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Silver.Bearing Sulfosalts from the Metamorphosed

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Silver.Bearing Sulfosalts from the Metamorphosed tM7 The Canadian M ineralo g i st Vol. 33,pp. 1047-1057(199s) SILVER.BEARINGSULFOSALTS FROMTHE METAMORPHOSED RAMPURA AGUCHA Zn-Pb-(Ag) DEPOSIT,RAJASTHAN, INDIA WOLFRAMHOLLER Insrttutfiir GeowissenscWen,Montananiversitilt Leobeu Peter-Turu'ter-StratJe 5, 8700 lzobeu Awtria STIBBARAJU MOHANDAS GANDHI HindasmnZinc Limited" Ya;hadBhavan, 3 I 3001 Udaipur, India AgsrRAcr The sediment-hostedRampura Agucha Zn-Pb-(Ag) depositin Rajasthan,India. containsassemblages rich in Ag-(Pb)-Sb sulfosalts. Freibergite Ag3.3_7.6Cq5_2.2Fel,_3.tzno_o.ssb3.7_4.6As0_o.4s13,pyrargyrite, stephanite, argentite, dyscrasite and various Pb-Ag-Sb sulfosaltsoccur either within or closeto large aggregatesof galena-Freibergite, pyrargyrite and stephanite alsooccur in galena-bearingveinlets in silicateminerals. Electron-microprobe analyses reveal an averageAg-content of 3 1 wt.7o in teibergite, whereasgalena is devoid of Ag. The sulfosaltsare Sb-rich end membersof the respectivesolid-solution series, with only limilsd As. The assemblageswere affectedby recrystallizationand re-equilibration during high-grademetamorphism and the subsequentcooling. The pyrargyritepresumably formed by replacementoffreibergite and ofPb-Ag-Sb sulfosalts,the stephanite,by decompositionof pyrargyrite and argentite,and the dyscrasite,by exsolutionftom galena Keywords: freibergite, pyrargyrite, stephanite,dyscrasite, argentite, boulangerite, Pb--Ag-Sb sulfosalts, metamorphism, RampuraAguch4 Rajasthan,India. Souramr k gisementiZn-Pb--(Ag) de RampuraAgucha au Rajasthan,en Inde, situ6dans un encaissantm6tasddimentaite, contient des assemblagesriches en sulfosels i Ag-(PbFSb. On trouve freibergite Agj:7.6Cn6.5.2.2Fe1.r3.12n0-o.8sb3.7-4.6Ase-4.4S13, pyrargyrite, st6phanite,argentite, dyscrasiteet autres sulfosels n Pb--Ag-Sb dans des amas importants de galdne, ou prbs de ceux-ci. Freibergite,pyrargyrite et st6phanitese trouvent aussi dans des veinules de galbnedans des mindraux silicatds. ks analysesh la microsonde6lectonique r6vdlentune teneurmoyenne en Ag de 317a(poids) dansla freibergtte,tandis que la galbnene contient aucunAg. ks sulfoselssont les termesb Sb des solutionssolides respectives, avec trbs peu d'arsenic. ks assenblagesont 6t6 affect6s par une recristallisation et un r6-6quilibrageau cours dun 6pisoclede m6tamorphisme intense et d'un refroidissementpar la suite. La pyrargyrite se serait form6e par remplacementde la freibergite et des sulfoselsl Pb--Ag-Sb,la st6phanite,par ddstabilisationde la pyrargyriteet de I'argentite,et la dyscrasite,par exsolutiondans la galdne. (fraduit par la R6daction) Mots-cl€s: freibergite, st6phanite,dyscrasite, argentite, boulangerite,sulfosels n Pb--Ag-Sb, m6tamorphisme, RampuraAgucha, Rajasthan, Inde. INTRoDUc[oN The deposit occursin the oldest part of the Bhilwara Supergtoup,at the contactwith the Archeanbasement RampuraAgucha is a stratiform, sediment-hosted ("Banded Gneissic Complex", BGC). The Bhilwara Zn-Pb--(Ag) deposit located some 220 km southwest Supergroup,consisting of a pile of metasedimentary of Jaipur in RajasthanState, lndia (Fre. 1). Since its rocks inftuded by igneousrocks, developedas a result discoveryra 1977,Rampura Agucha has becomeone of inffacratonicrifting of the Archeanbasement about of the most significant base-metaldeposits in India, 2.0 Ga ago. The deposit was formed by convective producing 900,000t/a of ore. The proven reservesare circulation of seawaterin zones of crustal extension 39.2 Mt probablereserves 13.8 Mt and possible during incipient rifting (Deb L992). The metals, reserves10.7 Mt (total 63.7 Mt), grading 13.6VoZa derived from the hydrothermalsystem, were accumu- l.9%oPb,9.58VoFeand 45 ppm AeGIZL Statr1992). lated in a trough with biological activity, as indicated Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/5/1047/3435461/1047.pdf by guest on 24 September 2021 1048 TIIE CANADIAN MINERALOGIST ARAVALLI- DELHI BELT W Deccanbasalts s| ErinpuraGranite El VindhyanSupagroup |ffi DelhiSupergroup AravalliSupergroup OREDEPOSTTS ffi I Khericopperbelt(Cu) ffi BhilwaraBelt 2 Saladipura (C\-7a) 3 Kho Oariba (Cu) f::Til JharolBelt 4 RampuraAgucha@-7n) 5 Rajpura Dariba (Zn-Pb-Cu) lll BerachGranite 6 Tawar (WZn) 7 Pipela (Cu-Zn) [::jj-:l BandedGneissic Complex E Deri-Ambaji (Pb-Z.;n{u) Frc. l. Simplified geologicalmap of the Delhi-Aravalli belt (after Sugdener aL 1990), including the location ofthe main ore deposits. by high contentsof graphiteand light 6r3Cvalues @eb westernmargin of the Bhilwara Supergroup,resulting l.992, Deb & Sarkar 1990).This suggestsa relatively in upper amphibolitefacies metamorphism(Deb et al. deep-water,reducing environmentduring sedimenta- 1989,Deb & Sarkar1990, Sugden et al. 1990)nthe