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

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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

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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.

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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) at the polished sections,indicating a synsedimentaryorigin. Instiote of GeologicalSciences, Leoben, using matix Remobilizationof galenaand sphaleriteand, to a lesser correctionsaccording to the proceduresof Bence & degree,of pyrrhotite is common;quartz and feldspar, Albee (1968). Acceleration voltage used for sulfide which are highly deforme4 exibit numerouscracks, analysis was generally 15 kV, and for Pb-bearing filled witr sulfidesand sulfosalts.Sandwich-like inter- phases,25 kY, with a beam current of 20 nA. Pure growths of sulfides with graphite and phyllosilicates metals (Ag, Cd, Co), alloys (SeCu) and nafural are very cornmon. The characteristic ball texture chalcopyrite@e, Cu, S), sphalerite(7n), galena@b), described by Ranawat et al. (L988), consisting of bismuthinite (Bi), and breithauptite(Ni) were used as rolled porphyroclasts of silicates and sulfides, has standardsfor the analysisof sulfidesand sulfosalts. been observed in several samples. Oxide minerals (rme Cr-V oxides, gahnite and pyrophanite) were Sut,rpss, Gnepnnr anp Surroserrs formedby interactionof silicateand sulfideminerals as a consequenceof high-grademetamorphism (H0ller & Sphalerite, which is the predominant sulfide Stumpfl1995). mineral,occurs as coarse-to fine-grainedaggregates in

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close associationwith galena"pyrrhotite and pyrite. minor constituents,such as Bi, Sb, Ag and Se,is below The coarse-grainedvariety locally retains evidenceof the detectionlimit of the electronmicroprobe. metamo{phic recrystallization: characteristic 120' Pyrite forms granular aggregates of polygonal triple-point grain junctions and homogeneous Fe crystals (up to 5 mm across),but may be intergrown contents(upto ll.SVoFe,0.5VoMn,0.37oCd). A small with pyrrhotite and marcasite.Large porphyroblastsof grain of sphalerite (15 pm across) with inclusions pyrite show brittle fractures filled with other sulfide of chalcopyritecontains considerableamounts of Cd minerals. Porphyroblastsof pyrite within cataclastic (up to 9 Vo) wrd Mn (up to LOEI).Aggregates of sphaleriteindicate the formation of at leastsome of the sphaleritecommonly contain inclusions of galenaand pyrite after cataclasis. graphite,as well as exsolvedblebs of pyrrhotite. Pynhotite generally forms coarse aggregatesof Galenaoccurs as coarse-grainedaggregates in close polygonal grains, displaying metamorphiccrystalliza- assosiationwith sphaleriteand pyrrhotite, or encloses tion. It occurs as fine-grained aggegates intergrown subroundedgangue minerals and other sulfides. with other sulfides or as exsolved inclusions Remobilizationofgalena along fissuresand cleavages in sphalerite. Pyrrhotite grains are often incipiently ofgangueminerals is widespread.The concentrationof (or completely) decomposedto a granular product,

Frc. 3. Modes of occunenceof Ag-rich minerals under reflected light in oil immersi6l. (a) Anhedral freibergite @rei) in remobilizeclgalena (Gn). @) Subhedralgrain of freibergitein galenaat the grain contactwith quare (Qtz). Adjacent grain is pyrrhotite (Po). (c) Flakes of boulaagerite(Boul) in galena. (d) Subhedralto euhedralexsolution-induced bodies of dyscrasite(Dys) in galena.(e) Pyrargyrite@a) intergrown with chalcopyrite(Ccp) and remnantsof pyrite @y), probably occurringas a pseudomorphafter freibergiteand pyrite andrimmed by galena(Gn) andbiotite @t). (f) Complexintergrowth of Pb-Ag-Sb sulfosalt (Lght gey, arrows) with surroundingpyrargyrite and boulangeritein galena.(g) Complex inter- growth of argentite(Arg) and pyraxgydte(Sa) relics in stephanite(Stp) adjacentto pynte (Py, poorly polished).Sulfosalt to the right is pure stephanite,rimmed by galenaand quartz(Qtz). (h) Intergrowtl of gudmundite(Gud), chalcopyrite(Ccp) and sphalerite(Sp), rimmed by galenaand plagioclase@).

