803
The Catndian M he raln gi st Vol. 32, pp.803-814 (1994)
ARGENTIAN.PENTLANDITE.BEARINGASSEMBLAGES IN METAMORPHICROCKS OFTHE KTB PILOT HOLE, OBERPFAIZ,GERMANV
AGNES KONTNY KTB-FeWlabo r, Postfafi 67, D -92667Windischz schenbach Germnrry
GTTNTIIER FRIEDRICH ANP PETER TIERZIG AacheU Instint fiir Mineralogie und Ingerstilftenlehre,Rheinisch-Westfdlische Technischz Hochschule Willlnerstr. 2, D-52062Aacheu Germtiny
STEFANKEYSSNER BayerischesGeoinsrttut, Universittlt Bayreuth. Postfach 10125, D-95440 Bayreuth Germany
ABSTRACT
Different assemblagesof argentianpentlandite occur in intermediate-grademetamorphic rocks fur,n the pilotlrcle of the GermanContinental Deep Drilling Program (KTB), Assemblage1 consistsof small inclusionsin chalcopyritewithin quartz veins.Argenrian pentlan&te is ahJ inteigown with chalcopyriG,pyrrhotite, pentlandite and pyrite (assemblagg2) glcobaltian occur in pentlandifu,chalcopyrite, pynhotite anOspnAerite or chalcopyriteand galena(assegrblage 3); assemblages2 and -3 paragneissicrocks.-'ibe Ag conteutof argintian pentlanditeranges from 10.3to 12.6vtVo. The exsolutionorigin of argentian 'ta\rcs, chalcopyritefrom assemblage1, ir"oti-Oit" fro- chalcopfrte, as well as inversiontwins andnegative 6sS of 4.7Vooin point to mineralization during retrogfade metamorphismat conditions of elevatedpressure and temperature.Pressures of )-3 kbar ancltemperatures of ibout 400oCare in agreementwith the modelingof the retrogradeP-T-t path for the-KTtsrocks. Temperatureand sulfur fugacity estimatesbasecl on sphaleriteand clLloritecompositiols suggestcoqditions of formation in -llVoo u.s".blug"s 2 and3 ator stightly below 260"C and at a value of a(S) less than 10-13.Bulk-rock 6sS values of ate consistentwith sulfur from a iedihentary source.Argentian pentlandite and associatedsulfides are attributedto hydrothermal mineralizationin which sulfur and the metalsare derivedfrom sedimentaryhost-rocks.
Keywords: argentian pentlandite, pentlandite, pyrrhotite, pyrite, chalcopyrite, sphalerite, chlorite, temperature, mineral chemistry,sulfur fugacity, KTB pilot hole, Oberpfalz,Germany.
Somaanr
Nous avonsddcouvert divers assemblagesdle mineraux contenant la pentlanditeargentifdre dans une suit€ de rochesm6ta- morphiquesdans le trou de forage exploraioireKTB du programmeallemand de foragesprofonds dans la cro0te continentale. t 'ass"mUtageI est fait de petitesinciusions de pentlanditeargentifbre dans la chalcopyriteincluse dansdes veinesde quartz. la pentlanditeargentifbre se prdsente aussi en intercroissanceiavec chalcopyrife, pynhotite, pentlanditeel pynte (assemblage 2), ou avec pentlinCite cobafufere,chalcopyrite, pyrrhotite et sphal6rite,ou bien avec chalcopyriteet galbne(assemblage 3)' On ttoou" 1o assemblages2 et 3 dansdes ioches paragneissiques. La teneuren Ag de la pentlanditeargentifbre vmie.de 10.3d 12.6To(pu poi Mots-cl6s: pentlanditeargentifere, pentlandite, pynhotite, pyrite, chalcopyrite,sphal6rite, chlorite, temp€rature,composition desmindraux, fugacit6 du soufre,trou de forageexploratoire KTB, Oberpfalz,Allemagne. 8M TIIE CANADIAN MINERALOGIST INTRoDUcrroN (1.977),Groves & Hall (1978),Mposkos (1983), Kozlovskiyet al. (1988)and Benvenuti (1991). These Argentian pentlandite was first describedby investigationshave shownthat argentianpentlandite is Shishkinet al, (1971) and Vuorelainenet al. (1972) not simply a memberof an isomorphoussolid-solution from Russianand Finnish sulfide deposits.Subsequent serieswith pentlandite,but a distinct mineral species, reports on the occurrenceocompositiono sfiucture and @e,Ni\*rAgrrSr. It is commonly in close association phaserelations of argentianpentlandite were pub- with chalcopyrite,either as small blebs or exsolution- lished by Scott & Gasparrini (1973), Hall & Stewart induced bodies or in direct contact with chalcopyrite, (1973),Karpenkov et al. (1973),Mandziuk & Scott with or without pentlanditeand pyrrhotite (Mandziuk FIc. l. Geologicalskerch-map of the westemrim of the BohemianMassif in northeasternBavaria. A. Variscanbasemenr out- crops in mid-Europe.RH: RhenohercynianZane,ST: SaxothuringianZote,MN: