373

The Canadfun Mburalo gistVol. 33, pp. 373-388 (1995)

ION.MICROPROBE ANALYSIS OF PYRITE, CHALCOPYRITE AND PYRRHOTITEFROM THE MOBRUN VMS DEPOSIT IN NORTHWESTERN OUEBEC:

EVIDENCE FOR METAMORPHIC REMOBILIZAf,ION OF GOLD

ADRIENNE C.L. LAROCQUE* rxr C. JAY HODGSONDepartmsnt of Geological Sciences, Quzen's University, Kingston, Ontario K7L 3N6

LOIIIS J. CABRI aNr JENNIFERA. JACKMANCANMEI, 555 Booth Street, Ottqwa" Ontario KlAqcl

ABSTRACT

TheMobrunZn-Cu-Ag-Au deposit,located in theNorandaDistict, in the Quebec segmentof theAbitibi Greenstone Belt,is hosted mainly by felsic volcanic rocls of the Archean Blake Rivo Group. Primary facies of mineralization in the MobrunoreMies resulted from deposition and reworking of sulfides by synvolcanic hydrothermal fluids. Secondary facies ofmineralization resulted from greenschist-grade metamorphisn and related deformation, and locally overprinted the primaryfacies. The 'invisible" gold contents ofpyrite, chalcopyrite and pyrrhotite in various facies were determined usinga Cameca MS-4f ion microprobe. Samples were spuffered with a Cs+ pdmar/ bea:q and negative secondary ions weremeasured. External standards of sulfides wsre imflant€d with lqAu. Mass interferences were eliminaled by operating inhigh-mass-resolution mode (lvI/AIvI in the range 22100 to 3900), giving rise to minimum limits of detections of 50 ppbw. Primarypyrite contains up to 10 ppmw gold" present as submicrometic inclusions of metallic gold, as well as very fine colloid-size orstrucnrrally bound gold, Concentrations of gold in associared secondary (recrystallized) pyrite range from 1 to 67Vo of theconcentrations in primary pyrite. The results indicate rhat syngenetic gold is present in the Mobrun orebodix, and thatmetamorphic recrystallization resulted in its release from pyrite. The remobilized gold was deposited in tectonic veins as easilyrecoverable electum and as "invisible" gold in secondary chalcopyrite. The partitioning of gold among various phases insecondary veins may have been influenced by the interaction between tie remobilizing fluid and the assemblages ofprimary oreminerals.

Keyword.s:'invisible" gold, refractory gol4 ion microprobe, SMS, pyrite, chalcopyrite, pyrrhotite, electrum, remobilization,metamorphic recrystallization, volcanogenic massive sulfide, Mobrun, Noranda, Quebec.

Souuenr

IJ gisement i Zn{u-Ag-Au de Mobrun" sif,r6 dans le district de Noranda" dans le secteur qu6b€cois de la ceinture deroches vertes de fAbitibi, est encaiss6 dans une suite de roches volcaniques, surlout felsiques, du Group de Blake River, d'fuearch6en. Les facies primaires de mindralisation dans les gttes de sulfiires massifs se sont form6s par ddposition et parmodification des sulfures par des fluides hydrothermaux synvolcaniques. ks facibs secondafues de nin6ralisation se sont form6spax metamorphisme dans le facids schistes verts et par ddformation contemporaine, et localement ont oblit6r6 les facibs

ks teneurs en or "invisible" (r6fractaire) de la pyrite, la chalcopyrite et la pyrrhotite dans plusieurs facibs demin6ralisation ont 6t6 detemrindes par microsonde ionique Cameca IMS-4f. ks dchantillons ont 6td bombard6s avec unfaisceau d'ions primaies de Cs+, et les ions n6gatifs secondaires 6mis ont 6tf mesur6s. ks 6talons externes de sulfire.s ont 6t6implantds avec l'isotop 19Au. On a pu dliminer le,s interf6rences de masse par offration dans le mode de haute rdsolution demasses (IWAM de 2&0 e 3900), ce qui a permis un seuil de d6tectisl minimum ds 50 ppb Goids). Ia pyrite primaire contientjusqu'l 10 ppm d'or, soit incorpord dans la structure, soit en inclusions submicromdtiques ou encore de taille colloidale. Lapyrite secondaire (rccdstallis6e) contient enhe I et 67Vo de la leneur en or de la pyrite primaire. ks r6sultats indiquent que l'orsyngdn6tique est pr6sent dans les gftqs de Mobrun, et que la regristallisation metamorphique a lib6rd I'or de la pyrite, etl'a depos6 dans les veines d'origine sous forme d'6lectrum, facilement recouvrable, et d'or "invisible" dans lachalcopyrite secondaire. La r€panition de I'or parmi les divers min6raux des veines secondaires ddpendrait possiblement deI'interaction ale la phase fluide et l'assemblage de minerais primaires.