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consistingof pyrite and marcasite.Pyrrhotite is invari- FROM TUE RAMPI.'NA AGUCHA DEPOSIT ably relatively pure, containingno more than tracesof AND OF FREIBBRGITEFROM MT. ISA Co and Ni. Ars Stpl Stp2 Pyal PJa2 Frell Frd2 Frci3 MLI$ Graphiteis a cornmongangue mineral in all samples s 14.88 14.88 15.86 17.88 16.67 2L.44 9.n 1958 20.40 of the graphite- mica - sillimanite schist.It occursas Fe 0.40 0.60 0.89 0.05 0.47 5.16 5.u 4.91 5.40 As 82.42 69J6 67,4 5923 60.16 30.58 34.50 37J6 33.N flakes (0.01-3 mm asfoss),or as fracture fillings and sb 2.51 14,61 15.08 21.64 n33 75.t9 25.83 EA 25,q along grain boundaries.Graphite also enclosesother As 0.05 0.36 1.01 0J9 0.65 1.39 0.82 0.70 0.40 Cu 0.50 - 0J1 - 0 14.95 13.2E 10.E6 13.30 ore and gangueminsrals, and occurs as small inclu- 7n - Lr2 0.61 O23 1.00 sions in quartz, feldspar and the ore minerals.Folded Pb - 020 0.22 0.34 graphite flakes and kink bandsindicale a differential 2vt9o 1M.82, l00rl 1m.64 99xs l@JS 99J:t 100.4 99J4 99.10 stress-and-straineffect in the area. s 1.00 3J? 3.E6 3.01 2.E5 1L73 12.26 t2'7 LL52 Arsenopyrite,a cornmonminor constituentin meta- Fe 0.02 0.@ 0.13 - 0.04 1J6 1.86 1J9 1,89 morphosedmassive sulfide deposits,foms subhedral Ag 1.91 5.18 4.gl 2.n 3.M 5.40 6.30 1.VJ 6.V2 sb 0.0J 0.96 o.st 0.96 1.01 3.% 4.1E 4.18 4.08 to euhedral crystals (up to 3 mm across) that are Ar - 0.04 0.10 0.06 0.04 0.36 O.m 0.18 0.10 occasionallyflattened parallel to the foliation in highly Cu 0.02 0.06 - 0.02 4.49 4.00 3.44 4.10 Za 0.01 0.33 0,20 0.08 0.29 deformedsamples. Occasionally bent crystals display Pb - 0.03 evidence plastic of deformation. Large crystals may 3.m 10.fi, 10.00 10.m 7.@ 29.N 29.00 29.M 29.& enclosea core of ldllingi.te @eAs). The arsenopyrite containstp to 0.5VoNi and O.45VoCo, and the Itillingite carriesup to 37oNi, 0.55VoCo and2.5Vo S. Chalcopyrttehas been found minor as a constituent Dysl Dys2 Sdfol Srlfo2 So|fo3Srlfo4 Boull Boul2 My (up to 100 ;rm across) and occurs as disseminated grains s t.tz - 16.32 1E.14 16.70 17.6 18.62 11.82 2155 axhedral htemtitial to tlre dominantiron sulfides Fe - o24 - 0,10 0.09 and silicates. It is also intergrown with either AC 69.M n.24 43.32 28.61 19.86 12.65 - LU 35.80 pyraxgyriteor gudmundite(Figs. sb 26.6 n.Io 21.68 30,J5 2t.64 8.m 25.@ U.53 42.4 3e, h). Gudrnundite AI 1.00 - 0.61 0:n o.49 0,4 0,2't 0.10 (FeSbS),breithauptite (NiSb), allmannite (NiSbS) and Pb 1.06 . 17.33 22.06 40.50 45.39 55.n 54.23

