UYTENBOGAARDTITE, Ag.Aus2, Ln the BULLFROG MINING

Home , Acanthite, Electrum, Supergene (geology)

Canalian Mineralogist Vol. 31, pp. 89-98 (1993)

UYTENBOGAARDTITE,Ag.AuS2, lN THEBULLFROG MINING DISTRICT, NEVADA

STEPHENB.CASTOR NevadaBureau of Minesand Geology, University of Nevads.Rem, Nevada 89557, U.S.A.

JAMES J: SJOBERG IJ.S.Bureau of Mines,Reno Research Center, 1605 Evans Avenue, Reno, Nevada 89507, U.S.A.

ABSTRAc"I

The rare mineral uyenbogaardtite,Ag3AuS2, has been identified in specimensfrom two mines in the Bultlrog district of southemNevada. The minesexploit gold-silver vein depositsof the low-sulfur type that arehosted by volcanicrocks of middle Mioceneage. At both mines,uytenbogaardtite occurs in local massesof high-gradeore within shallowly to moderatelydipping systemsof quartz-caxbonateveins emplacedduring late Miocene volcanismand extensionaltectonism. Uytenbogaardtite from the Bullfrog district is chemicallyand structurallyidentical to previouslydescribed synthetic and naturalAg3AuS2. It occursin associationwith two types ofelectrum: relatively coarse,early electrurn,with approximatelyequal molar amountsofgold and silver, andlate gold-rich electrum.It is also associatedwith acanthite,copper-bearing sulfides, and products of oxidationsuch as limonite and chrysocolla.Textural evidenceindicates that the uytenbogaardtitereplaced early electrum,formed contemporane- ously with late electrumand acanthite,and that its depositionmay have overlapped,in part, with that of mineralsformed in an oxidizing environment.Equilibrium relationshipsamong electrum, acanthite, and uytenbogaardtite in specimensfrom theBulllirog district seemto beconsistent with thosepredicted experimentally. On thebasis ofexperimental work by others,the uytenbogaardtite + acanthite+ electrumassemblageinthe Bullfrog districtisconsideredto haveformedatalow temperature (<113"C). Alate-stage hypogeneor supergeneorigin, following early, higher-temperaturedeposition of electrum and sulfide, is proposedfor the assemblageuytenbogaardtite + acanthite+ gold-rich electrum. Keywords:uyenbogaardtite, electrum, gold-silver vein deposits,Bullfrog district, Nevada.

SOMMAIRE

Nous avonsidentifid I'uytenbogaardtite,Ag3AuS2, espBce rare, dans des Echantillons provenant de deux rninesdu district de Bullfrog, dansle suddu Nevada.Ces mines exploitent des gisements fissuraux d'or et d'argenti faible teneurensoufre, localis6es dansdes roches htes volcaniques d'ge miocbnemoyen. Aux deux endroits,l'uytenbogaardtite se trouve dansdes amas localis6s de mineraienrichi dansdes systbmei de veinesi quartz+ carbonatei pendagefaible ou moyen,mis en placelors du volcanisme miocbnetardif et d'un 6pisoded'extension. L'uytenbogaardtite est identique,tant du point de we chimiqueque structural,aux exemplesnaturels et aucompos6 Ag3AuS2 d6crits ant6rieurement. Elle estassoci6e d deuxtypes d'6lectrum: une premibre, pr6coce et i grainsrelativement grcssiers, contient une proportion h peuprbs dquivalente d'or et d'axgent;l'6lectrum tardifest dche en or. Elle est aussiassoci6e i I'acanthite,des sulfures cuprifdres, et desproduits d'oxydation commelimonite et chrysocolle.D'aprds l'dvidence texturale,I'uytenbogaardtite aurait remplac6l'6lectrum pr&oce et serait contemporainede l€lectrum tadif et de l'acanthite;elle pourrait I'tre aussipar rapportaux min6rauxcaract6ristiques du milieu oxydant.lrs relationsd'6quilibre parmi dlectnrm,acanthite et uytenbogaardtitedans le district de Bullfrog semblentconcorder avec celles qui sont pr6ditesi partir des expdriences.A la lumibre des travaux exffrimentaux publi6s, l'assemblageuytenbogaardtite + acanthite+ dlectrumdans ces 6chantillonsse serait form6 d une faible tempdrature(<113"C). Une origine hypog6ndtiquetardive ou superg6ndtiquesemble indiqudepour l'assemblageuytenbogaardtite + acanthite+ 6lectum riche en or suiteh la d6positionant6rieure, h temp6ratureplus 61ev6e'd'dlectrum et de sulfure' (rraduit par la R6daction)

Mots-cl6s:uytenbogaardtite, 6lectrum, gsements fissuraux, or argent,district de Bullfrog, Nevada.