Mots-cl6s: or "invisible", or r6fractaire, microsonde ionique, spectrom6trie de masse des ions secondaires, pyrite, chalcopyrite,pyrrhotite, dlectrum, remobilisation" recristallisation metamorphique, sulires massifs volcanog6niques, Mobrun, NorandaQudbec.

e hesetrt addrcss: Departnent of Geological Sciences, The University of Manitoba, Winnipeg, Manitoba R3T 2N2.

TIIE CANADIAN MINERALOCIST

lurnoousnoN

Tlvo main types of Archean gold deposits occur intheAbitibi Greenstone Belt in northeastern Ontario andnorthwestern Quebec: (1) volcanogenic massive-sulfide (YMS) deposits, which formed synchronouslywith the host volcanic rocks and which codtain gold;(2) structurally controlled vein and replacementdeposi*. The origin of gold in Archean YMS depositsis contoversial, and three main views are held: (1) thegold was introduced syngenetically (e.9., Huston &Large 1989), (2) tlte gold was introduced syngene-tically, but its present distribution is the result ofremobilization that occuned during metamorphism andtectonic deformation (e.9., Tourigny et aI. 1989, 1993,Healy & Petruk 1990), and (3) the gold was introducedepigenetically, as in most structurally controlled gold-only deposits (e.9., Marquis et al. L990).

Ion-microprobe analysis of sulfide minerals hasbeen used for some time by process mineralogists forthe characlerization of sulfide ores (e.9., Chryssouliset al. 1985,1986,1987, Chryssoulis 1990, Chryssoulis& Cabri 1990, Cabri et al. L991, Marion et al.1991,Ca}ri L992). However, the geological application ofsecondary-ion mass spectrometry (SMS) to the ana-lysis of sulfides for trace metals has been more recent(Arehart et al. L99L, 1993, Bakken et al. L99L,Hickmott & Baldridge 7991,1-ayne et al.1991, Marionet al. L992,Pen91,992, Larocque et aI. 1992, 1993b, c,Neumayr et al. L993). The purpose ofthis paper is topresent the results of ion-microprobe analysis of sul-fide minerals from the Mobrun VMS deposit in north-western Quebec, and to discuss the implications ofthese results for the origin of gold in the deposit.

Evonwce FoR SYNGENE"TTcAND REMOBILMD GOLD

The clemest evidence of syngenetic deposition ofgold in massive-sulfide deposits comes from the iden-tification of gold in actively forming and recentlyformed deposits on the seafloor. Hamringlon et al.(1986) documented anomalously high contents ofgold(up to 1500 ppb) in samples from polymetallic snlfidedeposits in the eastern Pacific Ocean. Samples contain-ing up to 10 ppm gold have been recovered from theManus Basin of Papua New Guinea (Scott & Binns1992). Sulfide samples from the East Pacific Rise,Galapagos Rift, and southern Juan de Fuca Rtdgecontain up to 200 ppb gold @ischoff et al. L983).Auriferous strlfides have been collected on the Mid-Atlantic Ridge and the East Pacific Rise (Hannington& Scott 1988, 1989, and references therein). In addi-tion, up to 200.ppb gold has been measured in sulfidesfrom hydrothermal plumes issuing from seafloor ventsalong the East Pacific Rise (Ha:mington et al. L991.,and references therein).

There is also evidence for syngenetic deposition of

gold in ancient VMS deposits. Kerr & Mason (1990)proposed that the deposit at the Horne mine in theNoranda camp was formed by synvolcanic hydro-thermal processes operating during a period ofArcheanfelsic volcanism. Although deformation resulted insmall-scale rqmobilization of sulfides and gold into latequartz-bearing veinlets, they interpreted auriferoussulfides to have been deposited tbrough subseafloorreplacement of rhyolite tuffs. Huston & Large (1989)summarized the mineralogy and distribution of gold inselected YMS deposits in Canada Australi4 Japan,Sweden and Cyprus, in which gold is considered tooccur mainly as inclusions or in "solid solution" inro11i6e minerals (Huston & Large 1989), indicatingthat it was deposited synchronously with the prinarysulfides.