native antitnony occttr as compact inclusions in ZwtEo n32 9914 99 1mJ7 99.19 Wg| 99.78 9.07 100.17 aggregates of massive galena, or form irregular s 0.1s - 6.91 752 7.68 8.05 10.98 10.71 r.9 intergrowths in remobilized veinlets with galena and Fe - 0.05 pyrargyrit€. A complex intergrowth of gudmundite AC 2.43 3.19 5.43 352 2:71 1.73 - 0.,10 0.98 sb 0.94 0.81 2.41 t.t6 2.62 2.9t 3.9 3.86 r.V2 and chalcopyritehas been observedat the margin of A3 0.06 - 0.11 0.r3 0.r0 0.f/ 0.01 large grains of galena (Frg. 3h). The gudmundite Pb 0.02 - 1.14 t.42 2.89 3.24 5.03 5.03 contains up to 0.5VoZn, 0.2VoAg and 0.67oNi; the 4.00 4.&l 16.00 r6.m 16.00 16.m 20.m 20.m 4.m ullmannite carries up to 03Vo Fe and 0.4Vo Ag. Slnhols: Arg: dEotits! SF: nrpiai@, Pyd ptftrgdie, Feir fteibergig ltte dys@itq $ilfo: Greerwckite (CdS) forms anhedral grains (<20 pm Pb-Ag-sb $ll@lt, Born: tul@gqitq Myr eye&dF. tha @ tlibdgib tsE M! Ie @ bem non Riley across)within galenaand commonlyis associatedwith Ot4). boulangeriteand freibergite. It containsup to L.ZVoMn, L.4VoFe and 0.9VoZn. Molybdenite eithet occurs as anhedralgrains (<30 pm across)in close association with galen4 or forms flakes and fracture fillings in grainsof the samesample may vary. The Ag-contentof sphalerite.A thallium-bearingmineral with the ideal- most grains rangesfrom 28 to 36Eo,with a maximum ized forrnula (Cu,Ag,Tl)FeS2occllrs as inclusions in arclumd3l%ot the Cu-contentranges from l0 to L8Eo, galena. It was first dessribedby Genkin & Schmidt with a maximum arotnd l5Vo (Iable 1). The lowest (1991). Cu-contentscorrespond to the highest Ag-contents, Freibergite (Ag,Cu,Fe,Zn)12(Sb,As)aS13is an suggestinga Ag-for-Cu substitution @g. 4). Three abundant phase in Ag-rich samples. It occurs as grains of freibergite show lower Ag-contents inclusion-free anhedral grains (5-50 pn across), (17-20Vo), and two grains show higher Ag- usually at the margin of coarseaggregales of galena contents (38 and 4l%o). Despite these differencesin gmins closeto contactswith silicates(Fig. 3b) or other Ag-content, these grains show the same textural sulfides, or is invariably included within remobilized crileria and color as grains with common Ag and Cu galena@ig. 3a). In reflectedlight its color variesfrom contents(around 3l7o Ag) in the samesample. There is olive-green to grey-green,without correlation to the tro correlationbetween the contentsof.Fe (4.L-8.3Vo), extent of Ag-for-Cu substitution.Electron-microprobe Za (V2.lVo), As (A-L.6Vo),and Ag or Cu. At Aguch4 analyses were carried out on forty-six grains of the freibergite is similar in compositionto freibergite freibergite from ten samples.In this deposig single from Mount Isa @iley 1974). A study of freibergite grains of freibergite are chemically homogeneous, from severalprofiles tbroughthe entire orebody,from and no marked compositional changes were footwall to hangng wall, did not reveal any Cu-for-Ag detectedacross individual grains, although different zoning.

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Curo(Fe,Zn)r(Sb,As)4s13 Pyrargyrite (Ag3SbS) is the predominant 10 Ag-bearingphase of the Aguchaorebody.It commonly occurs as anhedral, disseminatedgrains (5-35 pm o across)in veinlets together with remobilized galen4 and Sb-minerals.It also forms complex o6 chalcopyrite intergrowthswith (or a nanow rim around)Ag-Pb-Sb sulfosalts (Frg. 30. Pyrargyrite also occurs in close associationwith pyrite and pyrrhotite, where it forms larger subhedralto euhedral grains (up to 100 p"m across),and it tendsto be associatedwith chalcopyrite (Fig. 3e). Individual grains are chemically homo- o geneous;although arsenopyriteis presentin the same l 2 3 4 5 6 7 8 910 ore zones,As-contents do not exceedIVo (table L). Ag Atoms Ag-PHb sulfusaltsare commonbut not as wide- spreadas pyrargyrite and freibergite.They are located ftc. 4, Variation of Ag and Cu in freibergite from Rampura invariably in the interior of large aggegatesof galena" Agucha calculatedon the basisof 29 atoms. where they typically occur as anhedral inclusions Pbs gn

