L37 The Canadian M inerala g i st Vol.33,pp. 137-151(1995) GOMPOSITIONAND OCCURRENCEOF ELECTRUM ATTHE MORNINGSTAR DEPOSIT, SAN BERNARDINOCOUNTY, GALIFORNIA: EVIDENCEFOR REMOBILIZATION OF GOLD AND SILVER RONALD WYNN SIIEETS*, JAMES R. CRAIG em ROBERT J. BODNAR Depanmen of Geolngical Sciences, Virginin Polytechnic h stitate and Stale (Jniversity, 4A44 Dening Hall, Blacl<sburg, Virginin 24060, U.S-A,. Arsrnacr Elecfum, acanthiteand uytenbogaardtite have been examined from six depthswithin the tabular quartzt calcite sockwork and breccia-filled veins in the fault-zone-hostedMorning Star depositof the northeasternMojave Desert, Califomia. Six distinct types of electrum have been identified on the basis of minerat association,grain moryhology and composition. Two types, (1) p1'rite-hostedand (2) quartz-hostedelectrum, occur with acanthite after argentite and base-metalsulfide minerals in unoxidized portions of the orebody; the remaining forr types, (3) goethite-hostedelectrum, (4) electnrm cores, (5) electrumrims and (6) wire electrum,are associatedwith assemblagesof supergeneminerals in its oxidizedportions. Pyrite- hosted quartz-hostedand goethite-hostedelectrum range in compositionfrom 6l ta 75 utt.7oAu and have uniform textures and no zoning. In lower portions ofthe oxidized ore zone, electrum seemsto replacegoethite and occursas small grains on surfacesof the goethite.Textural evidencefavors supergeneremobilization of Au and Ag, which were depositedas electrum on or replacinggoethite. This type of electrumis identical in appearanceand compositionto prinary electrum,In the upper portions of the oxidized zone,secondary electum occursas a gold-rich rim on a core of elechum and as wire-like grains,both with acanthiteand uytenbogaardtite.Such secondaryelectrum contains from 78 to 93 wt./o Au. Textural relations and asso- ciated minerals suggestthat the primary electrum was hydrothermally depositedand partially remobilized by supergene processes. Keywords:electrum, gold (Au), silver (Ag), uytenbogaardtite,supergene, remobilization, Moming Star deposit,Califomia. SovtrvIARE Nous avons examin61'61ectrum, l'acanthite et l'uytenbogaarditeproveuant de six niveaux dans le stockwerk i quaru t calcite et le systdmede brbchesen veines du gisementde Moming Star, mis en place da:rsune zone faill6e dansle secteur nord-est du d6sertde Mojave, en Califomie. Il s'y trouve six types d'6lectrum, identifiables par l'association de min6raux, la morphologiedes grains et leur composition:(l) dlectrumpi6g6 dansla pyrite, (2) 6lectrumpi6g6 dansle quartz, ces deux types associ6si l'acanthiteremplagant l'argentite et aux sulfires desm6taux de basedans les partiesnon oxyd6esdu gisement, (3) 6lectrumassocid d la goethite,(4) noyau des grains,(5) bordure des grains et (6) 61ecfiumen filament, ces quttre demiers associ6saux assemblagesde min6rauxsupergbnes dans les partiesoxyddes du gisement.L'6lectrum dansla pyrite, le quartzou la goethite a une teneur en Au compriseentre 6l *75Vo (Wids) et une texture uniforme, sanszonation chimique. Dans les partiesinf6rieures de la zone oxyd&, 1'6lectum sembleremplacer la goethite,et est dispos6een petits grains sur les surfaces de celle-ci. D'aprbs l'6vidence texturale,il sembley avoir eu remobilisationsupergdne de Au et Ag, et ddpositionsous forme d'6lectrum sur (ou en remplacementde) la goethite. Cette g6n6rationsecondaire d'6lectrum possEdeune apparenceet une compositionidentiques i l'6lectrum primaire. Dans les partiessup6rieures de la zoneoxyd6e, l'dlectrum secondairese pr6sente sous forme d'un liserd de bordure sur un coeur d'6lectrum, ou bien en filaments, les deux en associationavec acanthite et uytenbogaardtite.L'6lectrum secondairea une teneur en Au entre 78 et 93Vo(poids). D'aprbs les relations texturaleset les rnin6raux associ6s,l'6lectrum primaire a 6t6 ddpos6dans un milieu hydrothermal et a 6t6 partiellement remobilis6 pax processussupergbnes. (Traduit par la R6daction) Mots-cl6s:€lectrum, or (Au), argent(Ag), uytenbogaardtite,supergbne, remobilisation, gisement de Moming Star, Califomie. * Presentaddress: Depaxment of Geology, 1-26Earth ScienceBuilding, University of Alberta Edmonton,Alberta T6G 2E3. 