The general view of remobilization brought aboutby metamorphism is that it is a process of dispersionrather than one of concentration. Howeveq somestudies suggest that the interaction of pre-existingmineralization with metamorphic fluids may haveresulted in mobilization and concentration of elementsinto economic bodies. The case for late gold beingderived by metamorphic remobilization of eady goldhas been argued by Tourigny et al. (1989,1993) for theBousquet No. 2 deposit in the Abitibi belt, where earlymineralization and hydrothermal alteration resulted inauriferous massive-sulfide mineralization and asso-ciated argillic alteration. They suggested that remobi-lization of early sulfides during subsequent regionalgreenschist-facies metamorphism formed structurallycontrolled vein-type gold deposits. At the Trout Lakemassive-sulfide deposit near Flin Flon, Mnnitoba,Healy & Petruk (1990) identified fwo textural types ofgold alloy, as well as minor amounts of "invisible" gold(as determined by SIMS analyses) in pyrite andarsenopyrite. According to Healy & Petruk (1990),gold was released from pyrite following metablastesis,crystalliaing as fracture-fillings in pyrite from locallymigrating late metamorphic pore-fluids. In addition,anastomosing masses along the folded contactsbetween laminae of massive chalcopyrite and chloriteschist formed from metamorphic remobilization ofcoarse-grained free gold in the chalcopyrite-rich foot-wall ore. Huston et al. (L993) have documented theeffects of metamorphic recrystallization on the compo-sition of pyrite in a number of VMS deposits inAusfialia. Using proton-microprobe data, they con-cluded that recrystallization resulted in the release ofgold and other trace elements from pyrite. Althoughthey did not identify a potential sink for the remobi-lized gold, they documented important evidence for therelease of gold during metamorphic recrystallization.

Gnotoctcer Ssrm{c oF THE MoBRUN MINE

The Mobrun deposit is one of a group of YMSdeposits in the Noranda mining samp hosted by

TTIE MEXAMORPHIC REMOBILZANON OF GOLD 375

interbedded mafic-to-felsic submarine volcanic rocksof the Archean Blake River Group @RG; de Rosen-Spence 1976). T\e lower part of the BRG in theNoranda camp is a shield volcano, and the upper part isa caldera complex (Gibson & Watkinson 1990). Mostof the massive-sulflde deposits of the camp occur alongsynvolcanic faults within and along the caldera margin.The Mobrun deposit occurs in the upper part ofthe stratigraphic succession (Cycle V of Spence & deRosen-Spence 1975) about 7 km northwest of thecaldera margin.

The structural history of the area includes two mainperiods of deformation @imroth et al. 7983u Hubertet al.1984).T\e Noranda camp is located on the gentlydipping upright limb of one of a number of large-scalenorthwest-trending folds (D1). Associated Dl faultstypically consist of bedding-parallel zones of schis-tosity. The main cleavage-forming event was D2,

which resulted in east-trending folds and an associateddomainal cleavage. Metamorphic grades range' fromprehnite-pumpellyite to greenschist facies (Jolly 1980,Dimrotb et al. 1983b, G€linas et al. 1984).Amphibolite-facies assemblages that overprint green-schist assemblages occur in contact-metamorphicaureoles of syn- to late-tectonic intrusions (G6linaset al. 1984). In the Noranda are4 greenschist-faciesassemblages dominate.

The Mobrun mine is located 30 km northeast ofRouyn-Noranda (Frg. 1). The deposit is hosted byfelsic, intermediate and mafic lava flows, and felsic andintermediate pyroclastic rocks, that strike 110" to L20"and dip subvertically to the south (Caumartin & Caill61990, Barett et al.1992, Larocque & Hodgson 1993).It consists of three massive-sulfide complexes: the1100 orebody, the Main lens, and the Satellite lenscomplex (Fig. 1).The respective grades and tonnages

rhyolite

01mf_____j-- pyroclasllc

m€ffes ^ /,/ l@16a _ _ r _ _ _ _ _ _

- rhyollte --------ryYfL-,\#*l

\.f.-/

8ooRorryn-N.faida

i____ilm

r05G lre t|'7tG rm tcff llG tlic t12ffi rrErE

Ftc. 1. Geology of the Mobrun deposil modifred from Caumartin & Cailld (1990). Orebodie.s in black are near-surface; orebodyat depxh is stippled. Insef location map.