INTRoDUcnoN Indonesia. the Altai Mountains of Russia and the Comstocklode, Nevada(Barton et al. 1978)"at three Uytenbogaardtite,Ag3AuS2, is arare mineral.We are sitesin China (Chenet al. 1979,as reported in Fleischer awareof only eightoccurrences in theworld: in Sumatra, etaI. 1980), attheMorning Starmine, Califomia (Sheets 90 THE CANADIAN MIIIERALOGIST

et al. 1988), and at an unspecifiedgold prospect in wallrockin andnearthe veins. At theLac Bullfrog mine, Montanaflil. Fuchs,pers. cornm., 1992). Recently, we chlorite- and carbonate-bearingassemblages have been identifiedthe mineral in high-gradegold-silver orefrom reportedin sedimentaryand metamorphic rocks at depth two mines in the Bullfrog district of Nye County, (Jorgensenet al. 1989). Nevada. Veinsand breccias at the two minesare similar. Types Following the discovery of the Original Bullfrog of vein quafiz include crustiform, fine granular to mine in 1904, gold and silver were produced from chalcedonicquarz, and fine- to medium-grainedcomb severalmines in the Bullfrog district until 1940,most quartzthat is locally amethystine.The veinsalso contain notably from the Montgomery-Shoshonemine. Re- coarseto finely bladedcalcite. Rare crustiform layers of newedexploration since the mid-1970sresulted in the very fine-grainedK-feldspar are present in veins at the discovery and developmentof the Lac Bullfrog mine Lac Bullfrog mine,and fluorite andbarite also have been (originallythe Bond GoldBullfrog mine), adeposit with reported(Jorgensen et al.1989) but arerare (B. Clay- goldproduction and reserves in excessof 6l x 106g (D. bourn,pers. comm., 1991). McClure, pers. comm., 1992).Uytenbogaardtite has The paragenesisofindividual veinsappears to follow been identified in specimensfrom both the Original the same sequencefrom place to place within the Bullfrog andLac Bullfrog mines. Original Bullfrog vein system:early calcite deposited along vein walls, followed by delicatelybanded chalce- GEoLocrcALSETTTNG donic to fine comb quartz (t gold, silver, and copper minerals), followed by late comb quafiz. Many veins containonly a portion of this sequence,but high-grade The Bullfrog district, which is about 175km north- veins typically contain all three stages. In places, westof LasVegas, contains low-sulfur type (cl Bonham fragmentsthat consistofpart or all ofthis sequenceare 1989) epithermalgold-silver vein depositsthat are caughtup in fenestralbreccia quartz+ scafteredover an areaof nearly 100km2. The veins cut cementedby fine calcite. Vein paragenesis rhyolitic ash-flow tuffs and associatedrocks erupted at the [,ac Bullfrog mine is similar except betweenabout 15 and l0 Ma. Theyare associated with that bladed calcite, which is mostly replacedby fine-grainedgranular quartz, occurs in the secondaryK-feldspar that yielded K-Ar agesof 8.7 t first two stages. 0.3and 9.5 + 0.2Ma (Jackson1988, Morronet al. 1977), Ransomeet al. (1910) reported that gold in the and with extensionalfaulting that was mainly active Bullfrog district characteristicallyoccurs as electrum in betweenll.3 and7.6Ma(Weiss eral. 1990). limonite after pyrite. We found elecffumof this type at The Original Bullfrog mine exploits a complex, the Lac Bullfrog mine, but alsofound electrumin other shallowly north-dipping systemof veins that underlies forms. Electrumof compositionAuaaAgru (mole Vo, an areaof about 0.1 km2 and appearsto be truncatedat SEM-EDX analysis)occurs in quartzveins as contorted its base by post-mineralization displacementalong a flakes betweenquartz grains, and late-stage electrum of regional low-angle fault (Ransome et al. 1910, Mal- compositionAureAgal occurs as irregular grains up to 2 donado1 990). The best exposures ofthe veinsystem are mm in diameterin the matrix of hydrothermalbreccia at in a small areaof open cutsthat exposea complexmass the Lac Bullfrog mine. Specimensof high-gradeore ofveins andsilicified breccia up to about10 m thick,as well as stockwork veins that extendupward from this mass into altered ash-flow tuff. At the Lac Bulfrog mine,gold-silver TABLE 1. TRACEELEMEMT @NTENTS OF SAI'PLES FNOM mineralizationoccurs in a moderately THE ORIOINALBULLFROC AND LAC BULLFF@ II'INE9 west-dippingsystem of veins that lies along a normal fault. Ore comprisesa central zone up to ?0 m Mlne orlghal Bulfno lilno thick SA!'PTE BGBH u9 BH20 qqua BH16 within the vein system.The ore zone containsaltered Ag 21000 a 4.6 1.6 1100 16.7 1.8 '12.'l volcanicrock, hydrothermalbreccia, and abundant veins Ar $.2 2.8