@l w,'l @l -f"-p@ cii fr(frei# bk rYoS.oia rfrk l&[lbmo @ *!** nd # le

s1 pya sry arg sb2s3 Agzs

Ftc. 5. Compositionalvariation of Pb-Ag sulfosaltsfrom the RampuraAgucha deposit (o) in the temary system Sb2S3-PbS-Ag2S3.Additional end-memberminerals (o) are derived from Crug et al. (1973), Goodell (1975) and Keighin & Honea (1969). Ideal compositions,symbols and formulas of the known sulfosaltsand sulfides are shown: gn: galena" PbS; geo: geocronite, 27PbS.7Sb2S3lboul: boulangerite, 5PbS.2Sb2S3;sem: semseyite, 9PbS.4Sb2S3;hefi heteromorphite, 7PbS.4Sb2$; plag: plagionite, 5PbS.4Sb2S3;rob: robinsonite,7PbS.6Sb2S3; zin: zinckenite, 6PbS.TSbrSr;ftil flil16ppite,3PbS.4Sb2$; dad: dadsonite,1lPbS.6Sb2S3; ow: owyheeite, 5PbS.3Sb2S3.Ag2S;fiz: fizelyite, 5PbS.4Sb2S3.Ag2S;ram: ramdohrite, 3PbS.3Sb2S3.AgrS;frei: freieslebenite, 2PbS.Sb2S3.Ag2S;dia: diaphorite, 4PbS.3Sb2S3.Ag2S;and: andorite, 2PbS.Sb2S3.Ag2S;si: stibnite, Sb2S3;my: miargyrite, SbrSr.Ag2S;pya: pyrargyrite, Sb2S13Ag2S;stp: stephanite,Sb2S3'5Ag2S; arg: argentite,Ag2s.

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35

30 s *j 25 F 20

15

10 0 pm Ftc. 6. Traverseacross a zonedgrain ofAg-Pb-Sb sulfosalt.

OA-25 pm across). Occasionally complex inter- DlscussroN growths with pyrargyrite (Fig. 3f) occur within large aggegaies of galena. Their color in reflected light The regional metamorphic history (Ranawat & resemblesthat ofgalena, but the reflectivity is slightly Sharma1990, Deb et al. 1989) includes a high-grade Iower. The Ag-Pb-Sb sulfosalts are strongly eventwith peak conditionsof 650-700"C and -6 kbar anisofropic and lack internal reflections. Microprobe in the Rampura Agucha area, atrd it affected the analysesreveal considerablecompositional variations stratiformorebody. The high-grademetamorphism was (Fig. 5,Table 1). A few grainsare significantlyzoned almostisochemical on a large scale,although a number (Ftg. 6). Sevendata points encountered@g. 5) plot of componentshave been redistributedlocally during near the idealized composition of freieslebenite metamorphism and retrogression. It is difficult to (AgPbSbS3). determine which features in the ore relate to the Boulangerite@b5Sb4S11) forms flakes (10-50 prn progradeevent and which reflect retrognde equilibra- across)in galena @g. 3c) and is commonly encoun- tion. The high-grade metamorphic event resulted in tered in complex intergrowths with Ag-Pb-Sb thorough recrystallization of most sulfldes, develop- sulfosalts and pyrargyrite (Frg. 3f). Although most ment of triple-point junctions, ductile deformation aggregatesare devoid ofAg, somegrains contain up to of pyrite and arsenoplrite, the development of 2VoAg (Fig. 5, Table l). Dyscrasite (AgrSb) forms porphyroblasts and the formation of characteristic tiny, single-phase subhedral to euhedral grains metamorphicminerals suchas sillimauite and gahnite. (<10 pm across) that are characteristically disse- himary sulfide minerals were homogenized,so that minated in the interior of large aggregatesof galena original compositionalzoning, if ever present,is no Grg. 3d). It is not found in cont4ct with other Ag longer observable.Only Ag-Pb-Sb sulfosalts,which minerals. Low contents of Pb" Sb and S have been seemto have formed during the 'ssnningstage of the determined(Table 1). Stephanite(Ag5SbS, (5-50 pm metamorphicevent, may be zoned (Frg. 6). The ore across)is found in a few Ag-rich samplesand occurs in minerals seem to have behaveddifferentially during close associationwith galen4 pyrargyrite and pyrite metamorphism.Pyrite, pyrrhotite andsphalerile tend to (Frg. 3g). Minor contents of As and Cu have been maintain a constantshape, whereas galen4 which was detected(table 1). Argentite (Ag2S)is rare and forms by far the most mobile sulfide, almost everywhere tiny anhedralgrains (<5 pm) in close associationwith exhibits evidenceof plastic deformationand remobi- pyrargyriie and stephanite(Fig. 3g). Polybasite (Ag, lization. To a lesser extent, sphaleriteand pyrrhotite Cu)t6Sb2Sr1(Mukherjee et al. l99I) and miargyrite were remobilizedinto the silicate minerals during and AgSbS2also are rare and occur as vein minerals in after high-grademetamorphi$m. All sulfide minerals sphalerite.Miargyrite containsup to 0.4VoAs, and all except pyrite commonly fill fractures in quartz, other traceelements are below the detectionlimit. feldspar, and mica; there, stephanite,freibergite and