376 THE CANADIAN MINERAL@IST

ofthe orebodies are listed in Table 1. The 1 100 orebodyis statigraphically lowest and consists offour separatemassive-sulfide lenses, designated A, B, C and D (seeLarocque et al. L993U Fig. 4). The B lens contains thebulk of the mineralization in the 1 100 orebody; it has alateral extent of 300 m. and exlends.downward from360 m below surface to an unknorrn depth. The Mainlens has a lateral extent of 350 m and a present verticalextent of approximalely 200 m (the upper part of theorebody has been removed by erosion). The Satellitelens complex comprises tbree small massive-sulfidebodies representing three parts of one originally con-tinuous lens offset by northeast-striking faults (Fig. 1).

Alteration affecting the rocks that host the orebodiesconsists of sericitization, silicification, and chloritiza-tion (Caumartin & Caill6 1990, Riopel et al. 1990,Burctt et al. L992). In addition, synvolcanic hydro-thermal carbonate afteration has been recognized in therocks hosting the 1100 orebody (Larocque & Hodgson1993). A regional schistosity (S1) oriented parallel tosftatigraphy is best developed in localized zones ofaltered rocks enveloping the lenses ofmassive sulfides,and is cut by a second schistosiry (S2) snikingeast-west and dipping ste€ply to the south (Caumartin& Caill6 1990, Riopel et al.1990).

Ons PSTRocRAPHY AND GE@rfiMLsrRy

Ths main sulfide minerals in the Mobrun orebodiesare pyrite, sphalerite, and chalcopyrite (Figs. 2, 3), withminor galen4 arsenopyrite, digenite and tetrahedrite.Electrum, mainly in veins, also occurs as inclusions inpyrite and sphalerite (Ianocque & Hodgson 1991,Larocque et aI. L993a). In the 1100 orebody, pyrrhotiteis abundant in the sfratigraphically lower part of thelens. Quartz, carbonate, chlorite and muscovite arethe main gangue minspfu.

Based on mineralogical, textural, and structuralcharacteristics. thirteen different facies of mnemJiza-tion have been identifled at Mobrun (,arocque 1993,Larocque et al. L993a). The end-member types ofmineralization summarized in Table 2 may showgradational or overprinting relationships. These facies

TAELB I. ORB RBSERYBS. MOBRT'N ldNIEl

ORSODY TONNBS % Cu ,6 Zr g/tAg g/tAo tu/Ag

sATeulS' 179.m Lrt 2.4t 4\n 4.96 0.12

represent mappable subunits of the massive-sulfidebodies, and have genetic significance, as they resuftedfrom specific depositional or deformational processes.Prinary facies resulted from deposition and reworkingof sulfides by synvolcanic hydrothermal fluids, and aresimilar to minepli26lion in modern seafloor andKuroko deposits [see larocque (1993) and Larocqueet al. (1993a) for specific criteria for identification offaciesl. Secondary facies formed as a result of meta-morphisn and deformation, and locally overprint theprimary facies. Examples of some primary andsecondary facies of mineralization are illusfrated inFigures 2 ard3 (se9 also Larocque et al.1995, Fig. 1).

Each ofthe orebodies exhibits zonation ofthe faciesofmineralization. In addition to large-scale zonation offacies (Larocque et a\.1993a, Figs. 4 to 6), texturalzonation has been observed on hand-specimen andthin-section scales @igs. 2, 3). There is excellentpreservation of the primary facies of mineralization inthe Main and Satellite lenses, given the state of defor-mation of rocks hosting the Mobrun orebodies.Although the 1100 orebody also contains primarytextures, the development of secondary facies is morepronounced, owing to its more severe deformation andmetamorphic recrystallization (Larocque et al. L993a).The distribution of. domirwnt facres of mineralization issimilar in each of the orebodies, although the disnibu-tion in the 1100 orebody is complicated by the fact thatthe B lens comprises two coalesced massive-sulfidelenses. In general, each lens or sublens comprises abasal zone of granular mineralization, a core zone ofmassive pyrite, and an upper zone of granular mineral-ization that gtades upward into banded pynte +sphalerite or massive sphalerite. Pyrrhotite-bearingfacies are abundant in the footwall to the B and Clenses of the 1100 orebody (Larocque 1993).Overprinting secondary facies consist mainly of tans-gressive veins developed toward the flanks ofthe coreof massive pyrite in the Main and Satellit€ lenses andthe western part ofthe B lens ofthe 1100 orebody. Theveins generally contain quartz and chalcopyrite, andmay conlain electrum. Recrystallized massive pyriledominaters in the eastern part of the B lens, and ispresent as patches in the flank areas of the Main andSatellite lenses (Larocque et al.L993a).