TABLE2. COUPOAMONOF i'YIENBOCAARDITTEAND ELECTFUIT FFOU T}IE BULI.FROGDISTBICT

Welght% Formula(rllh 25) s Ae _.lq A! Au

12.9 51.9 35.2 2.4 0.9 72.5 27.1 BGB5D 12.5 63.0 34.8 2.5 0.9 74.7 6.3 BGB 1 13.6 54.0 V.4 2.4 0.8 75. 24.7 BGBAI (mEgoot19)t 12.6 60.8 36.6 2.4 0.9 71.7 28.3 BGB 2 12.5 U.2 3S.5 2.6 0.9 74.7 25.3 BGB A2 (atrorageot 10) t 12.2 53.9 S3.9 2.6 0.9 74.4 25.6 BGB5 12.2 66.6 31.2 2.4 0.8 76.8 4.2 BGP1B 11.7 53.2 35.1 2.7 1.0 7S.5 26.5 BGP 3E 1r.8 .63.6 9.7 2.7 1.0 73.7 4.3

BCB IA 9.7 65.3 49.2 50.8 BOB 5A 93.1 66.9 47.4 62.6 BGBS 33.0 67.0 47.3 52.7 BGBAS (avenge ol 3q I 34r.7 66.3 48.1 51.9 BGB A4 (awlagp of 23) t 35.0 66.0 49.6 50.4 BGP.IA 31.3 61.7 48.1 51.0 BGPsA 90.5 69.5 4.6 55.5 gGP30 4.1 7!.9 42.8 57.2 BGBA 34.0 65.0 €.5 51.5

BGB18 17.9 e..7 27.7 72.5 BGBP/O t 11.4 88.6 19.0 8t.O BGBP50 t 20.8 m.2 4,4 67.6

BGB 1C 17.1 82.9 27.1 72.6 BGB 58 ts.6 u2 25.5 74.6 BCB 4 16.7 84r.4 4.7 73.3 BGPSB 14.2 85.8 8.2 76.8 BGBA 5 (awngo ot 7) t 17.1 82.9 27.4 72.6

BGBA10 f 17.O 7S.S A.1 71.5

ORIGINALBULLFROG UINE

10.9 57.4 91.7 3.1 0.9 76.8 4.2 20204 t 10.7 57.0 31.7 3.2 t.0 76.6 4.2 20208 11.4 54.5 94.t 2.8 t.0 74.6 S.5 ?o?0,c 11.0 54.5 9.4 s.0 1.0 74.4 5.6 202OCt 12.2 53.6 34.3 2.4 0.9 74,O 6.0 BH2OH 11.9 54.7 3S.4 2.7 0.9 74.9 25.1 ry BH 2ON 39.2 60.8 3t.l ,15.9 BH 2OT ,tll.o 57.0 56.0 42.O BH 2OA 3!1.2 66.8 47.6 52.4 BH 2OF 42.5 '/.5 57.4 42.6 BH 2OI 40.9 69.1 55.8 44.2 BH 20F (averageof 9) | 37.4 62.6 52.2 47.8

