parametersthat affect Ag contents, distribution and Ag host in many YMS deposits. mineralogy include temperatureopH, salinity, HrS Studies of the composition of tetrahedrite- concentrationsand redox state. tenwrntitein Ag-rich epithermalvein (e.9., Hackbartl & Petersen1984) and VMS deposits(e.9., Jambor & Speciationof Ag, Sb and Bi in volcanogenicfluids Laflamme1978, Miller & Craig 1983,Ramsden et a/. 1990) demonstrateda strong positive correlation In volcanogenic fluids above 200oC, Ag is betweenAg and Sb contentsof tetrahedrite-tennantite. transported as chloride complexes (mainly AgCl2; Hackbarth & Petersen(1984) proposeda "fractional Seward 1976). Antimony is transportedby a neutral crystallization" model for the covariations of Ag hydroxyantimonite complex [Sb2S2(OH)2]below with Sb. By assuming that Cu and As fractionate 275'C (Krupp 1989)and a neutral hydroxide complex preferentiallyinto tennantiterelative to the fluid, they lSb(OfD3l above275"C (Wood er al. 1981).Bismuth duplicatedvariations observed at gain and ore-deposit is probablytransported as a neutralhydroxide complex scales: early or proximally precipitated tennantite is [Bi(OH):; Wood et al. 1987].In all cases,a decreasein enrichedin Cu and As, whereaslater or more distally temperatureor an increasein HrS concentrationcauses precipitated tetrahedrite is enriched in Ag and Sb. precipitationof metals.Changing pH has no effect on Experimentalresults at 500"C wereinterpreted by Sack Sb and Bi solubility, but increasing pH causes & Loucls (1985) to be consistentwith the fractional precipitationof Ag. crystallizationmodel of Hackbarth& Petersen(1984). For a 350oCvolcanogenic fluid, the solubilities of The effect of fractional crystallization may be Ag (relative to AgrS) and Sb (SbrSr) are 23-40 ppm enhancedby a (crystal-chemisrycontrolled?) correla- and 340-1900 ppm, respectively(calculated from tion of Ag/(Ag + Cu) with Sb/(Sb+ As), noted in ttre compositionaland thermodynamicdata mentionedin electrum-bufferedexperiments of Ebel & Sack(1989). the above two paragraphs).Hence, it is unlikely that Fractional crystallization seems to be an important Ag or Sb were saturatedin higher-temperaturefluids control on the composition of the tetrahedrite- that formed Cu-rich ores. As a consequence,both Ag tennantite-groupphase. and Sb wereprobably dissolved from the Cu-rich cores Of the threemajor hostsof Ag in VMS deposits,the of the depositsand moved outward to the upper and crystal chemistryof Ag in chalcopyriteis leastknown. lateral portions, where they precipitated as the fluid Experiments by Prouvost (1966) showed that quenchedwhen it mixed with seawater.This process, chalcopyritecan contain up to 5-8 wt%oAg in solid oozone termed refining" @ldridge et al. 1983), also solution, probably substituting for Cu. However, in appliesto Pb andZn, hencethe commonassociation of nature, levels of Ag in chalcopydteare much lower. Ag wifh Pb in the lateral and upper portions of VMS Sandecki(1983) recorded up ro l.8Vo Ag in chalco- deposits. pyrite from the GrapenburgNorra depositin Sweden, but in most deposits the amount of Ag is below Crystal-chemicalcontrols on Ag solubility in 2000ppm (e.9.,J.F. Riley in Knights1983, Harris er a/. major ore-formingminerals 1984, Cabri et al. L985, Larocque et al. L995; thts study). Cabri & Harris (1984) suggestedthat the high Silver occurs as a trace or minor element in ore- concentrationsreported by Sandecki(1983) probably forming minerals in VMS deposits;thus the crystal result from Ag diffrrsion that forms a thin film of Ag2S chemistryof theseminerals plays an important role in at the surfaceof chalcopyriterather than