Hand samples containing primary facies of mineral-ization contain up to 5 ppm gold in the Main lens andup to 10 ppm gold in the Satellite lens complex. In boththe Main and Satellite lenses, primary facies contain10 to 60 ppm silver. In the 1100 orebody, pdmaryfacies contain up to 10 ppm gold and up o 160 ppmsilver. In all orebodies, samples with secondary veinsexhibit significantly higher concentations of gold,silver, or both, owing !o the presence of electum. Themaximum concentrations of gold and silver are lowestin the Satellite lens complex (23 and 137 ppm, respec-tivd) and highest in the 1100lens (245 and 780 ppm,respectively; Larocque et al. 1993a),

MAINZ

u@r

955,m 0.81 2.4 30.30 2.n O.()7

7572,W 0.84 t.87 42-g 1.75 0.(X

8,706,m 0.& 5.42 4t.t2 l.E7 0.05

t hovlded by Resures fuftsy bc, 2 Poven Fsuvos. B&!6y l, l9S.' Plobsblo rosdvoc Odd€r Zt, 191-

TI{E MEIAMORPHIC REMOBILZANON OF GOLD 377

FIc. 2. Examples of primary facies of minenlization from the Mobrun deposit. A1l photos except for B are reflected-lightphotomicrographs. Abbreviations are as follows: pynte (py), pyrrhotite (po), chalcopyrite (sp), iphalerit€ (sp), galena (ga),chlorite (ch). Scale bars in lower left corner; lengths of bars in parentheses. A) Granular pyrite with concentric colloformlqdiog of chalcopyrite, in a matrix of chlorite. Arrow indicates location of secondary-ion image in Figure ll (250 pm).B) Back-scatter scanning electron microscope photograph of spheroidal aggregatl of pyrite with colloforrn banding defined!y thalcopynte and sphalerite (100 pm). C) Massive fine pyrite with colloform bands of sphalerite and galena tiOO pml.D) Pyrite partly replaced by chalcopyrite in massive pyrite - chalcopyrite facies (200 pm). E) Massive sphalerite with thinbands of pyrite (5fft Um). F) Stringer mineralization consisting of chalcopyrite and pyrrhotite with "swiss cheese" textgrereplacing altered wallrock (100 trm).

378 TTIE CANADIAN MINERALOGIST

Frc. 3. Examples of secondary facies of mineralization from the Mobrun deposit. Abbreviations as in Figure 2. A) Primarycollofornbanding truncated by -assive coarse (recrystallized) pynte (500 pn). B) Prinaly massive fine pyrite (mfp) cutby secondary vein containing coarse euhedral pyrite (arows), chalcopyrite, and sphalerite (250 pm). C) Foliated pynte inmatrix of carbonate, chlorite, and quartz (500 U.m). D) Vein of electrum (arrow) in sphalerite (200 Um).

MErtoDol.ocY

Sampling and balk chemical arnlyses

In situ samplTng of ore was carried out in accessibleundergtound areas of the Main and Satellite lenses in1990 and 1991. In addition. drill core from the Satellitelens complex and 1100 orebody was logged andsampled (core from tle Main lens had been used formetallurgical testing and wa$ not available forsampling). Splits (5G-300 g) of bulk samples wereanalyzed for Cu, Zn, and Ag (by atomic absorptionspectroscopy) and Au (by fue assay) at the lab atMobrun. Polished sections of ore were prepared forpetrographic study and analysis by electron microprobe(EPMA), scanning electron microscope (SEM), andsecondary-ion mass spectrometry (SMS).

I on-microprob e analy s is

The calibration and operation of SMS have beendiscussed by Mclntyre et al. (1984), Chryssoulis el c/.(1989), Cabri & Cbryssoulis (1990), and Cabi et al.(1989, 1991), among others. For this study, a CamecaIMS-4f ion microprobe was used to quanti! the goldcontent of sulfide minerals from the various lenses ofore at the Mobrun mine. Bxperimental parameters arcsummarized in Tbble 3. For their ion-microprobe studyof gold content of pyrite, Chryssoulis et al. (1987)operated in high-mass-resolution mode with an_g-nergyoffset to eliminate a mass interference with lrCsS;(Fig. ) and achieved minimum 4s1ss1i6a-limi1!(MDL) of 500 ppb. For our study, operation in high-mass-resolution mode alone yielded good dynamicrange (peallbackground) and lower detection-limits