BH20U 11.7 88.3 19.5 80.5

BH 20M 2?.2 V.9 g2 65.8 BHzOM t 21. 78.7 gr.'t 67.0

Mostt SEI&EDX data; wt. 70 normalizedto 1oo%; t denotG micmpob€ adyss; s Indudos 9.7 wt. % Cu. found in a small areas at the Lac Bullfrog and trum andcopper minerals has high copperand antimony Original Bullfrog mines contain irregular gains of contents,along with moderatelyelevated bismuth, arse- electrum associatedwith limonite, malachite,chryso- nic, lead and cadmium(Table 1). Vein quartz without colla and sulfide in crustiform quartz. At the Lac visible electrumand copperminerals has relatively low Bullfrog mine, such ore contains irregular massesof contentsof theseelements. electrum up to 3 mm in diameter disseminatedin Ore from the Lac Bullfrog mine has relatively low pillow-shapedmasses of quartz,limonite, chrysocoll4 contentsof trace metals if comparedwith epithermal and sulfide. precious-metaldeposits elsewhere in Nevada(Castor & In additionto high concentrationsof gold and silver, Weiss 1992).A sampleof unusuallyrich ore containing ore from the Original Bullfrog mine with visible elec- 0.9wt.7o gold and2.l wt.Vosilver from theLac Bullfrop 92 THE CANADIAN MINERALOGIST mineis an exception,containing high levelsof coppeq TABLE 3. COMPARISONOF C-OMPOSMONALDATA lead,antimony, zinc, selenium and tellurium (Table 1). OBTAINED BY SEM AND ELECTRONUICROPFOBE The temperatureof the low-salinity fluids associated SEITUEDx UICROPROBE with the mainstage of mineralizationat theLac Bullfrog AlAgSAuAgS and Montgomery-Shoshonemines, during which elec- E, LBM 66.5€ 32.83 66.95 3l'.4 0.01 E, LBM 81.14 16.21 &.66 16.61 0.03 trum and sulfide were deposited,was slightly above U, LBM 3t'.40 rt.00 1220 35.% 51.77 12.92 200oC,according to fluid inclusion data (Jorgensoner U, OBM 34.79 55.34 11.15 32.10 58.32 10.87 12.60 al. 1989).The maximum depth of mineralizationwas U, OBM 3it.01 59.76 11.34 35.65 55.71 about 2000 m, judging from the total thickness of Corf,entradonsin vrt. 7o. Comparedanalygea aro ot adiacgnl volcanic units that predate mineralization and occur site8 in the 6ams minoral graln. E, eloctrum;U, uylenbogaatdt'te: LBM, Lac Bulltrog mlne: OBM, OriginalBllltrog mlno. stratigraphically above host units for both the Lac Bullfrog and Original Bullfrog mines. Mineralization overlappedtemporally with extensionaldisplacement within the Bullfrog district, assuggested by radiometric dateson secondaryK-feldspar, stratigraphic and structural relationsin the district, mineralizationcontrol by The SEM-EDX analyseswere performed atthe Reno normal faults, and evidencefor fracruring and breccia- ResearchCenter of the U. S. Bureauof Mines using a tion concurrentwith vein deposition(Noble er a/. 1991, JEOL T-300 scanningelectron microscopeequipped Castor & Weiss 1992). The depth of mineralization with a hinceton Gamma-TechSystem 4 X-ray analyzer. would be lowered from the maximum indicated by Electron-microprobe(ARL) analyseswere performed at stratigraphyif extensionalthinning took placeprior to, the Twin Cities ResearchCenter of the U. S. Bureauof or during, mineralization. Mines usinga JEOL Model 733 electronmicroprobe. In both cases,pure metals and analyzedsulfide samples PROPERT'rESoF UYTENBOGAARDTIE preparedin the samemanner as the ore specimenswere FROM THE BULLFROG DIST?JC| used as standards.Compositions of adjacentsiles in grainsofuytenbogaardtite and electrum, obtained using In the Bullfrog district, uytenbogaardtitehas been both methods, are compared in Table 3. Replicate found only in ore specimensthat contain macroscopic analysesperformed at the samesite usingeither method electrum;it is generallyintergrown with electrumand resultedin consistentlylower sulfur and higher silver acanthite.On freshly polishedsurfaces, it is bright gray valuesfor successiveanalyses because of volatilization with a faint pink tint, andtarnishes over a periodof days of sulfur in the electronbeam. In the following discus- to a brassycolor. Freshly polished surfacesof uyten- sions,mineral compositions are given in mole 7o,except bogaardtiteare very similar to thoseof acanthite(which as noted. is bright gray with a faint blue tint), but the fwo minerals can be distinguishedwith difficulty when comparedin TEXTI,RES AND MINIERAL ASSEMBLAGES OF the samefield of view. Anisotropyis very weakfor both UYTENBOGAARDTTE-BEARn,IG ORE sulfides.The hardnessof uytenbogaardtiteseems to lie betweenthat of acanthiteand electrumka.2.5 on the Mohs scale)based on polishing hardness.Uytenbo- Specimensof high-grade ore from the Original grains gaardtitefrom the Bullfrog district is relatively brittle if Bullfrog mine contain irregular of uytenbo- comparedwithacanthite, asnotedbyBarton et al. (1978) gaardtiteup to 200 pm acrossand acanthite grains up to for specimensfrom otheroccurrences. X-ray-diffraction 0.5 cm in diameter associatedwith coarse electrum dataobtained on uytenbogaardtitefromtheLac Bullfrog (Au52Ag* to Au42Ag56,Table 1) up to 2 mm in mine areidentical to thosefor syntheticAg3AuS2, which maximumdimension. Uytenbogaardtite, acanthite, and is tetragonal(Graf 1968),and to uytenbogaardtitefrom coarseelectrum comprise irregular massesrimmed by - other naturaloccwrences (Barton et al. 1978). chrysocolla,malachite, unidentified antimony bismuth - On the basis of SEM-EDX and resultsof electron- copperoxide phases,and limonite. Inegular grainsof microprobe analyses, the composition of uytenbo- gold-rich electrum(Au6eAg2e) up to 30 pm acrossare gaardtitefrom the Bullfrog district is consistentwith sparsely to abundantly scatteredin uytenbogaardtite microprobe data reported by Barton et al. (1978); it adjacent to electrum in some masses,and gold-rich containsbetween 50.8 and 57.6 vtt.Vosilver. 31.2 and electrumof similar compositionforms a rim up to 3 pm 36.6 wt.Vogold, and 10.7and 13.5wt%o sultur (Table thick on coarseelectrum adjacent to uytenbogaardtitein 2). Copper,which is reportedto occurin minor amounts places (Fig. lA). In addition, veinlets of electrum in some examplesof uylenbogaardtite(Bwton et al. (AueoAg:a)up to 2 pm acrosswere found cutting both 1978,Chen et al. 1979,as reporiedin Fleischere/ a/. acanthite and uytenbogaardtite(Fig. lB). Acanthite, 1980), was not noted. Normalizedcompositions of along with tracesof electrum,also occurs in cavities uytenbogaardtiteand electrum fromthe Bullfrog district lined with quartzcrystals and in late-stageveinlets with arereported in Table 2. chrysocolla.In addition, acanthiie,limonite, malachite, TJYTENBOGAARDTTIEIN THE BULLFROG MINTNGDISTRICT, NEVADA 93