the true Ag determiningthe abundanceof Ag in host minerals,but contentof the chalcoppte (cf.Chen et al. 1980). that role is not uniform. Although the solubility of Ag2S in galena is A modelfor the concentrationof Ag in VMSdeposin insignificant below 500'C (Van Hook 1960), signifi- cant amountsof Ag can dissolvein galenaby coupled Amcoff (1984) presenteda model for the distri- substitution with Sb or Bi. Above 200'C, AgBiS2 bution of Ag in YMS depositsin which he emphasized forms a complete solid-solution with galena (Van the importanceof coupled substitutionof Ag with Sb Hook 1960), and above 390'C, AgSbS, forms a or Bi in galena. According to his model, a small complete solid-solution with it (Amcoff 1976). At fraction of Ag precipitates in chalcopyrite af high temperaturesbelow 390'C, AgtsiS2 is more soluble temperature(>300"C), but most Ag precipitates in Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/3/529/3435587/529.pdf by guest on 26 September 2021 DISTRIBUTION OF SILVER, AUSTRALIAN VMS DEPOSITS 541 galem; higher temperaturesfavor coupledsubstitution Reaction (3) indicates that Ag fractionatesmore into with Bi ratherthan with Sb. Although Amcoffls (1984) chalcopyriterelative to galenafrom reduced,Bi-poor model is consistentwith observationsin depositsin or H2S-poor fluids. A limiting factor in the above which galenais the dominanthost to Ag, it does not reactionsis the abundanceand stability of galena.Most account for the disnibution in deposits where tefta- ArcheanVMS depositsare Pb-poor, and galenais only hedrite is an importanthost. a minor to trace mineral in parts of Cu-rich zonesin On the basisofdata from this and previousstudies, many Phanerozoicand Proterozoicdeposits. we havedeveloped a geochemicalmodel for the spatial andmineralogical disnibutions of Ag in VMS deposits. Conditionsof oreformation This model extends the original model of Amcoff (1984) to include depositsin which galenais not the In addition to the well-documenteddifferences in majorhost. temperaturesof deposition among VMS ore types, there may also be differencesin redox conditions.The Chernicalfactorsthat affectpanitioning of Ag among best evidencefor redox variations within depositsis major hosts the conunonpresence ofbarite in the upperportions of Proterozoic and Phanerozoic VMS lenses, but its Although not precisd describing the partitioning absencein the lower portions of the same deposits. of Ag, these reactions illustrate the most important Additional mineralogicalevidence for redox variations controls: include changesin iron - sulfur - oxygenminerals and in the FeS content of sphalerite. Pyrrhotite occurs 8AgSbS2(ss) + 12CuFeS,+ 4ZnS+ mainly at the baseof the Millenbach deposit(Knuckey 6H2S(aq) *3O2= et al. L982),and trace quantitiesofpynhotite occur in ZCu6AEFnzSbaStu+ 12FeS, + 6HrO (1), Cu-rich zonesobut not in overlying zonesat Rosebery (Huston & Large 1988). At Rosebery, the FeS AgSbS2(ss) + Bi(OH), (aq)= contentof sphaleriteis commonlyhighff (l-1 4 mol.Vo) AgBiS2(ss) + Sb(OH)3(aq) (2), in barite-free massive sulfide zones, and lower (4 mol.Vo)in badte-bearingzones (Green et al. L981, and Khin Zaw & Large 1996). Green et al. (198L; Rosebery) and Ramsden et al. (1990; Hellyer) 8AgFeS2(ss) + 8Bi(OID: (aq) + observedthat the FeS content of sphaleritedecreases 16H2S(aq) *2O2= from the Cu-rich baseto the Zn-rich top. Thesedata 8AgBiS2(ss) + 8FeS,+28HrO (3). suggestthat in well-zoned deposits,Cu-rich ores tend to be most reduced,whereas barite-bearing zones are In all reactions,AgSbS2 (ss) and AgBiS2 (ss) occur as most oxidized. solid solution in galen4 and in reaction (3), AgFeSt (ss) occun as solid