THE METAMORPIilC REMOBILZATION OF GOLD

TABLE 2. FACIES OF MINERALUATION*

379

PRIMARY FACIES Formed by depositioa and reworking of sulfides by synvolcanichydrothermal fluids (syngenedc./diagenetic)

Bedded [ritic Mineralization

e'raNlar $ritic Mineralizdion

Nodular Pgitic Mineralization

Massive h/rite-Chalcopyrite

Massive Fine grrite

Massive Sphalerite

Banded ryrite-Sphalerite

Banded Pyrrhotite-Sphalerite

Banded Pyrrhotite-Pyrite

Stringer Mineralization

- prn-scale pyrite euhedra in chlorite matrix- apparent layering, 'graded bedding"

- Fm- to rnm-scale pyrite spheroids and spheroidal aggregates inmalrix of sphalerite or gangue

- ioternal colloform banding defined by porous pyriG,chalcopyrite, sphalerite, galena

- mm- to cm-scale spherical bodies of pyrite in a sphalerite matrix- ndiaring internal stucture

- massive pyrit€ with unoriented replacement veins of chalcopyrite orpseudomorphic replacement of colloform./spheroidal pynte by chalcopyrite

- microcrystalliae to crypfocrysta[ine pyrite- colloform textures defined by porous pyrite, minor chalcopyrito, galena,

sphalerite

- massive fine-grained sphalerite

- bands of massive sphalerite up to l0 cm wide, alternating with bands ofgranular, massive, or nodular pyrite

- bands of massive pyrrhotite intedayered with massive sphalerite

- bands of massive fine-grained pyrrhotite interlayered with granular pyrite

- veins, disseminations, and patches of chalcopyrite and/or pyrrhotite in alteredwallrock

SECONDARY FACIES Fonned during metamorphism and deformation

Massive C.oarse P)'rie

Foliated Mineralization

Transgrassive Veins and BrecciaZonqs

- massive coarse-grained pyrite with granuloblastic texture (triple junctionsformed by recrystallization), some sutured grain boundarias

- traosgressive wispy bands of fne, abraded pyrite and discontinuous baods ofangular pyrite

- subparallel and orthogonal sets of veins and veinlets, anastomosing networksof veinlets, vein breccias

- contain any of pyrite, chalcopyrite, sphalerite, galen4 electrum, quar%chlorite, carbonale

'From larocque et at. (1993a)

than cited previously (Fig. 5, Table 3). Details of datareduction are given in Appendix I ofLarocque et al.(1995). In dl, 178 analyses of pyrite in 78 sections,59 analyses of chalcopyrite in 39 sections, and 15 anal-yses of pyrrhotite in 9 sections were carried out fordifferent facies of mineralization from all three ore-bodies at the Mobrun mins. lea-micrsplefue analysesof pyrite and chalcopyrite in samples from the Mobrundeposit had been ca:ried out previously (Cook &Cbryssoulis 1990). However, preyious investigators

analyzed samples from tailings, possibly resulting in abiased sample-population.

Rrsu.rs oF IoN-MIcRopRoBE ANALysEs

Pyrite

The frequency diagrams of gold content of pyritefrom the Main lens show log-normal disributions, withmaxima in the 1-2 ppmw range (Fig. 6), similar to

380 THE CANADIAN MINERALOGIST

TABLE 3. HPERIMENTAL PARAMETERS FORION.MICROPROBE ANALYSIS

GENERAL PAR,AMETERSI

Instflrment

Stsndardiztion

Cameoa MS-4f ioo micropde

exteraal (on implantation)

ION IMPLANTATION

Ion sou$e

Nominal ion energier

Operati4g ion energies

Implanted ryeoies

Irylantation dosc

Mineral ryecies

low-pressure kqAton dc themral ionization sourceconstnrolcd at Chalk River Iaboratories, Canada

300 - 20(D kev

l MeV

reAu

2.5E13 ions/cd

pyrite, chalcopyrite, pyrhotite

OPER,ATING CONDITIONS

Bean source

Primary-ion polarity

Secondary-ion polarity

Primary beam cument

Impact energy (primary ionr)

Sample-charge cornpensation

hirnary-beam diametEr

Cmtor size

Analysis diameter

Mass hterference

Energy offret

Masr resolution

Minimum detection limits (Au)

Depth of analyzed profile

Dynamic rango (peak/back$ound)