Frc. l. Back-scatteredSEM imagesof specimensfrom the Original Bullfrog mine.A. Coarseelectrum (er) witb gold-richelectrum (e, rim, uytenbogaardtils(i) with inctudedgold-rich electrumgrains. Chrysocolla (c) is intermixedwith uytenbogaardtite an? goh-rich electrum.B. uytenbogaardtiteand acanthite(a) cut by gold-rich electrumvein (E).

and tracesof electrumnll thin tabular cavitiesthat are bogaardtitedoes not accompanyacanthite that is more consideredto haveformed by the dissolutionof calcite than I mm away from coarseelectrum, although small blades. amountsof high-gold elecffum is found replacing or Some vein specimensfrom the Original Bullfrog rimming suchacanthite. Copper-bearing sulfide consists by mine containstromeyerite t chalcocilethat is generally of chalcopyriteand stromeyeritepartially replaced rimmed and partially replacedby platy covellite and chalcocite,covellite, limonite, malachite,chrysocoll4 The relationshipbetween chalcopyrite sunoundedbychrysocollat malachitet coppersulfate. andbrochanthite. in places,chalcopyrite Tiny corrodedgrainsof elecfrum(Au56Aga) werefound and sfromeyeriteis ambiguous; in othercases, stromeyerite rims included in covellite in one specimen, but uylen- rims strolneyerite,and Covellite partially replacesboth bogaardtitewas not found adjacentto copper-bearing andveins chalcopyrite. of high-goldelectrum (Aun\gza) sulfide. Grains entirely composedof stromeyeritethat phases.Filiform veins blebs of naumannite(Ag2Se with are encapsulatedby quartzalso arepresent. iccompaniedby tiny casein chalcocitereplacing In additionto copperand iron, productsof oxidation minor Cu) werefound in one oxidationgenerally rim uyten- in Original Bullfrog ore containatsenic, bismuth, lead, stromeyerite.hoducts of however, uyienbogaardtite antimony, and zinc. Theseelements were detectedby bogaardtiteand acanthite; with limonite in places SEM-EDX techniquesin extremelyfine-grained mix- and acanthiteare interlayered Bulllrog ore,very fine-grained tures of oxide and silicate minerals that could not be (Fig. 2C). As in Original in uytenbogaardtite-bearingore at identified. productsofoxidation tracemetals including lead, In specimensfrom the Lac Bullfrog mine, coarse the Lac Bullfrog mine carry electrum (Au57Aga3to Au5eAgro,Table l) occurs in antimony,and zinc. grains up to 3 mm across, and is associaledwith uylenbogaardtitein grains up to 300 pm across.The Panecpwrstsaxl PHeseEQUrmRIA uytenbogaardtiterims and seernsto replacethe coarse electrum,which has a thin gold-rich rim (Au72Ag2)in On the basisoftextural evidencefrom reflected-light many placesalong contactswith uytenbogaardtite.In microscopyand SEM imagery,the sequenceof mineral addition, the uytenbogaardtitecommonly is studded deposition in high-grade ore at both occurrencesof with tiny gold-rich grains of electrum (Au7Ag4 to uytenbogaardtitein the Bullfrog district is the same. Au1Ag27) that in some cases form a halo around Early-formedelectrum, with compositionsapproximat- includedcoarse electrum (Ftg. 2A). ing Au5s,Ag5s,was deposited during or betweenepisodes Specimensof high-gradeore from the Lac Bullfrog of deposition of comb quartz t calcite. Stromeyerite mine containacanthite in inegular massesup to 400 pm (+ chalcopyniteat the Lac Bulllrog mine) seemsto have across,as mixed grainswith uytenbogaardtite(Fig.2B), been depositedat approximatelythe sametime as this andin laie cross-cuttingveins with chrysocolla.Uyten- electrum.Uytenbogaardtite and acanthitethen crystal- 94 THE CANADIAN MINERALOGIST