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pyrargyritemay be minor constituents. Rajpura Dariba Rajasthan(Kanika Basu e, al. 1984) Galena-rich samples from the Rampura Agucha and Cofer, Virginia (Miller & Craig 1983), the tetra- orebodycontain eight definedAg-bearing ore minerals hedrite-tennantite-group mineral decomposes to and a number of intermediate compositions in the arsenopyrite,chalcopyrite, gudmundite and sphalerite. ternary system PbS-Ag2S-Sb2S3. Sphalerite- At Lingdal Sweden (Karup-Moler et al. 1989), dominated samples tend to be devoid of any freibergite hasreacted to pyrargyrite and chalcopyrite. Ag-bearing minerals. Pyrargyrite, freibergite and An upper stability limit of gudmunditeof 280 t 10"C Ag-Pb--Sb sulfosalts representthe most widespread in the system Fe-Sb-S (Clark 1966) indicates that Ag-minerals.Stephanite, argentite, miargyrite, dyscra- reaction (1) took place at the end of the retrogmde site and polybasite are less common. Sulfosalts are stageof the metamorphicevolution. highly susceptible to retrogression and generally Pyrargyrite and stephanitemay also have formed undergocontinual re-equilibration to low temperatures by a breakdownofAg-Pb-Sb sulfosalts.Replacement (Vaughan& Craig 1978).Freibergite, boulangerite and of Ag-Pb--Sb sulfosalts (presumably freieslebenite; argentite are most probably prograde sulfosalts at Fig. 3f) by pyrargyrite and boulangerite in galena Rampura Agucha, present at the peak of meta- suggeststhe following retrogmdereaction: morphism. The upper limits of stability for iron-rich tetrahedrite (6.9 atVo Fe) and S-rich argentite are Ag3Pbr.rSbr.nSr.6-+ 661'C in the system Cu-Fe-Sb-S (Tatsuka & Ag-Pb-Sb sulfosalt Morimoto 1977) and 622"C in the system Ag-Sb-S Ag3SbS3+ PbS + 0.45PbrSboSlr (3) (Keighin & Honea 1969), respectively. At higher pyrargyrite galena boulangerite temperatures,these minerals melt. Boulangeritehas an upper stabiliry limit of 637'C in the systemPb-Sb-S A similar reactionis repor0edfrom other localities: (Salanci 1979). The limited solubility of Sb2S3in for example,at Tetyubke,Russiao andorite is replaced galena(3 mol%oat 640'C; Salanci1979) would also by pyrargyrite, owyheeite and stephanite (Natarov suggestthe appearanceof a separateSb2S3-bearing et al. L972);at Riverree, New SouthWales, owyheeite phase at this stage.However, interpretationof these is replacedby pyrargyriteand galena (Lawrence L962); stability limits could be ambiguous. The stability at the Meerschaummine, I\41.Wills, Victoria, andorite curvesare pressure-dependent,in the order of G-10'C is replacedby miargyrite and owyheeite(Birch 1981), per kbar or less in the systemPb-Sb-S (Garvin 1973), and at Morea, Nevada, andorite is replaced by and additional elementssuch as Zn or As also may diaphorite, diaphorite by owyheeite, and owyheeite result in a shift of the stability curves (Craig & by galena and pyrargyrite @illiams 1968). Alhough Kullerud 1968, Tatsuka & Morimoto 1973, 1977). some compositions of Ag-Pb-Sb sulfosalts from P1'rargyrite,the most cornmon Ag-mineral, has an Agucha plot close to the ideal2ed composition of upper stability limit of 485oCin the systemAg-Sb--S freieslebenite(AgPbsbsr), most display intermediate (Keighin & Honea 1969) and thus formed during the compositionsbetween the end-memberminerals. A retrogade part of the P-T path. Although there is no few grains aro zoned,and no obvious trend in zoning direct evidence for a breakdown of freibergite, the pattemswas observed,although an influence of reto- occurrenceof gudmundite,chalcopyrite and sphalerite grade metamorphismon the zonation of Ag-Pb-Sb grains @g. 3h), as well as pyrargyrite-chalcopyrite sulfosalts cannot be ruled out. Hoffinan & Trdlicka intergrowthsthat form large, subhedralpseudomorphs (1978) compared compositions of owyheeite (presumably after freibergite) in galena (Fig. 3e), (AgrPb5Sb6S6)from seven deposits, and showed suggeststhat the following retrogradereactions took that the chemical compositions are highly variable place (not balanced): (40.7746.45 wt.VoPb). Mo0lo e/ al. (L984) showed that substitutionof the major cations(mainly 2Pb2+<+ (Cu,Fe,Zn)12SbaS13 -) + FeS2 Ag+ + Sb3+)is responsiblefor the significant variation pynte tetrahedrite in the composition of owyheeite from thirteen ore (1) CuFeS2 + FeSbS + ZnS deposits. The lowest Ag-contents of some zoned gudmundite chalcopyrite sphalerite Ag-Pb--Sb sulfosaltsfrom the Agucha deposit corre- (Ag,Cu,Fe,Zn)i2Sb4St3+ FeS2 -) spond to the highest Pb- and Sb-contents,suggesting freibergite pydte mainly a Ag-for-(Pb+Sb)substitution (Ftg. 6). CuFeS, + Ag3SbS3 t ZnS (2) Stephanile,a relatively commonretrogtade mineralo chalcopyrite pyrargyrite sphalerite is found in small amountsin many Ag-deposits.It is a typical secondarymineral phase that formedfrom other It has beendemonstrated at severalmetamorphosed Ag-sulfosalts.Stephanite has an upper stability limit of deposits that freibergite, tennantite and tetahedrite 197 t 5'C in the systemAg-Sb-S (Keighin & Honea breakdown during cooling.At BrokenHill, New South 1969),indicating formation at the end of the retrograde Wales @limer 1980, Both & Stumpfl 1987), tena- metamorphicpath. Relics of argentiteand pyrargyrite hedrite reacts to chalcopyrite and gudmundite. At in stephanitegrains @g. 39) indicate the following