Cs

positive

negative

50 - 150 nA

5.5 keV (10 kev PAPr, 4.5 keV SAts)

notre

5 0 - 6 0 p m

approximatsly 200 pm x 200 pm

60 lm.terCssz

none

2400 - 3900

30 - l0O ppb

0.3 - 1.3 pm

3 decades

I kimary Acceleratiog Potential2 Secondary Accelerating Poteotial

results obtained by Cook & Chryssoulis (1990). Thefrequency distribution of gold content in pyrite fromthe Satellite lens complex is bimodal, with the princi-pal maximum in the 1-2 ppmw ftnge and a secondmaximum in the 8-16 ppmw nmge Gig. 6). Thefrequency distribution for pyrite in the 1100 lens isbimodal, with the principal maximum in the 1-2 ppmw

range and the second maximlrm in the 0.06{.12 ppmwrange (Fig. 6). The positive skew for sarples from theSatellite lens complex may be due to sampling biasbecause the samples were selected for SIMS analysison the basis of their high gold content in bulk assays,whereas assays for some samples from the 1100 lenswere not known prior to doing SIMS analysis.

TIIE METAMORPIilC REMOBILZANON OF GOLD 381

ttc"si

l9Ar'

198.8 198.9 197.0

MASS (anu)

Maln Lensl l c 6 6

1100 Lenst l=75

0.03 0.06 0.12 0.25 0.5 1 2 4 8 16Au (ppmw)

Frc. 6. Frequency distribution diagrams for gold content ofpyrite (in ppmw), as determined by SIMS.

single sections containing homogeneous, inclusion-free, massive fine pyrite, the variation in gold contentranges from 0 to 2OVo. In sections containing granularpyrite, the variation in gold content ranges from 0 to387o. Small spheroids ofpyrite consistently have lowerconcentrations of gold than larger ones. In differentthin sections from the same hand specimen, the vari-ability in gold content of various facies of pyrite is ashigh as 507o.

Figures 7 shows the concentrations of ooinvisible"gold in pyrite in various facies of mineralization. TheIowest concenfiations of gold generally occur inmassive recrystallized pyrite, whereas high concentra-tions of gold occur in granular and massive fine pyrite(Ftg. 8). The spatial relationship among the variousfacies of mineralization in individual hand samples andthin sections is shown in Figures 2 and3. The range ofgold content in samples containing both primary andsecondary facies of pyrite is much larger than thosecontaining only primary facies. Massive recrystallizedpyrite and pyrite in secondary veins cowistently exhibitlower concentrations of gold than associated primarypyrite @g. 7). For example, recrystallized granularpyrite contains between I and 50Vo of associatedprimary concentrations of gold. Recrystallizs6l massivepyrite contains between 1. and6T%o of the gold concen-

1 5ln,

zI

6z

uo

rd

lol

(Jco=oE

ll-

1 5

1 0

oco5ItEtl.

Ero=ctI

ll.

Flo. 4. High-mass-res_olution spectrum showing Au peak andinterfering mass ltozCsS;)

9E

ztrtrzuJozo

mF"s

refu

0.0 02 0.4 0.0 0.8 1.0 12 1.4

DEprH (pm)

Flc. 5. SIMS depth-profile through extemal pyrite standardimplanted with lryAu. 9FeS was monitored to ensureinstrumental stability and sample homogeneity.

However, the I 100 lens has the lowest gold grades, andthe Satellite lens complex has the highest gold grades(Table 1), consistent with the low and high secondmaxima and the low and high skewing of the principalmaxima in the respective frequency-distributions(Frg. 6). Results of ion-microprobe analyses also areconsistent with assays of hand specimens of ore(Larocque et al. L993a).

The variable gold content of pyrite in differentsamples reflects the bulk zonation of gold throughoutthe orebodies. The reproducibility of analyses ofpyritein individual thin sections depends maidy on theiextural homogeneity of pyrite. Tbble 4 summar.izesthe relationship between gold content and reproduci-bility of analyses in relation to facies of pyrite. In

102

lo t

100

1or

382 TTIE CANADIAN MINERALOGIST

TABLE 4. RANGE OF COLD.CONTB.ITS OFPYRITE (BY SIIvIS)

q/rite-Boaring FadesNumber of Averag€ Mean o/oAnalyses Gold€ontent Devlatlon Devlation

(pprnw) (ppmw)

granularmasslve, slightly recrystallized py

massfusflne pymassMe flne py wllh Induslons

granular recrystalllzed pymas6lve recrystalllzod py

masslve flne py wlth lnclusionsmasslve flne py wlth lncluslons

nodular pygranular py

masslve llne pymasslve flne py

Satelllte Lens Complex

granular py wlth Incluslonsgranular py wlth Inclusions

granular pygranular py with c,olloform cpy

granular pymasslve recrystalllzed pymasslv€ flne colloform py

rnasslve flne py wlth lnclusionsgranular py

massfue flne lo granular pycoalae granular py

flne granular pymassive flne py with lncluslons

1 10O Len6

Maln Lens

1*

3'