lized, uytenbogaardtiteforming, at least in part, as a result of replacementof early electrum.Late, relatively gold-rich electrum (Aq6Ag26-Au7Ag27)formed con- temporaneouslywith uytenbogaardtiteas a rim on eady electrumadjacent to uytenbogaardtiteand as tiny irregular grains in the uytenbogaardtite.Contemporaneous deposition of thesephases is consistentwith stability relationsreported by Bartonet aL (1978),which indicare that uytenbogaardtiteand acanthiteform together in equilibrium with electrumthat contains20 -30 moleVo silver. An isothermalsection for the systemAg-Au-S at 100'C showselectrum with approximatelyAu75Ag25 to be in equilibriumwith acanthiteand uytenbogaardtite (Barton I 980),and the small grains ofgold-rich electrum in uytenbogaardtitefrom the Bullfrog district cluster around this composition (Fig. 3). The occurrenceof grains of gold-rich electrum in hatos about partially replaced early electrum (Fig. 24) sugges8 that the gold-rich electrumgrains are relics of emlier rims left behind as replacementof early electrum by uyten- bogaardtiteadvanced. Acanthite and uytenbogaardtile seemto havecocrystallized @g. 2B), butlate cross-cur- ting veins carrying acanthite with a little gold-rich electrum but no uytenbogaardtitemay postdate the period of depositionof uytenbogaardtite. Thereaction of AuroAgroelectrum with sulfurto form uytenbogaardtiteoacanthite, and gold-rich electrum (Au75Ag25)is predictablefrom thephase diagram for the systemAu-Ag-S at 100'C, as determinedby Barton (1980);Figure 3 showsthe phaserelationships along with compositions of the minerals in the Bullfrog district. The reactionsthat take place when sulfur is added to Auso,Agsoelectrum can be predicted from Figure3 alonga straightline connectingthe sulfur corner andAu5eAg5s. Initial additionof sulfur would movethe bulk compositioninto the acanthite+ electrum field. Extraction of silver from elecfium to form acanthire would cause the composition of electrum to move towardthe gold corner.When sufficient sulfur is added, the bulk composition can be expectedto reach the

Frc. 2. Photomicrographsof uytenbogaardtitefrom the l,ac Bullfrog mine.A. ReflectedJightimage of coarseelectrum (e1)rimmedby uytsnbogaardtite(u) containingsmall grains ofgold-rich electrum(q) arrangedin zonesthat areroughly parallel to uytenbogaardtite- coane electrumcontacts. B. Back-scatteredSEM imageof intergrownuytenbogaardtite (u) and acanthite(a) with minor gold-rich electrum. C. Back-scatteredSEM imageofcoarse electrum (el) partially rimmed by uytenbogaardtite(u) and malachite(m). Thin bandsofuytenbogaardtite (u) andacanthite (a) areinterlay- eredwith limonire (L). UYTENBOGAARDTITE IN TIIE BULLFROG MINING DISTRICT' MVADA 95

Ag Au A

Frc. 3. A. Compositionsofuytenbogaardtite and electrum from the Bullfrog district(Table 2) plottedon the isothermal_section for the systim Ag-Au-S at 100"C,adapted from Barton(1980): uytenbogaardtite (O); coarseelectrum (A); smallelectrum grainsin uytenbogaardtite(I). B. Compositionsof uytenbogaardtitefrom thet ac Bullfrog mine(O) andthe Original Bull- irog mine(O) plottedon a portionof Figure3A. Theoreticaluytenbogaardtite composition: x.