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retrogradereaction: Rmenm,lces

Ag2S + AgrSbS, -r Ag5SbSo g) BeNcE,A.E. & Amm, A.L. (1968): F.'npirical correction argentite pyrargyrite stephanite factors for the electron microprobe analysis of silicates and oxides.J. Geol. 76, 382403. The formation of stephanite at the expense of pyrargyrite and argentite is also reported from Bncrr, W.D. (1981):Silver sulphosaltsfrom the Meerschaum GarpenbergNorr'4 centralSweden (Sandecki 1983). mine, Mt Wills, Victoria, Australia. Mineral. Mag, 44, 73-78. Dyscrasite,a common Ag-bearing inclusion in galena (Sharp & Buseck 1993) fomrs tiny inclusions BorH, R.A. & Sru"pFI. E.F. (1987): Distribution (40 pm, Fig. 3d) uniformly distributedin the interior of silver in the Broken Hill orebody.Econ. Geol. U,1037-1M3. of large aggregatesof galena(>3 mm). Thesetypes of inclusions have been interpretedas due to exsolution CIIRK, A.H. (1966): gudmundite. of phase Heating experimentson an Ag-Sb rich from galena,attributed to a M ineral. Mag. 35, 1.123-1 125. decreasingsolubility of these elementsin galena at lower temperatures. On the basisof their experimental Cnarc, J.R. & Kuu,mup, G. (1968): Phaserelations and work, Hall & Czamanske(1972) nd Sandecki & mineral assemblagesin the copper-lead-sulfur system. Amcoff (1981) suggestedthe formation of Ag-Sb Am-Mineral. 53" 145-161. phasesby exsolution from galena between 350 and 400'c. CneNc, L.L.Y. & t{EEs, W.R. (1973): Investigationsin the Pb-Sb-S system.Cdl4 Mineral. 12, CoNclustons 199-206.