solution in chalcopyrile.knportant The madel confiols on Ag pafiitioning includetemperature orrilJzs, mSb(OII)/ntsi(OlI)1, and the redox condition of the On the basis of the abovedifferences in the condi- fluid. tions of ore formation, likely reactionsgoverning the Reaction (1) suggests that mH2S, redox and partitioning of Ag, and observationsof the miner- temperatureof the fluid confiol partitioning of Ag alogical occlurenceof Ag, Figure 8 illusfratesa model betweenSb-rich galenaand tetahedrite. As nH2S is for the concenfiationof Ag in YMS deposits. fairly constantin VMS systems(cf. Lwge L977), the At the thermal peak of ore formation, higher main controls appearto be redox and temperature.At temperature(300'-350'C), relatively reduced Cu-rich constanttemperature, Ag is partitionedinto tetrahedrite fluids interact q/ith the baseof a pre-existingmassive undermore oxidized conditions.In addition, empirical sulfide lens. The fluids dissolve and replace 7n-rich evidence suggeststhat Ag is partitioned into tera- oresto form Cu-richores @)didge et aL.1983).During hedrite at lower temperature. this process,there is a net gain of Ag in the fluid phase, Reaction (2) suggeststhat the mZBilmZSbratio of as the Ag gradesof Cu-rich zonesare generallylower the fluid determinesthe dominant semi-metal with than those of Zn-rich zones.This results mainly from which Ag enlersinto lir:ked substitutionin the structure the dissolution of tetrahedriteand galena. However, of galena. At temperatues below 275"C, where someAg is retainedin chalcopyriteand relict galena. hydroxyantimonite complexes predominate over Sb The temperatureand redox condition of the fluid hydroxidecomplexes, increasing nH2S favors coupled probably govem the proportion and mineral siting of substitutionwith Bi: Ag retained in Cu-rich zones.Reaction (3) suggests that chalcopyrite will be the preferred host of Ag Sb2S2(OlI)2(aq) + 2AgBiS2 (ss) + 4H2O= relative to galena under reducing conditions. In 2B(OII)3 (aq) + 2AgSbS2(ss) + 2H2S(aO ( ). addition. reduced conditions are more favorable for Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/3/529/3435587/529.pdf by guest on 26 September 2021 542 THE CANADIAN MINERALOGIST Fluid Oxidizedfluids Reducedfluids Temperafure >AsDsb with low with hloh ratio )B/)Sb ratios )B/)Sb r-atios Low Low Enrichmentof tetrahedritein Ag and Sb due to fractional crystallizationor chanoesin High tempe-rature. High High Flc. 8. Schematicdiagram showing hypothesized controls on the distribution and mineralogyof silver in volcanogenicmassive sulfide deposits. precipitationof Ag into chalcopyritefrom the fluid: The relative importance of Sb or Bi in coupled substitutionwith Ag in galenadepends on the XBiDSb 4AgFeS2(ss) + 8Cl- + 4W + Or= value of the hydrothermalfluid. High XBiDSb values $gclz- + 2HrO + 4FeS2 (5). favor coupled substitution with Bi. Because Bi generally is concentratedin Cu-rich zones and Sb This may account for the higher Ag content of generally is concentratedn Zn-rJ.chzones (Smith & chalcopyrite in reduced depositsrelative to oxidized Huston 1992),the XBiDSb valuein this fluid decreases deposits,and for the decreasein the contentsof Ag in as it passesthrough the massivesulfide lens. Coupled chalcopyriiefrom the relatively reducedCu-rich to the substitutionof Ag with Sb is more imFortant toward relatively oxidized barite zones at Rosebery. Silver the top (Fig. 6), but as Sb-richgalena cannot dissolve may be concentrated in the Cu-rich core of the as much Ag as Bi-rich galen4 Ag concenfations in Millenbach (Knuckey et al. 1,982)and enriched in galena decreasetoward the top of ore lenses.Silver- chalcopyriteat Izok