acanthite-€lectrumjoin, and uytenbogaardtitewould earlyelectrum (atAu56Ag5e), following 8Aue.5Ago.r + S, begin to form, in equilibrium with acanthiteand elec- = 4Auo.usAgo.z,+ Ag3AuS, and conservingboth gold trum (Au75Ag25).Continued addition of sulfur would and silver, is approximately40:60 (usinga densityof causethe bulk compositionto enterthe three-phasefield uytenbogaardtiteof8.3 g1cm3:Barton et al.1978)-Ina acanthite+ uytenbogaardtite+ electrum (Au75Ag2), few cases,the amount ofgold-rich electrum in uyienbo- with the proportionof the phaseschanging. Eventually, gaardtiteseems to approachthis amount(Fig. 1A)' but acanthitewould disappear,and other phases could form. our examinationsindicate that most uytenbogaardtite Evidence from natural assemblagesin the Bullfrog gtains contain much lower amountsof gold-rich elec- district indicate that the amount of sulfur remained trum (Fig. 2A) and, in some cases,none at all. The sufficientlylow for acanthiteto persist,or that silver was formation of variable rnixtures of uytenbogaardtitet addedalong with sulfur. The agreementbet'ween phase acanthitet gold-rich electrum is consistentwith the relationships observed in Bullfrog district ore and addition of variable amountsof silver and sulfur to predictionsfrom phasestudies is remarkable,but may Au56Ag5e. be fortuitous consideringuncertarnties inherent in such Equations can also be written for the reaction of predictions. electrum,acanthite, and sulfur that will producerela- Mass-balancecalculations show that the conversion tively large volumesof uytenbogaardtite(e.8- 10Ag2S of early electrum to uytenbogaardtiteand gold-rich + 36Auo.5Age.s+ 7S2 = \2Ag:AuSz + 8Au0.75A90.25, electrumin ore from the Bullfrog district was accompa- producingabout 70 vol.7o uytenbogaardtiteand-30 nied by the infoduction of both silver and sulfiu. The vol.7o gold-rich electrum). However, we did not find expectedvolume-ratio of a mixture of gold-rich elec- early-formedacanthite in ore from the Bullfrog district, trum (at Au75Ag25)and uytenbogaardtite produced from such as that reportedby Barton et al. (1978) in uyten- 96 T}IE CANADIAN MINERALOGIST

bogaardtite-bearingspecimens from Indonesiaand the uytenbogaardtite,indicate that both silver and sulfur Comstocklode. were added during the formation of uytenbo- As shownby Barton (1980),addition of sulfur rc gaardtite. Other elements that seem to have been gold-rich electrumat 100'C would result in the forma- enriched in uytenbogaardtite-bearingore include tion of uytenbogaardtiteand AuAgS (Fig. 3). The laner lead, antimony, and bismuth (Table 1), which mainly phase,recently discoveredin Kazakhstanas a rim on occurin productsof oxidation,as shown by SEM-EDX native gold and namedpetrovskaite (Nesterenko el a/. scans. 1984,as reported inDmnet al.1985),has subsequently Hypogenefluids were undoubtedlyresponsible for been found with uytenbogaardtitein gold ore from the initial deposition of electrum and stromeyerite( Montana(W. Fuchs,pers. comm., 1992).Measured chalcopyrite)1n the Bullfrog district, possibly at tem- compositionsof silver-gold sulfide from the Bullfrog peraturessimilar to thoseindicated for quartzdeposition district do not deviatesignificantly from the theoretical (ca.200"C). Uytenbogaardtitemay haveformed during compositionofuytenbogaardtite (Frg. 3), anda sulfide the waning stagesof hydrothermalactivity, whendepo- with a compositionapproaching that of petrovskaitewas sition of acanthiteand chalcocitewas taking place.The not found. formation of gold-rich electrum(70-80 mole Vo Ati rn Replacementof stromeyerite+ chalcopyriteby chal- the Bullfrog disrict would be due to the partial equili- cocite t naumanniteand by covellite may have taken bration of early-formedelectrum (ca. 50 mole Vo Au) place concurrentlywith depositionof uytenbogaardtite with uytenbogaardtiteand acanthite(cf. Banon et al. and acanthite.Electrum veinlets that cut acanthite+ 1978). A similar relationshipbetween early electrum, uytenbogaardtite(Fig. 18) or chalcocite have gold late-stageelectrum with high gold contents,acanthite, contentssimilar to thoseof the inclusionsof high-gold andcovellite in ore that may containuytenbogaardtite is electrumin uytenbogaardtite(Table 2) but mustpostdate ascribedto hydrothermalprocesses at the Mondng Star sulfide deposition(at least in the enclosinggrains). depositin southernCalifornia (Sheetset al. I988).T-he Mineralsthat formed underoxidizing conditions(limo- productsof oxidation in ore from the Bullfrog disficq nite, chrysocoll4 malachite,and brochanthite) generally suchas limonite and chrysocolla,could haveformed as appearto have postdatedsulfide deposition,forming a result of in sita leachingof sulfide or transportfrom rims aroundsulfide electrum.However. intimate inter- higherlevels. mixturesof uytenbogaardtitewith productsof oxidation, An alternative supergeneorigin for the uytenbo- including gold-rich electrumthat seemsto have been gaardtite+ acanthite+ gold-rich electrumassemblage is depositedon uytenbogaardtitewith chrysocolla @g. suggestedby texturalrelationships that indicateoverlap 1A) and crustiform interlayersof acanthiteand uyten- with mineralizationproduced under oxidizing condi- bogaardtitewith limonite (Fig. 2c), suggestthat the tions in the Bullfrog district. In addition,the presence of depositionof uytenbogaardtite,acanthite, and gold-rich chalcocite and covelliie is consistentwith supergene electrumoverlapped with oxidizing conditions.Deposi- depositionof sulfide, althoughtypical supergeneforms tion of late silic4 calcite, and electrumas components (sooty chalcocite and blaubleibender covellite) are of hydrothermal breccia matrix clearly followed the absent.Since proposedby Emmons (1912) for the deposition of early electrum and copper-bearingsul- Bullfrog district and other areas,supergene enrichment fides, but the temporalrelationship between the forma- ofprecious-metaldeposits has been supported by others tion of hydrothermalbreccia and uytenbogaardtiteis (e.9.,McKay 1944,Webster & Mann 1984,Stoffregen unknown. 1986, Moeller 1988). An uytenbogaardtite-bearing assemblagein a specimenfrom Russiais consideredto DIScUSSIoN have been,at leastin part, of supergeneorigin (Barton et al. 1978). Mobilization of gold during supergene Data on fluid inclusions in quartz veins from the processesis ascribedto thepresence ofcomplexing ions, Lac Bullfrog mine suggestformation at temperatures particularly thiosulfate(Webster & Mann 1984),which of about 200'C (Jorgensonet al. 1989). However, is formed by the breakdownof pyrite in environments the work of Graf (1968) indicates thar uytenbo- in which pH of the fluid is maintainedat relatively high gaardtitecan form in equilibrium with acanthiteonly levelsbythepresence of calciteQ-alanet al.1974). Such below ll3oC. In addition,the lack of polysynthetic conditions are met in vein systemsof the Bullfrog twinning in acanthitesuggests formation at temperatures district, which contain vein calcite as well as pyrite in below 140'C (Taylor 1969). On the basis of this aliered wallrock. However, thiosulfate complexesare information and the textural data presentedabove, the relatively unstable, and Stoffregen (1986) proposed assemblageuytenbogaardtite r acanthite+ gold-rich redepositionof gold at Summitville, Colorado,within electrumin the Bullfrog district representsa relatively metersor centimetersof the initial site of gold dissolu- late and low-temperature stage of mineralization. tion. In the Bullfrog district, redepositionof gold as Mass-balanceconsiderations, along with tle presence uytenbogaardtiteapparently occurred within millime- of significant amountsof acanthitethat seemto have ters or micrometersof the site of dissolutionfrom earlv beendeposited, at leastin part,contempoianeously with electrum. UYTBNBOGAARDTITE IN T}IE BULLFROG MINING DISTRICT, NEVADA 97