138 TTIE CANADIAN MINERALOGIST Iltnorucnon approximately7.3 million metric tonnes of ore aver- agng 2.057 g/tonne(Ausburn 1988).The mine was Chemical models used to describethe distribution operatedintermittently in this century, with the last of gold and silver minerals between oxidized and episodeof mining occurringfrom 1987to mid-1991; unoxidizedportions of gold depositsremain equivocal heap-leachextraction of gold and silver continues for many deposits.Gold and silver remobilization by today. Additional subeconomicprecious-metal miner- low-temperatureoxidizing groundwateris suggested alizationoccurs thro0ghout a seriesof prospectpits in as an important supergeneprocess leading to the KewaneeHills areq south of the mine, and along secondaryenrichment of someprecious-metal deposits the Sunnysidefault, to the west of the mine @g. lB). (Emmons 1,972,Boyle L968, 1978,Larkin et al. The IvanpahMountains are underlainby Jurassicto 1974). Also, paning-stagehydrothermal activity may Cretaceousweakly peraluminous,magnetite-series remobilize gold and silver (Barton et al. 1978). granitic plutons of the Teutoniabatholith, which Increasedfineness of electrumduring mineralizationis constitutespafr of the Mesozoicmagmatic arc that commonlycited as evidencefor remobilizationof gold intruded the southernportion of the Mesozoicforeland andsilver (F.--ons 1912,MacKay 1944,Koshman & fold-and-thrustbelt (Burchfiel & Davis 1972,1975, Yugay 1972), but recent studies (Mann 1984, 1988,Beckerman et al. 1982,Barton et al. 1.988). Stoffregen1986) convincingly arguefor remobi- Subeconomicporphyry copper-molybdenumand lization of gold and silver without an increasedfine- relatedskam mineralization is associatedwith portions nessof the depositedelectrum in certain chemical of the batholith in the adjacentmountains (Ntiamoah- environments.Other investigatorshave suggestedthat Agyakwa 1987), and tin, tungstenand copper skarn- supergeneremobilization of gold and silver may gpe mineralization surroundsthe Ivanpah Mountains actually lead to a decreasein finenessofthe electrum Qlewett 1956, Thompson 1978). Detailed description in specificcircumstances (Webster & Mann 1984). of the regional geological setting and associated Other mineralsof gold and silver may be produced mineralizationis given in Sheets(1995). during late-stagehydrothermal or supergeneoxidation of precious-metaldeposits. For example,uytenbo- Gsolocy oF THEMoRNTNG Sran Dnposn gaardtite(AgrAuS), a rare silver-gold sulfide, has beenreported in a small number of deposits,where it The areaaround the Monring Star depositis under- occurs as a late-stagehydrothermal or supergene lain by coarse-grained,equigranular Ivanpah granite phase@arton et al. L978,Dunn et al. 1985,Castor & of Jurassicage (Fig. 1B). Mineralization occurs in Sjoberg 1993). Uytenbogaardtitehas recently been foliated to mylonitic Ivanpahganite situatedbetween identified in the upper levels of the Morning Star tle Morning Star and Sunnysidethru$t faults, both deposit(Sheets et al. 1988,1989).Stability relation- orientedroughly N20'W/35"SW. Each fault is marked ships for uylenbogaardtite,acanthite and electrum in by a zone of ductile deformation(mylonite and proto- the supergeneenvironment are not currently available, mylonite) overprintedby brittle deformation. but low-temperature(100'C) hydrothermalequilibria Precious-metalmineralization at the Morning Star predict the formation of uytenbogaardtite and deposit is hostedby the Ivanpah granite in the upper acanthiteby sulfidationof electrum(Barton I 980). plate of the Morning Star thrust fault (Figs. 1B, C). This paper describesthe distribution and chemistry The fault crops out at the mine site as a zone of fault of precious-metalmineralization in the vicinity of the gouge consisting of bleachedand rounded granite Morning Star deposit and places constraintson con- fragmentsof variable size surroundedby anastomos- ditions attending gold and silver remobilization. As ing foliated, clay-rich selvages.Very rare galenaand previously stated,tle remobilizationof gold and silver chalcopyritehave been found in fault-gougesamples, may occur either with an increasein finenessof elec- but gold and silver mineralization is truncated at trum (Mann 1984) or with a constantor slightly the gougelayer. The upper edge of the Morning Star decreasingfineness (Webster & Mann 1984, diorite dike. which underliesthe thrust fault. is nun- Stoffregen1986). The Morning Star depositis unusual cated by the Morning Star fault, and fragmentsof in that textural and chemical evidence support both altereddiorite are found in samplesoffault gouge.A trends in the compositionof electrum, at different series of gouge-filled E-W- to ENE-WSW-nending levels within the oxidizedhorizon. vertical to sleeply north-dipping unminslalized sftug- tures cut the orebody,Morning Star fault and Morning LocAnoN ANDllrsrony Star diorite. Displacementon the E-W structures appearsto be left-lateral normal oblique-slip, with The Morning Star deposit is located on the south- maximumthrow of 5 meterson eachE-W structure. easternflank of the Ivanpah Mountains, northeaslern Disseminatedbase- and precious-metalmineraliza- Mojave Desert, California, approximately 100 km tion at the Morning Star deposit is structurally southwestof Las Vegas,Nevada (Fig. lA). Proven controlled within a tabular zone of quartzI calcite reservesat