MoldanubianZnne. B. Geologicalmap with tectonomorphicunits. l: aysql]in9 nappeccmplexes, Z: lasal units of the nappecomplexes, MM: Mtinchbirg nappe complex, ZEY: Zone of Erbendorf-VohenstrauB,ZTT: Tnne of Tepla-Taus,3: Sixothuringian; 4: Moldanubian of the OberpfHlzerWald; 5: late- to post-tectonicgranites; 6: KTB drilling site. 7: overfirust; ZTM: Tane of Tirschenreuth- Miihring; W: Winklarn (IVeber 1990). ARGENTIAN PENTI.ANDITE, KTts PILOT HOI,B, GERMANY 805 & Scoft 197D. Groves & Hall (1978) describedan sive complex of basallic composition (Schalkwijk occlurence in which pyrrhotite and pentlandite are 1991).The basicrocks are suggestedto be Ordovician replacedby argentianpentlandite in associationwith in age on the basis of Rb-Sr, U-Pb and Sm-Nd galen4 galenobismutite(PbBl2Sr, native bismuth and isotopic systems(von Drach & Kdhler 1990,von parkerite(Ni:Brzs). Quadt 1990).Paragneissic and metabasicrocks under- Natural argentian pentlandite ranges in Fe/Ni went intermediate-grademetamorphism (675'C and atomic ratio from 1.34 to 2.57, and in metaUsulfur 7 kbar, Reinhardtet aI. 1989)at about380 Ma (Teufel ratio from 1.096to 1.173.The crystal stucture deter- 1988).An earlier high-presswe,granulite-facies meta- mined by Hall & Stewart (1973) is face-centered, morphismis sporadicallydocumented in the metabasic spacegroup Fm3m. Hall & Stewart (1973) demon- rocks. Relict paragenesesimply pressuresup to strated that the octahedrally coordinated sites in 12kbw (Rith e/ al. 199O,Schalkwijk 1991). Incally, argentianpentlandite are occupiedexclusively by Ag there is a greenschist-faciesoverprint. Deformationin atoms.Groves & Hall (1978) suggestedthat Fe and Ni the brittle field resulted in the formation of catacla- occupy specific tetrahedrally coordinated sites; the sites,which arepartially graphitized. structure of argentianpentlandite may thus be based The lithostritigraphy of the KTB pilot hole is on the primitive space-groupP43rc. shown in Figure 2, and can be subdivided into four Experimental investigationsof Mandziuk & Scott characteristicunits (Rohr et al. L99O):(1) 0-460 m: (1977) showed that argentianpentlandite is a stable phasein the systemAg-Fe-Ni-S below 455'C. The authorsconfirmed that the breakdowntemperature of Fe-rich argentian pentlandite undersaturatedin Ag (less than 1 atom per formula unit) is slightly lower than for Ag-saturatedargentian pentlandite. 0m In magnatic Cu-Ni deposits,argentian pentlandite is resticted to Cu-rich zones.Experimental investiga- tions of Mandziuk & Scott (L977) bave shown that subsequentsubsolidus reactions between Ag-bearing tr Lqend intermediatesolid-solution (/ss), chalcopyrite and aftsmtio of Fe-Ni sulfidesbelow 455'C can produceargentian l-t fagBefus / Bdbasio lEl pentlandite either at grain boundariesor as exsolved F;l ldagleiss bodies in /ss or chalcopyrite. Both textures occur in lfiX)n naturalargentian pentlandite. However, Groves & Hall l-^^ ^l me{abasicmk (1978)and Benvenuti(1991) assumed that argentian l- ^-l hmblndeembs pentlandite could also be formed by replacement processes. tr aFl iD qusrtzwin Argentian pentlandite occurs in various mineral aA in lmpropbyre assemblagesand lithologic units of the pilot hole of t a!0 in pqagrsis the German Continental Deep Drilling Program. o Argentian pentlandite was formed not only during 20110m retrograde metamorphism at conditions of elevated I pressureand lemperature,but also underhydrotherrnal I conditions;it is not of magmaticorigtn. Gsol-octcAr.Sn'rrncc a The 4000-m pilot hole of the GermanContinental I Deep Drilling hogram (Emmermannet al. 1991) is 3lXX)m locatedat the westernma€ln of the BohemianMassif, in the Zone of Erbendorf-Vohenstrauss(ZEV, Oberpfalzregion, Fig. 1). This areais interpretedto be a suture zone between the Moldanubian and Saxothuringian units of the mid-European Variscan A AA fold belt (Franke 1989).The ZEY, a sectionof the AA A AA metamorphicbasemeng is mainly composedof para- ,t/\ gneissand metabasicrocks (amphibolite,metagabbro and meta-ulfiamaficroctrs). The paragneissicrocks are derivedfrom greywacke(Wimmenauer 1991), and the Flc. 2. Lithological profile ofthe KTB pilot hole' with occur- metabasicrocks are essentiallyderived from an intru- rencesof argentianpentlandite (apn). 