Lake (Harris er al. 1984)because andBirich galenais concentratedin Cu-rich zonesand the reducedconditions of formation, as imFlied by the the lower parts of Za-rich zonesowhereas Sb-rich abundanceofpyrrhotite, may havefavored retention of galenawith lower Ag contentsis concentratedin the Ag in chalcopyriteduring zonerefining. upper parts of Zn-rich zones and in barite-bearixo In most other deposits,Ag has been leachedfrom zones. Cu-rich zonesand depositedin galenaor tetrahedritein As the temperaturecontinues to decreaseand the Zn-ich upper and outer zones.As the fluids cool and fluids become more oxidize4 argentian teilahedrite become more oxidized, Ag initially precipitates in becomesmore important according to reaction (1). galena, mainly by coupled substitution with Bi. Incorporationof Ag into tetrahedriterather than galena Reaction (1) implies that galenais preferredas a host also may be favoredby evenlower XBiDSb valuesin over tetrahedrite under reducing conditions, and the fluid. Valuesof XCuDAg andXSbDAs in the fluid empirical evidence suggeststhat galena, particularly also influence the importanceof tefahedriie as a host Bi-rich galena, is favored over tennantite at higher to Ag (Hackbarth & Petersen1989, Sack & Loucls temperafure. 1985). Fractional crystallization of tetrahedrite- Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/3/529/3435587/529.pdf by guest on 26 September 2021 DISTRIBUTION OF SILVER. AUSTRALIAN VMS DEPOSITS 543 tennantite, and chalcopyrite and arsenopyrite sulfosalts in Ag-rich, Zn-ich, barite-free massive precipitation in the lower portions of ore lenses, sulfide samplesat Roseberymay result 1to6 similar increase both ratios such that the crystallization of processes. Ag-rich tetrahedriteis favored in the upper portion of Lastly, Waterloo is an unusual VMS deposit ore lenses. becauseof the presenceof significant quantities of telluride minerals.This implies that the Waterloo ore Exceptionsto the madel fluids were enriched in tellurium, which favored precipitationof hessiteover otherAg-bearing minerals, The abovemodel for the distribution of Ag in VMS particularly as the paucity of Bi minerals and the low depositsaccounts for most of the depositssu-marLed Bi contentsin galenasuggest low levels of Bi in the in Tables I and 2. The most important exceptionsare mineralizingfluids. Kidd Creek.Heath-Steele and Waterloo. At Kidd Creek,native silver is a significant host in CoNCLUSIoNS chalcopyrite-richzones (i.e., "N' or4, and perhapsin Za-rjch zones (Cabri 1988). In the bornite zoneo Data from this and previous studies indicate the naumanniteis a siglificant host of Ag (t\orpe et al. following generalized distribution of Ag in 1976).Although pyrite and pyrrhotite areboth present volcanogenicmassive sulfide deposits: pynhotite is more common in Cu-rich stringer zones (1) Ag is generally enrichedin the Zn-rich, upper than Zn-ich massive sulfide zones (Walker & portion of the deposits. In a few deposits, Ag is Mannard 1974). T\e following reactionstnay govem enrichedin the Cu-rich, basalportion. stability of native silver in equilibrium with Ag-bearing (2) Galenaand tetrahedriteare the most important chalcopyrite: hostsof Ag in most deposits.Tetrahedrite is commonly most important in the uppermost, barite-bearing AgFeS2(ss) = 4g * P"t, (6), portions,whereas galen4 particulady Bi-rich varieties, is commonly more important deeper in the lenses. and Chalcopyriteis a significanthost to Ag at the baseof a few deposits. 