89 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 <l 355 1.6 7.7 Au EA 452 4.2 3.4 117 1.9 0.1 in stockworks and sheetedswanns. The ore zone is Ca 4415 6t.4 37.6 38.4 5160 60.8 121 generallybounded by faults that separateit from sparse Hg <0.10 <0.10 <0.10 <0.10 <0.10 <0.t0 Mo 8.8 2.3 z'l 3.2 8.t 2,2 4.5 vein stockworks above and below (Jorgensenet a/. Pb 381 I 8 8 91 3" 8 1989). High-gradegold ore is mainly composed sb a6 0.4 0.5 0.4 4g'4 16.6 ZO of n 4.5 <0.25 <0.2i <tr.?s 0.8 hydrothermalbreccialying along the bounding faults (8. 2n 179 o 27217 Claybourne,pers. comm., 1991). This brecciaconsists Bl 0.4 <0.25 <0.23 19.1 <0.25 1.3 cd 0.42 <0.10 <0.10 <0.t0 1.€ <0.10 <0.10 of vein andwallrock fragmentsin matricesof cnrstiform Ga <0.5 <u.c 4.c a.c to spongyquartz t calcite. Ss 173 <1 <1 <1 <1<1d To '12 0.7 <0.5 <0.5 <0.5 Alteration assemblagesnear the veins at both mines Type 1 2 3 4 134 consist of K-feldspar + + albite illite. This alteration CorEentadorEoxpreoo€d ln ppm. Analy8€sby ICPemissbn spett@py. gradesoutward into illite + calcite + quartzt albite t Roc&ty!€: I - Hlgh{rade ore containhgol€clrum, uylonbogaardno, ard copperrrdnemls: 2 - Qua|? veh wth ma@pic