806 TIIE CANADIAN MINERALOGIST garnet amphibolite with calc-silicate lavers. common interlayering of amphibolite, calc-silicate muscovite-biotitegneiss, biotite-hornblende gneiss rocks and sillimanite-biotite gneiss.This sequence and minor marble; (2) 46A-160 m, L6l0-2470 m and resemblesunit (l). 2690-3575 m: monotonousgarnet-bearing biotite Argentian pentlanditein the KfB pilot hole occurs gneisswith kyanite or sillimanite (or both), intersected in quartz veins at a depth of 509.77 m and, further- by dikes of Late Variscan lamprophyreand aplite; more, in paragneissat a depth of 2533.21 and (3) l160-1610 m and 3575-4000m: amphibolite, 3546.78 m, as well as in lamprophyredikes that cut metagabbroand minor meta-ultramaficrocks. and the paragneissat 2046.52,2083.62,2244and g) 247A-2690 m: biotite-hornblende gneiss with 2811.68m depth(Frg. 2). Ftc..3. A. Argentian pentlandite(apn) occursas small patchesin chalcopyrite(cp) wim inversion-twin lamellae. Assemblage l, s09J7 m, oil immersion, parallel polars. B. Detailedsketch of the markedarea in A. ARGBNTIAN PENTI.ANDITE, KTB PII.OT HOLE, GERMANY 807 TsruRAL REI-ATIoNSHPS sphalerite, galena and argentianpentlandite in para- gneiss (assemblages2 and 3) and in fine-grained, The occurrencesof argentianpentlandite in dif- unmetamorphosedlamprophyre. The textural features ferent lithologies of the KTB pilot hole can be related of argentian pentlandite in the three associationsas to different stagesof hydrothermal mineralization seenin reflected light microscopyare described @riedrich et aI. 199L,Kontny 1994).One type is pre- below. servedin quartz veins (assemblage1). A secondtype Argentian pentlanditeof assemblage,loccurs as is characterizedby dispersedsulfide mineralization small (3-10 pm) reddishbrown patchesor stringsin including pyrite, pyrrhotite, chalcopyrite,pentlandite, chalcopyritein a deformed quartz vein at a depth of Frc. 4. A. Pydf€ (py) and pyrrhotite (po) with flamelike pentlandite(pn) intergrownwith argentianpentlandite (apn) and chalcopyrite (cp). Assemblage2' 2533.2L m, oil immersion, plane-polarizedlight. B. Detailed sketch of the marked area in A to characterizeintergrowth relations. 808 TIIE CANADIAN MINERALOGIST 509.77m. The characteristicfeature of the chalco- intergrown with, pyrrhotite. Textural relationships pyrite is the developmentof twin lamellae.The inclu- indicatethat pyrite is youngerthan pyrrhotite. sions of argentianpentlandite are mainly oriented Assemblage3, consisting of argentianpentlandite, along the twin lamellae (Figs. 3A', B), suggestingthat cobaltian pentlandite, chalcopyrite, pyrrhotite and they may haveexsolved from chalcopyrite. sphalerite,occurs in biotite-sillimanite gneiss at a Assemblage2 occurs in biotite-hornblende gneiss depttrof 3546.78m (Figs. 5A, B). Argentian pent- at a depth of 2533.21m. Here, argentianpentlandite is landite is intimately intergrown with cobaltian intergrownwith chalcopyriteand occursin closeasso- pentlandite.Pyrite occurs as inclusions in pynhotite. ciation with pyrrhotite and pyrite @gs. 44, B). The The sulfide aggregatesare aligned along the cleavage pyrhotite containsflame-like lamellae of pentlandite, planes of biotite, which has been altered to chlorite. which are not in direct contact with argentian pent- The textural featuresimply a genetic relationship landite, and the pyrite containsinclusions of, and is betweenthe formation of chlorite and the sulfides. FIc.5. A. Complex aggregateofthe sulfideschalcopyrite (cp), pyrrhotite (po), cobaltian pentlandite (cpn), argentian pentlandite (apn), and sphalerite (sl). Assernblage3, 3546.78m, oil immersion, plane-polarizedlighr. B. Detailed skerchof the marked areain A. ARGENTIAN PENTLANDITB, KTB PILOT HOLE, GERMANY 809 At a depth of 3537.83 m, argentianpentlandite is pure metals (Co, Ag). ZAF correctionswere made intergrown with chalcopyrite and galena (biotite- using a modified version of program MAGIC IV sillimanite gneiss).In this sample,pyrite is the domi- (Colby 1968). nant