This study of Ag-bearing sulfosaltshas established Dm, M. (1992):Lithogeochemisfy of rocls aroundRampura the mineral compositionand paragenesis of freibergite, Agucha massive zinc sulfide ore-body, NW India - implications for the evolution of a Proterozoic pyrargyrite, stephanite, dyscrasite, various *Aulakogen". Ag-@b)-Sb sulfosaltsand severalminor impsrtanl6t /z MetallogenyRelated to Tectonicsof the Proterozoic Mobile Belts (S.C. mineralg from the Agucha deposit. The present Sarkar, ed.). Balkema Rot0erdam,The Netherlands(1-35). Ag-mineralogy reflects the metamorphicevolution of this stratiform premetamorphic deposit. The Ag- (1990): minelals, present & SARKAR,S.C. hoterozoic tectonic at the time of formation of the evolution and metallogenesis in the Aravalli-Delhi deposit, remain unknown owing to the event of OrogenicComplex, northwestern lndta- Precamb. Res. 46, high-gade metamorphismthat transformedboth the 115-1,37. silicate and sulfide assemblages.Freibergite, as well as argentite,which were apparentlypresent at the peak THoRpE,R.I., Cwnvmqc,G.L. & Weciw" P.A. of metamorphism,were partialy consumed during (1989): Age, sourceand stratigraphicimplications of Pb retrogression. As a result, a low-temperature isotopedatafor conformable,sediment-hosted, base metal assemblage comprising pyrargyrite, stephanite, depositsin the hoterozoic Aravalli-Delhi Orogenicbell gudmundite and chalcopyrite formed in close asso- northwesternIndia- Precamb,Res.43. 1"-22, ciation with galena. Pyrargyrite also formed by decomposition of Ag-Pb--Sb sulfosalts. Dyscrasite GANDrfl,S.M., Pauwal H.V. & BnerueceR"S.N. (1984): exsolvedfrom galenaowing to retrogrades6qling and Geology and ore reserveestimates of RampuraAgucha decompression. lead-zinc deposit Bhilwara Districl J. Geol. Soc. India 25.689-705.

Actoqowmncurm.ms GARvtr{,P.L. (1973): Phaserelations in the Pb-Sb-S system. NeucsJahrb. Mineral.. Abh. 118. 235-267. We thank Hindustan Zinc Ltd. for providing drill-core samplesfor this study,and Hetnut Miihlhans GnNrnt, A. & Scuvor, S.H. (1991): Preliminary data of for assistancewith the electron-microprobeanalyses. a new thallium mineral from the lead-zinc ore deposit I am grateful to B. McElduff, who performedprevious Agucha, India. Neues Jahrb. Mineral., Monatsh., work on the sulfosalt assemblage,as well as R.T.M. 256-258. Dobbe, of the Free University, Amsterdam,and E.F. Stumpfl, F. Melcher and J. Raith, Mining University GoopeLL, P.C. (1975): Binary and Grnary sulphosalt Leoben, for helpful discussions. The constructive assemblagesin the Cu2S-Ag2S-PbS-AqS3-Sb2S3-Bi2S3 system.Caz. Mineral, 13, criticism of two reviewersas well as financial support n 42. of the Austrian Research Council (FWF) through IIAIT, W.E. & Czavaxsrc, G.K. (1,912): grant No. 10322-:I:EC Mineralogy and to E.F. Stumpfl are gratetuUy trace element content of the Wood River lead-silver acknowledged. deposits,Blaine County, Idaho. Econ Geol.f1,35G361.