tion, in which bacterialreduction of sulfatesto sulfides RampuraAgucha area, would havebeen crucial for mineralization.Pb isotope The resultsreported herein form part of an ongoing studiessuggest a model age.of 1.8 t 0.04 Ga for the geoscientificinvestigation directed toward a characler- RampuraAgucha deposit Qeb et al. 1989).,At1.5 Ga ization of mineralogy,geochemisnry and genqsisof the the lower crustalrocks of tle BGC weretlrust over the RampuraAgucha deposit.This work is being carried Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/5/1047/3435461/1047.pdf by guest on 24 September 2021 Ag- BEARING SULFOSALTS,RAMPURA AGUCHA tMg out by staff of the Institute of Geological Sciences, et al. 1984).The deposithas an oxidized gossanand a Mining University, Leoben, Austria, in collaboration small zone of partially oxidized material between with Hindustan7.nc Ltd. The aim of this paper is to gossanand protore. Since the greaterpart of the areais reportthe resultsof a detailedstudy of the ore minerals, cappedwith soil cover, and fresh outcropsare scarce, their distribution and chemistry within the Rampura much of the detailed geological information was Agucha deposit,in order to enhancerecovery ofthese obtainedfrom drill cores. metalsduring ore extaction and concentration.A study The mineralization occurs predominantly as dis- of the texturesand the relation of the ore minerals to seminationsin a schist, consist'ng of quartz, feldspar metamorphismis anotherimFortant part of this paper. (alkali feldspar and plagioclase),sillimanite, graphite and various micaceousminerals (muscovite, biotile Geor.ocrcar Ssrm{c ANDORBoDy and chlorite). Sphalerite,by far the most widespread sulfide mineral, occurs with galena, pyrite and The depositoccurs in a doubly plunging synformal pyrrhotite in varying proportions, with numerous structure of elliptical shape, comprising sillimmile- inclusionsofrounded to subroundedgrains offeldspar and graphite-bearingmica schist enclosedin gamet - and quartz. Graphite, a common gangue mineral, biotite - sillimanite gneiss, with minor bands of represents6 to 10 vol.7a of the bulk of averageore. A amphibolite and calc-silicaterocts, as well as aplite, large variety of minor sutftdephases occurs within the granitic pegmatiteand mylonite. The orebody,hosted ore, the most important being pyrargyrite, freibergite by a graphite- misl - sillimanite schist,is lens-shaped, andAg-(Pb)-Sb sulfosalls.Ore microscopyof samples with a northeast-southweststrike length of 1600m and from 20 drill-core intersectionsdid not reveal any a width varying from a few metersin the northeastto zonationof ttrosemetals within the orebody,although as much as 100 m in the centraland southwestsection the Zn:Pb:Feproportions vary within meters.Although (Frg. 2). The orebody dips between 50 and 80o sphaleriteis the most important base-metalsulfide at (averagedip of approximately60') southeastand has RampuraAgucha" small sectionsof the orebodycan be been proved by driling to a depth of 370 m from the dominatedby either galena or pyrrhotite. Peak meta- surface(Gandhr et al. 1984).The semimassiveorebody morphic conditions of upper amphibolite to granulite exhibits sharp contacts with the footwall and the facies were estimated by garnet-hornblende and hanging-wall rocks. No major faults have been garnet-biotite thermometry (650-700'C), sphalerite detectedin the orebody, although minor faults have barometry and fluid-inclusion studies (-6 kbar) beentaced in somesegments of the ore zone (Gandhi @anawatet al. 1988,Deb 1992).The metamorphism s700 s600 s5m N3OO N€O Conrolower@ Anphibolite @Orebody Pegmatite [MCatcsiUcare Gneisses,schists- -Shears s7m s600 s5m s400 s300 s200 s1m 00 N100 N200N300 N4m FIc. 2. Simplified geologicalmap ofRampura Agucha orebodyand surroundingrocks. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/33/5/1047/3435461/1047.pdf by guest on 24 September 2021 1050 TIIE CANADIAN MINERALOGIST was isochemical, but led to a redistribution of the ME"rr{oDsoF INVESTIGATIoN sulfide minerals within the orebody.High-grade peak metamorphismwas followed by a clockwiseretrograde Detailed ore petrography of 97 drill-core chip P-T cooling path. The deposit also underwent samplesfrom acrossthe strike length and depth of the polyphase deformation @ay 1980). The high-grade orebody has been documentedto investigate metal metamorphicevent resuhed in a high degreeof recrys- 2ening in the orebody and Ag-zoning in tallization (aggregatesof sulfides with grain sizes tetrahedrite-tennantite-gloupminetals, aad simply to exceedingI mm) and an obliteration of most of the establishthe identity of the Ag minerals.The sulfides primary sedimentary textures. Primary mineral were analyzedwith an ARI-SEMQ electron micro- banding is very rare, but can be observedin a few probe (wavelength-dispersionspectrometry)
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