Fe-bearingsulfide. Chalcopyriteand galenaalso Resultsof the analysesare summarizedin Table 1. occur in fractures of pyrite, whereaspyrrhotite is The stoichiomery of argentianpentlandite was calcu- absent. lated on the basis of eight atoms of sulfur and is In lamprophyres,argentian pentlandite is invariably comparedwith publishedinformation in Table 2. The associatedwith chalcopyrite,and in placeswith Ag contentof the KTB material rangesfrom 10.28to pyrrhotite and pentlanditeor galena.These grains of 12.6 vtt.Voand amountsto less than 1 atom of Ag per argentianpentlandite are too small for investigations formula unit (0.82-0.91) This Ag deficiency is not a by electronmicroprobe. courmon feafure in natural argentianpentlandite, but was describedpreviously by Vuorelainenet al. (L972) MNSRAL CHH\,flSTRY and by Friedrich et bI. (1989). The Fe/I.{i atomic ratio variesfrom 1.75to 1.9 and lies within the rangedeter- The chemical compositionof argentianpentlandite mined for other examplesof argentianpentlandite, and associatedchalcopyrite, pentlandite, pyrrhotite e.g.,L.34 to 2.57 Mandziuk & Scott1977). However, and pyrite was determinedby electron-microprobe the FeA.{i ratio is typically higher tlan for stoichio- analysis(ARL-SEMQ, 20 kV, 10 nA samplecurrent, metric argentianpentlandite (1.67). The metaUsulfur 10 s counting time) using the following standards: ratio rangesbetween 1.102 and 1:148,in agreement pyrite @e, S), niccolite (Ni), chalcopyrite(Cu) and with publisheddata (1.09G1.173). TABLE I. RESIJLTSOF EL,ECTRON_MCROPROBEANALYSES OF SELECIED MINBAIJ , FROM ARGM ASSE\dBLAGES, KIts PIldTf HOI,E. OBRPFAI,:Z. GERMANY Co C! Ag S loal FdI\i M6 dq' *% AsseNnbhgeI alrn x 35.86 19.86 -- r39 t1.94 3t2t 1m.20 1.90 1.15 &(L r22 r.vz - 0,16 0.09 033 cpx 30.99 0.08 0.03 v2A -- 35.00 rm34 1.04 s.(L o29 0.08 0.0r a2A -- 0r0 As€mblage2 apn x y.8 20J6 0.(B 0.43 rLffi 3rJl 100.01 1.75 t.ll s.d. o.(B 0.04 0.01 0.03 0,05 0.05 x 29.88 35.43 0.t2 33J3 98,96 0.89 1.09 8.{L 0.o7 0.r4 0.m 0.13 x 59.15 l.16 0.05 3955 99.95 0.88 s.& 0.19 005 0.01 0.01 x q.@ 0.r5 0.09 51.93 99.42 os2 8.(L 030 O.ffi o.vz 022 x 29.93 0.(X 0.04 33.86 3551 9958 1.00 &d. 0.97 0I)8 0.01 0.rs 094 As$nblage3 apnx3 33.11 t932 3,t2 0.65 tO.?A 3155 98.03 1.86 1.10 &d. 0.63 023 0.E2 0.m 050 0.r6 cfx3 2'J.39 4.42 m.65 0,(X 33.53 9839 0.93 r.Ul sd. 0.18 0.61 0.10 0.01 0.08 pox3 59,5 0.6 0a 0.05 38.99 99.13 0.88 sd. 0I)3 0.03 0n2 0.04 0.r0 c? x 3 30.89 0.07 0,07 3t.14 0.03 35,32 99.49 s.d. 0.16 0.04 0.02 0-56 0.03 o20 p-!tE.dt*.: l,I/S: metayrofir ratio; synbofs: af EEpdie e€fftde4 op dalcagfito' cf !"qqan F - bslor, limit 6d-d6, p" p!fu;, iry nrtre, nmtq of inatyses,x avgragp,s.d. stsodad dsviatlo, if detecdo.t sixrneo re--aonyzeoby B. Chffin' Nedtdds' Pdth' wesm Atrsmli& 810 TIIE CANADIAN MINERALOGIST TABIA 2 CAI,CUI,ATD @MPOSIIOI\IS OF ARGBVIIAN PE\NLINDIIE extraordinaryhigh Qs cont€nt,3.L2 wt.Vo.This might FROMTHE KTB PIIOT}!OLE also be an analytical artifact as appearsto be the case for Cu, but elevatedCo contentsin the associated Fo$gNZ?9CbO.tBAgO9lS8 KrB ldld lolePbspeL (qbhgo 1) chalcopyrite and pyrrhotite (0.07 and 0.23 wt.Vo, Fo4g7Nnd5AggJ4Sg fm Frtbore^)be(0elz (,ghlqge 2) Foa.9a&a66Cq32Ag.pS3 KrB dorblq{tbq@lz (aMttsge 3) Table 1) and the occlurenceof cobaltian pentlandite imply a strongrelation betweenchemical composition &n yZt{aCa(Co,Cuh.UagOSgSt Scttbnd. Gffimy (Fri€dlcl €aaL 1989 and paragenesis.The pyrrhotite contains 46.94 at.Vo Ra3s$3.67Ag1SS8 86qIbly (Band r91) Fe* and has antiferromagneticproperties (Keyssner Fo4.72M29Ag1.00$i rob ussnGdnadyd at t9E6) 1992),T\e presentborehole temperafure at a depth of Eo,CqCs)4.92M3.BAgl.635S Kmda, Gc€Orpodc 1983) (Zoth 6oco,6)533rt2.6Ag1.ffi Ks@da, Gmor@lrc r9$) 3500m is about110"C 1990).This temllerature FoJ.fSM2.79Agr.mSS qnlim(Gry6&R!nD78) is in good agteemetrtwith that necessaryfor the for- Fe52aNi2.31lgt91SS Cltldan(Gffi&q'tr f978) mation of hexagonalpyrrhotite with an NC supercell Fe5:2Nt19Ag13353 Kandrh l!fm, JaF Qlrlko €r aL l9'R) (Nakazawa& Morimoto 1971,Kissin & Scott1982). 8e4.