ACKNOWLEDGEMENTS JAcKsoN, M.R., JR. (1988): The Timber Mountain Magmato' Ther'mal Event: an Intense Widespread Culmination of Hydrothemnl Activity at the Southwestern The writers areindebted to MichaelL. Boucherof the Magmatic anl NevadaVolcanic Field.M.Sc. thesis,Univ. Nevada, Reno, Twin Cities ResearchCenter of the U.S. Bureau of Nevada. Mines for performingthe electron-microprobeanalyses. Brian W. Claybournof Lac Bullfrog, Inc. provided ore JoRGENsEN,D.K., RANKn{,J.W. & WrlKtrrs, J., JR.(1989): The specimensand information on the Lac Bullfrog mine. geology, alteration, and mineralogy of the Bullfrog gold The managementof Angst, Inc. and Lac Bullfrog, Inc., deposit, Nye County, Nevada. Soc. Min. Eng., Preprint are thanked for providing accessto mine workings. 89.13s. FrankW. Dickson of the University of Nevada Renois thankedfor patientdiscussions about phase equilibria in Lerw, H.W.,Cunrnt, G.C. & Hurenr, A.E. (1974):Geochem- goldinthe cycle.U.S. Geol. Sum., BulI. sulfide systemsand for a review of this paper.We also istryof weathering 1330. benefittedfrom reviews by StevenI. Weiss, Dhanesh Chandra andJonathan G. Price, University of Nevada" MALDoNADo,F. (1990):Structural geology ofthe upperplate Reno,MarkD. Barton,University of Arizona andJames of the Bullfrog Hills detachmentfault system,southem R. Craig, Virginia PolytechnicInstitute and StateUni- Nevada.Geol. Soc. Arn BuIl.102,992-1006. venity. McKav, R.A. (1944): The purity of native gold as a criterion REFFRENCES in secondaryenrichment. Ecoa Geol. 39,56'68.

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