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Horruamr, V. & TRDUCKA,Z. (19'78): Owyheeite from & Snanve, N.K. (1990): Petrology and geo- Kutnd Hora - a new mineral for Czechoslovskia..lfaues chemistry of the hecambian Pb-Zn deposit Rampura Jahrb.Mineral., Monatsh.,45-57 . Agucha, India. In Regional Metanorphism of Ore Deposits and Genetic Implications (P.G. Spry & L.T. H0ui& W. & Srur'pt'l-, E.F. (1995): Cr-V oxides from the Bryndzk, eds.).VPS, Zeisg The Netherlnds (197-227). RampuraAgucha Pb--Zn-(Ag) deposit"Rajasthan, lndia. Can.Mineral33" RAy, J.N. (1980):An evaluationof the tectopicframework of the Rampura-Agucha lead-zinc deposit, Bhilwara HZL SrAFF(1992): Rampura-Aguchamine. Mining Mag. District, Rajasthan.Indian Minzrals y, 19-21. 167,372-375. RtrjEy, J.F. (1914): T\e tshahedrite-freibergiteseries, with Kenup-Mou-Bn,S., Merovrcrv, E. & PnsmOv, G. (1989): referenceto the Mount Isa Pb--Zn-Ag orebody.Mineral. Mineralogy of the sulfosalt zone at the l^ingrlal deposit Deposita9, ll7-124. Boliden district, northernSweden. Neues Jahrb. Mineral., Abh.t60"299-3n. Sar.aNcr,B. (1979): Beitriige zum System Pb$-Sb2S3in Beziehungzu natiirlichen Blei-Antimon-Sulfomineralien. KANTKABAsu (SANYAL),BoRrNtr(ov, N.S., Mrsme, B-, NeuesJahrb. Mineral...Abh. 135. 315-326. Moorcnrur,r, A., MozcovA, N.N. & TsEpn{,A.I. (1984): Significance of transformationtextures in fahlores from Samectc, J. (1983): Silver-rich minerals at Garpenberg Rajpura-Dariba pollmetallic deposit, Rajasthan,India. Non'4 central Sweden.Neues Jahrb. Mineral., Monntsh, NeucsJahrb. Mincral., A.bh.149, 143-16I. 365-374.

KHcHD{,C.W. & Hor.rsA,R..M. (1969): The systemAg-Sb--S - & AMcorr, O. (1981): On the occurrenceof silver- from 6@oCto 200'C. Mirural. Deposita4,153-171. rich tetrahedriteat GarpenbergNorra central Sweden. NeaesJahrb. Mbural, AblL 141.32+3q. Lewn.r.xcr, L.J. (1962): Owyheeite from Rivertree, New SouthWales. Mineral. Mag.33, 315-319. Snenp,T.G. & Busecr, P.R. (1993): The distribution of Ag and Sb in galena:inclusions versus solid solutions.r4rz" Mnren, J.W. & Cnerc, J.R. (1933): Tetrahedxite-Gnnantite Mircral.78. 85-95. series compositional variations in the Cofer Deposil Mineral District, Y trgjna. Am, Mbural, 68, 2n -n4. Sucow, T.J., Ds, M. & WD.TDI.BY,B.F. (1990):The tectonic settingof mineralizationin the ProterozoicAravalli-Delhi MoH,o, Y., MozcovA, N., hcor, 8., Bonrwncov, N. & orogenic belt N.W. lndru In PrecambrianContinental VRUBLBVSKAIA,Z. (1984): Cristallochimie de Crust and its Economic Resources(S.M. Naqui, ed.). I'owyheeite: nouvelles donn6es. TschermalcsMineral. Elsevier,Amsterdam, The Netherlands(367 -390). Petrogr. Mift. 32, 27| -2U. Tersure, K. & Monnaoro, N. (1973):Composition variation MurtlrnrsE, A.D., SA!trilTA Gulrmr, & Bnarracrunve, and polymorphismof tetrahe&itein the Cu-Sb-S system H.N. (1991): Silver-bearing sulphosaltsfrom Rampura below 400'C. .4LnMiwral, 58. 425434. Aguchamassivesulfide depositofRajasthan. ./. Geol.Soc. India37,132-135. & - (1977): Tetrahedritestability relalions in the Cu-Fe-Sb-S system.,4on. Mineral. A,lL0l-L109. NATARoV,A.G., SvEsHNrtr(ovA,O.L. & GAr-yrr(,v.A. (1972): First find of andoritein the USSR.DokL Akad.Nauk SSSR VAUcHAN,D.J. & CRAIc,J.R. (1978): Mineral Chemistryot 216, 1,89-192 (in Russ.). Metal Saffides.Cambridge Univ. Press,Cambridge, U.tri

Pmm, I.R. (1980): Hydrothermal mobilization of silver Wnuavs, S.A. (1968): Complex silver ores ftom Morea, during retrograde metamorphism at Broken Hill, Nevada.Can. Mirural. 9. 478484. Ausfalia. NeuesJahrb, MineraL, Monatsh, 433439.

Rarvawer, P.S., Bueruecen, S.N. & Snanv^r,N.K. (1988): Metamorphic chaxacter of Rampura Agucha Pb-Zn Received February 8, 1995, revised manuscript arcepted deposigRajasthan. GeoI. Soc. India, Mem.7,397409. June 21. 1995.

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