$Nt3.fAg0.99S8 Bil RivB, lrm (grtl & Slcxet f 973) Fe4.96M323Ag1.g3S8 EtdRha,lrm (Scd&G'qlddit l ) DISCUSSIoN Fer.AMt.OZAgr.o:Sr SudhySarpolwaal f93) FeS.etMeg2A&.gtSS lhrb daosfra(ltodeeo €! aL rYt2) rej.SjMZ.rrACr.r0SS TeJEI(h,Glts(Sldlhtdn €t aL l9?l) Known phase-equilibria in the systems Fe451Ni337491.n989 Xbf,@te, CIUS(sHrhkln et a[ rtl) Ag-Fe-Ni-S and Fe-N({o)-S, as well as the com- position of associatedsphalerite and chlorite, allow constraintsto be placedon the environmentof deposi- tion of hydrothermal sulfide (--chlorite) assemblages containing argentianpentlandite. In the system Ag-Fe-Ni-.S, the assemblageargentian pentlandite + AssemhlageI pentlandite+ monosulEdesolid-solution (= pyrrhotite) + Ag appearsto be stable at defined sulfur activities Argentian pentlanditeoccuring as inclusions in (stability field of pynhotite) and temperaturesbelow chalcopyritecontains lL.9 ttrt.VoAg. The Cu conteng 455'C (Mandziuk& Scott1977; see Fig. 6). 1.39 *.Vo, doesnot seemto be incorporatedinto the Argentian pentlandite is commonly interpreted to sftucture of argentianpentlandite. Rather, it probably be an exsolutionproduct of chalcopyrite(Shishkin is an artifact of the microprobesampling region below et al. 197L,Vuorelainen et al. 1972,Scott & the surfaceof the grain. A very strongnegative corre- Gaspanini 1973, Mandziuk & Scott 1977, Groves & lation was detectedbetween the Cu content and the Hall 1978). Argentian pentlanditein assemblageL at size ofthe argentianpentlandite grains. 509.77m occrusas inclusionsin chalcopyritecharac- teized by inversion twins, also pointing to an origin Assemblage2 by exsolution. Twin lamellae in chalcopyrite with a metaVsulfur ratio greater than I form at the cubic- Argentianpeftlandile intergrownwith chalcopyrite, tetragonaltransition, which occurs below approxi- pyrrhotite and pyrite, has a higher FeA.[i (1.75) and a mately 550'C (Yund & Kullerud 1966).Although the Iower metaVsulfuratomic ratio (1.107) than stoichio- inversiontemperature of chalcopyriteis known to be a metric argentianpentlandite (L.67 and 1.125),and function of composition and pressure(Yund & contains L2.6 wt.VoAg. The Fey'Niratio of pentlandite Kullerud 1966),the twin lamellaealong (102) have (0.89) correspondsto pentlanditein associationwith also been producedby compressiotrat a confining p)trrhotite,or pyrrhotite atrd pyrite, as documentedby pressureof 3 kbar and a temperatureof 400oC Harris & Nickel (1972) and Misra & Fleer (L973). (Hennig-Michaeli & Couderc 1989) without involve- Pyrrhotile has a meaVsulfir ratio of 0.876 and a high ment of any phasetransformation. Therefore, the Ni content(1.2'aft-Vo). Peftographic examinations of a observedtwin-lamellae in chalcopyritecould have polished samplecoat€d with a magneticcolloid have formed during the retograde deformationof the KTts shown that almost all pyrrhotite is ferrimagnetic.The rocks under conditions of moderate pressureand ma netic charact€rand the Fe* contentof 46.67 at.Vo temperature.This is also consistentwith an exsolution (Fe*= Fe + Ni + Co) suggestthat pyrrhotite is of the origin of argentianpentlandite below 455oC. monirclinic4C modification(Keyssner 1992). Unfortunately,the fluid inclusionsof the argentian- pentlandite-bearingquartz vein werc found to be too Assemblnge3 small for study. However,in the rocks of the KTB, an alkaline palaeofluid systemwith low salinity is Argentian pentlandite, intimately intergrown with describedby Reutel (L992); it showstemperatures in cobaltianp€ntlandite, has an Ag contentof 10.3 wt.Vo, excessof 400'C at pressuresof about 2 kbar. This the lowest value of all three assemblages.In this para- fluid systemcould be responsiblefor the forrnation of genesis,argentian pentlandite is characterizedby an argentianpentlandite of assemblage1. ARGENTIAN PENTLANDITE, KTB PILOT HOLE, GERMANY 811 -10 c! U) CB a0 300 400 temperature("C) Frc. 6. Ternperature- sulfur fugacity diagram with sulfidation curve for the pyrite-pyrrhotite buffer from Scott & Barnes (1971) and extrapolatedfield of stability of argentian pentlandite to lower temperaturesin the system Ag-Fe-Ni-S, modified from Mandziuk & Scott (1977). Striped fietd: chlorite (chl) from