2AgFeS2(ss) + 2HrO=2Ag+ (3) In a small proportion of deposits,Ag sulfosalts, 2FeS+ 2HrS + O, (7). telluride mineralsor nativesilver areimportant hosts of Ag. For oresrich in pyrite relative to pyrrhotite,reaction (6) The partitioning of Ag between the three major suggeststhat the stability of native silver dependson mineral hosts is probably controlled by temperature, the solubility of Ag in chalcopyrite.In conftast,in ores fluid redox conditions, fractional crystallization of richer in pynhotite, reaction (7) indicates that redox tetrahedrite-tennantite.and >Di/ZSb valuesin the fluid also is important: native silver is favored by more phase.High temperatureand reducedconditions favor reducedconditions. Hence, one possiblereason for a the partitioning of Ag into chalcopyrite,followed by significant proportion of native silver, particularly in galena. Silver is partitioned into tetrahedrite under Cu-rich stringerzones, is the reducedcharacter of the lower temperatureand relatively oxidized conditions. ores. As an alternative,the presenceof native silver In mature VMS deposits,upwelling hydrothermal may result from metamorphicrecrystallization. fluids leach Ag from the baseof the lens during zone At Heath-Steele,New Brunswick, a large portion of refining, although a portion may be retained in the total Ag residesin Ag-Sb sulfosaltsand acanthite, chalcopyrite,particulady in more reduceddeposits. As and acanthite is commonly intergrown with native the fluid passesthrough the sulfide lens and cools, Ag antimony (Chen & Petruk 1980). Chen & Peruk precipitatesinto tetrahedriteor galena.Lower XBiDSb (1980) atso found that galena contains little Sb valuesin the fluid phase,lower temperaturesand more (<500 ppm), andthat Ag in galenawas takeninto solid oxidized conditionsfavor tefahedrite over galenaas a solutionby coupledsubstitution with Bi. In the absence host for Ag. Consequently,galena is favored as a host of significant coupled substitution of Ag and Sb in in the lower part of Zn-rich zonesowhereas t€trahedrite galen4 possibly as the result of low temperaturesof is favoredin barite-bearingzones. deposition,reaction (1) suggeststhat Ag-Sb sulfosalts [e.g., miargyrite (AgSbS)] are stabilized relative to ACIOIOWIjDGH\/a.ITS argentiantetrahedrite under reduced conditions. Heath- Steeleand other VMS depositsin the Bathurstdistrict DLH was supportedduring this study by an ARC are characterizedby the very rare occrurenceofbarite Australian Postdoctoral Research Fellowship. This (It4cCutcheonI992)a hence, a low temperatureand communicationwas written during DLH's tenure as a relatively reducedconditions during the formation of Visiting Fellow at a CanadianGovemment Laboratory the Heath-Steeledeposit may have stabilized Ag-Sb at the Geological Survey of Canada.Final revisions sulfosalts. By analogy, the presence of Ag-Sb were completed at the Australian Geological Survey Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/3/529/3435587/529.pdf by guest on 26 September 2021 5M TIIE CANADIAN MINERALOGIST Organisation. Analytical costs were supported by an Cr*.yssours,S.L. & SuRGF,s,L.J. (1988):Behaviour of tetra- ARC Grant and by a CSIRO - University of Tasmania hedritein mill circuits of Brunswick Mining and Smelting - Collaborative Grant. Don Harris and Ralph Thorpe Corporation, Ltd. /z Silver Exploration, Mining and Treafinent.Inst. Mining Metall., Iondon, U.K. reviewed an early version of this manuscript. The Q05-21,6). comments of Louis Cabri and Mark Hannington EsnL,D.S. & Sacr, R.O.(1989): Ag-Cu andAs-Sb exchange improved the final version substantially. energies in tetrahedrite-tennantitefahlores. Geochim. Co smo chim. Acta 53. 2301,-2309. RtrERENCEs ELDRDcE,C.S., BARroN, P.B., Jn. & Onrraoro,H. (1983): AMcoFF,O. 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