assemblages2 and 3, triangle: sphalerite(sl) from assemblage3. The sulfides associatedwith argentianpentlandite assemblage3. In the sulfur fugacity - temperature of assemblage2 and 3 are oriented parallel to the diagram(Fig. 6), all chlorite dataplot very closeto the metamorphicfoliation, and are mostly associatedwith pyrite-pyrrhotite buffer. This could explain the phyllosilicates and titanite. Chlorite, which is inter- common occurrenceof pyrrhotite and pyrite in the grown with the assemblage-3argentian-pentlandite- samples,as well as the suddenchanges in proportion bearing sulfides, is a product of alteration of biotite of pyrite andpyrrhotite over a few meters. and can be classifiedas magnesianchamosite (Fe2* Sphaleriteassociated with argentianpentlandite of dominatesMg2*; Bailey 1980).In associationwith assemblage3, which is intergrownwith chlorite, assemblage2, two varieties of chlorite can be distin- contains 12 mole VoFeS. The log activity of sulfur guished:Mg-rich chlorite that formed from alteration calculatedfrom FeS contentin sphalerite(Scott 1973) of hornblende and Fe-rich chlorite showing textural and a temperatureof about 260'C determined from featuresconsistent with hydrothermalformation. Both the associitedchlorite is about 10-12.This value of types of chlorite can be classified as magnesian a(S) is higher than those obtained from the chlorite chamosite.Applying the o'six-componentsolid- data.A possiblereason for this deviationis loss of FeS solution model" of Walshe (1986), temperaturesof in sphaleritethat is not enclosedin pyrite. Therefore, chlorite formation are calculatedto be 160'-260'C in the FeS content determinedin sphaleritemay be too assemblage3 and 185'-250oCin assemblage2. These low, and the calculatedactivity of sulfur correspond- calculations are based on a pressureof 1 kbar. This ingly too high. However, the values of sulfur activity pressureis in accordancewith results of Borchardt & indicate that chlorite and sphalerite have formed Emmermann(1989), who found that the formation of within the stability field of pyrite (Ftg. 6). alterationminerals in the KTB pilot hole took place at According to the solid-solution fields determined presflresof lessthan 1.5kbar. by Kanedaet al. (1986), the chemicalcomposition of Sulfur activities have been calculatedassuming cobaltian pentlandite intergrown with assemblage-3 quartz - chlorite - pyrite equilibrium following argentianpentlandite points to a temperatureof f.orma- Walshe(1936). The log a(S/ valuesrange from 10-18 tion of less than 200'C. Thus it is assumedthat at to 10-14in assemblage2 and from 10-20to 10-13in least cobaltian pentlandite and sphalerite have been 8t2 TI{E CANADIAN MINERALOGIST deposiledunder hydrotherrnalconditions at a tempera- genesis.The Ag content of argentian pentlandite ture of 200o-260'C and a low fugacity of sulfur, less langesfrom 10.3 to 1,2.6wt,Vo, the Fe contentfrom than 10-13.These data are close to the extrapolated 33.1 to 35.9'xt.Vo,and the Ni contentfrom 19.3 to upper stability-limit of argentianpentlandite (Frg. 6), 20.8wt.Vo. which cross-cutsthe pyrite-pyrrhotite equitibrium at Two mechanisrnsare propo$edfor the formation of about260oC flog a(S) 10-ta]and seemsto reflecrthe argentianpentlandite: (1) lextural featuresof argentian environment of deposition in which argentian pent- pentlanditefrom assemblage1., occurring as inclusions landite of the KTB pilot hole has equilibrated. Thus in inversion-twinnedchalcopyrite in a quartz vein, are the temperatureof final equilibration of argentian consistentwith an origin by exsolution. This type of pentlandite must have been about or slightly below mineralizationhas probably been formed by remobil- 260'C. This correspondsto the experimentaldata ization from the host rock during the retrograde from Mandziuk & Scott (1977), which imply that deformation of the KTB rocks. Negative 6eS ratios reaction rates in the solid state of the svstem of chalcopyrite (-6.74%0)point to a mobilization of Ag-Fe-Ni-S decreasesignificantly in the range sulfur from metasedimentaryrocks (Berner et al. between200" and 300'C. 1991).The normal retrogradeP-T path inferred by Argentian pentlandite of assemblage3 in associa- Reutel (1992),giving conditionsof 2-3 kbar and tion with chalcopyrite,pyrrhotite, pentlandite and about 400oC,is consideredrealistic. (2) Subsolidus sphaleritehas also been describedfrom paragneissic reactionsbetween Ag-bearing chalcopyrite and Fe-Ni rocks in the Black Forest, Germany (Fiedich et al. sulfides are proposedfor the formation of argentian 1989)and the Kola deepdrilling, Russia(Genkin ar a/. penflanditeof assemblage2, which is intimately inter- 1984, Kozlovskiy e/ al. 1988). In the latter case, grown with chalcopyrite, pyrite and pyrrhotite. The argentianpentlandite occurs in carbonate-quartzvein- sameprocess probably is responsiblefor the presence lets in metabasicrocks containing chlorite, epidote, of argentianpentlandite of assemblage3, in which it is pyrite and pyrrhotite; the assemblageis interpretedto in close associationwith cobaltianpentlandite, chalco- result from hydrothermal mineralization in zones of pyrite, pyrrhotite and sphalerite. Textural relation- retrogrademetamorphism. Kozlovskiy et al. (1.988) ships, as well as est''natesof temperatureand sulfur assumedthat the presenceof argentian pentlandite actiyity basedon the chemicalcomposition of points to an epigeneticorigin of hydrothermalsulfide associatedchlorite, point to formation of argentian- mineralization.The'temperature of formation pentlandite-bearingsulfide-chlorite assemblages estimatedfrom the distribution coefficient of cobalt in underhydrothermal conditions. The conditionsoffinal the pyrrhotite-pyrite intergrowth rangesfrom 180 to equilibrationare assumedto be about260oC or slightly 323"C (Bezmenet al. 1975).This is in good agree- lower, at pressuresofabout l-2 kbar and at log a(S) ment with the postulatedtemperature of forrnation of less than 10-13.Negative 63aS values (-11.48 and ttre hydrothermal sulfide mineralization in the KTB -10.957oo)are consistentwith thosefrom hydrothermal pilot hole, as estimatedfrom sulfide and chlorite ther- environments,and further suggestthat sulfur and mometry (200-260'C). Values of 63aSdeterrnined for metalswere remobilizedfrom sedimentaryhost-rocks. the KTB rocks containingsulfide assemblages2 and 3 are depleted(-11.48 and -L0.95Voo).and are differenr AcKr.IowLEDcEl[ENTs from data on argentianpentlandite mineralization in the Kola deep drill-hole, which are in the range of This study was carried out within the framework of -l to +2Voo.This may reflect the different host-rock the German Continental Deep Drilling Program lithology of argentian-pentlandite-bearingmineraliza- (KTB). Thanks are due to the German Ministry of tion in the Kola deepdrill-hole (metabasite)and in the Researchand Technology@I!{FD and to the German KTB pilot hole (mainly paragneiss).The negativesul- ResearchFoundation @FG) for financial support.We fur isotope ratios of the argentian-pentlandite-bearingalso thank Z. Berner (Karlsruhe) for providing the rocks in the KTB pilot hole are consistentwith an sulfur isotopedata, the drilling qrew atrd coworkersin origin of sulfur from a sedimentarysource. Thus the KTB-Feldlabor in Windischeschenbachfor logis- remobilization is assumedto be responsible for the tical and scientific assistance.Thanks are due to formation of sulfides,with sulfir derived from a sedi- K. Benle (Giittingen), S. Hall @erth) and D. Schiips mentaryhost-rock (Aachen)for their constructivecriticism" and to S. Hall and B. Griffin (Perth)for re-analyzingsome of Sunruanv the argentianpentlandite specimens.A special thalk eNo Coxcr_ustoNs you is given to S.D. Scott, R.F. Martin and an anony- mous reviewerowhose commentshave been very Petrographicand electron-microprobestudies of useful in lmproving an earlier version. Finally, we different argentian-pentlandite-bearingassemblages gratefirlly acknowledgethe help of our lechnical assis- occurring in the KTB pilot hole show a variation in tants at Aachen University of Technology and the composition of the argentian pentlandite with para- Field Laboratoryof Windischeschenbach. ARGENTIAN PENTI,ANDITE, KIts PILOT HOLE, GERMAI'IY 813 Rrrmmlcx HArr S.R. & SrewARr,J.M. (1973): The crystal structureof argentian pentlandite, @e,Ni)6Agft comparedwith the pentlandite,(Fe'Ni)eSs. Can Mincral' BAILEv,S.W. (1980): Summary of recommendationsof refined structureof AIPEA nomenclaturecommittee, Clay Minerals 15, 12.169-177. 85-93. Hanns, D.C. & Ntcrn, B.H. (1972): Pentlanditecomposi- in some mineral deposits. Can. BnryBnrurn,M. (1991): Ni-sulphidesfrom the Bottino mine tions and associations (fuscany,Italy). Eun J, Mincral3,79-84. MtupraL11, 861-878. & Coupmc, I.J. (1989): TEM study of BERNBR,Z.,BLuM, N. & PucIrELr,H. (1991): Schwefel- Havrvro.Mrcresu, C. experimenally deformedchalco- Isotopie kontinentaler Kruste im KTB-Zielgebiet mechanicaltwinning in (CuFeS/ crystals.tr. Strain-induced Oberpfalz. In Forschungsergebnisseim Rahmen des pyrite single {102} DFG-Schwerpunktprogra-ms'KTB", I 98Gl 990 (R. twins.Ear. J. Mineral,1,295-310. Emmermann& J. Lauterjung, eds.). KIB Repon 9l-1, S. & Suol, T. (1986): stability of 379-392. KANEDA,H., Tar QuADr,A. vor (1990): U-Pb-zircon and Sm-Nd analyseson TEUFEL,S. (1988): VergleichendeU-Pb- und Rb-Sr- metabasitesfrom the KTB pilot borehole. In Beitrage Altersbestimmungenan Gesteinendes Ubergangsbe- zum 3. KTts-Kolloquium (GieBen)@. Emmermann& p. reichs Saxothuringikum / Moldanubikum, NE-Bayern. Giese,eds.). KTB Report!N4,545 (absrr.). GdttingerArb. GeoI.Paldont. 35, 1-87. Rrnuanor, J., KrrBuemt, U., Br,thmu p. & SqrnByrn. W. VUoRELATNEN,Y., HAKLT,T.A. & PApuNpr,r,H. (1972): (1989): Geothermobarometryof metapelitesas a key to Argentianpentlandite from someFinnish sulfide deposis. the lnessureand temperaturehistory of the ZEV (Zone of Am Mineral. 57, 137-145. Erbendorf-Vohenstrau8),NE Bavaria.In Beitfdse zum 2. KTB-Kolloquium (Gie8en)@. Emmermann&1. Gies", WAISHE,J.L. (1986):A six-componentchlorite solid solution eds.).KIB Report 89-3, 2+32. rnodel and the conditions of chlorite formation in hydrothermaland geothermalsystems. Econ. Geol.8I, REUTEL,C. (1992): Krustenfluide in Gesteinen und 681-703. Lagerstilftenam Westranl der BrihmischenMasse. Ph.D. thesis,Univ. Giittingen,Giittingen, Germany. WEBE&K. (1990): Observationsof the ductile deformation path of the paragneissesof the KTB pilot hole. 1z KTB ROHR,C., KoHL,J., HAcrim' W., Krvssr.rm, S., MfiutR, H., Pilot Hole, Results of Geoscientific Investigation in the STcMUND,J., SrRoH,A. & Zur_eur,G. (1990):German KTB Field Laboratory,0-4000.1 m (R. Emmermann, continental deep drilling program (KTB) - Geological H.G. Dietrich, J. Lauterjung & T. Wdhrl, eds.). KIB survey of the pilot hole "KTB Oberpfalz VB-. In KTB Repon90-8, J3-J19. Pilot Hole, Results of Geoscientific Investigation in the KTB Field Laboratory,0-4000.1 m (R. Emmermann, WIMMENAUER,W. (1991): Geochemieder metamorphen H.G. Dietrich, J. Lauterjung & T. W6hrl, eds.). KIB Sedimentgesteinein der KontinentalenTiefbohrung und Repon9O-8,B1-855. ihrem Umfeld. In Forschungsergebnisseim Rahmendes DFG-Schwerpunktprogramms'KTB", I 986-1990(R. Scor, , S.D. (1973): Experimentalcalibration of the spha- Emmermann& J. Lauterjung, eds.). KZB Repon 9l-1, lerite geobarometer,Econ GeoI. 68, 466474, 106-135. & BARNFs,H.L. (1971): Sphaleritegeothermome- YuNo, R.A. & Kur,lrntro, c. (1966):Thermal stability of try andbarometry. Econ Geol.6,653-669. assemblagesin the Cu-Fe-S system.J, Petrol.7, 454488. & Gesraxnnr, E. (1973): Argentian pentlandire, (Fe,Ni)rAgSr, from Bird River, .Manitoba.Can. Mincral, Za'r|J, G. (1990): Temperaturmessungennach AbscbluB der t2, t65-t68. Bohrarbeitenin der Bohrung KTB Oberpfalz VB. 1z Grundlagenforschungund Bohrlochgeophysik(Bericht 8) Scrurrwur, G. (1991):Metabasites in the pilot Boreholeof (J.K.Draxler, ed.). KTB Report90-1,299-313. the German Continental Deep Drilling project, Windischeschenbach,Eastem Bavaria. Structures and Fabics as Documentof Crastal Evolution Ph.D. thesis, Univ. Bochum,Bochum, Germanv. SHrsHKrN,N.N., MlrENKov,G.A., Mrrerlova, V.A.. SrDoRovA,A.F., KanpENrov,A.M. & Bunvo" I.M. \1971): An Ag-rich pentlanditevariety. Zap. Vses. ReceivedJuly 11, 1993,revised manuscript accepted Mineral. Obshchest.l00, 184-191. March 13.1994.