56#78o

1 01 11 21 314151 61 7

o1 1142000

466t)o

3tl2020

o.7 0.04.5 0.53.5 0.51.3 0.3t.'t 0.01.5 0.02.1 1.01.3 0.82.O 0.01.5 0.50.s 0.10.6 0.1

I

2s43o7II1 01 1 "1 2

12#

56#78',oI O

t 1 #1 21 3

1 31 7o00

20503!!0

1 32AT 433430

25o

granular py wlth flne lncluslonsgranular py wlth coarse lncluslons

masslve flne pymasslve recrystalllzed pywith @y

massfue flne pymasslve flne py wlth Incluslons

massfu e recrystalllzed pymassfue flne py wlth incluslonsmasslve flno py wlth lncluslonsmasslve ffne py wlth lncluslons

granular recrystalllzed pyrnaaslve, sllghtly recrystalllzed py

lollated pymassfue flne py with Incluslons

granular recrystalllzed pymasslve coarse recrystallized py

masslve flne py

1 .52.5't .6

10.50.90.52.O2.O1.01.01 .50.5o.9

't.2

3.02.11 .52.12.2o.4o.81 .5't.2

o.51 .8o.20.4o.40.0o.4

0.0o.70.60.5o.20.10.10.50.oo.o0.00.00.3

o.20.50.00.00.0o.4o.20.30.0o.20.10.30.14.20.00.00.0

o28s8

o1 820

5

25o00o

8Ii

* same thln secuon as enty llsted below' samo hand speclmen as thln s€ctlon llsted below

trations in primary massive fine pyrite. The goldcontent of coarse euhedral pydte in secondary veinsranges from 3 ta 1,4Vo of that in associated massive finepyrite. The large ranges of gold concentrations insecondary pyrite likely are a function of the degree ofrecrystallization, the lowest gold content being present

in pyrite that has undergone the most extreme recrys-tallization (e.g., coarse massive pyrite, and euhedralpyrite in veins).

With respect to the siting of gold in pyrite, it appearsthat two types of gold are present. Figure 9 shows twodepth-profiles through pyrite, one of which is flat; the

TTIE MEIAMORPHIC REMOBILZANON OF GOLD 383

t ,

oaY 0.1

E 1oo

$'ot60EaoEE 2 0=( ) O

E loo

$*!60Eao=E 2 05( ) o

E 1oo

$'otuo6 4 0E520oo

0.01

0.0011 @

B rE

: 0.1

0.01

t0EE Ieq

= o.'l

Salsllhg Lffi

l l00 LeE

1 2 3 4 6 0 7 8 0 1 0 1 1 1 2

Prlrrwy Facles: E Granular hdte I Masslv€ Fhg h/rit€S€condaty Faole$ l% Granular R@ryfr[[z€d hdts

N tr{dye R€cry€talnzed pyriteEl Veh! hrrlts

FIG. 7. Gold content of various pyrite-bearing facies coexist-ing within individuall rhin sections. (ecrystallizsd massivepyrite and pynte in secondary.veins consistently exhibitlower concenfrations of gold than associated pdmaxypynte.

other shows a distinct peak. The peak is caused bythe presence of an inclusion of electrum about 0.2 pmin diameter. The flat profile represents the content of"invisible" gold (<0.1 pm; Haycock 1937), whichconsists of very fine colloid-size gold or structurallybound gold. Other types of analysis are needed todistinguish between these two possibilities (cf Marionet al. 1,992). The gold contents in Figures 6, 7 and 8 andTable 4 represent levels of "invisible" gold.

Chalcopyrite

Fewer analyses were done of chalcopyrite than ofpyrite because of the relative proportion of eachmineral in the deposit. It seems, however, that thefrequency disftibutions of gold in chalcopyrite aredifferent from those ofgold in pyrite. Frequency distri-butions for pyrite in all orebodies show a maximum infhe l-2 ppmw mnge (Fig. 6). In contast, the principalmaxima in the frequency disnibutions of gold inchalcopyrite (albeit based on fewer samples) are vari-able @ig. 10). The distributions of gold in

0.0:l 0.08 0,12 025 0.5 1 2 4 I 16Au/Ag (ppmw)

IRecr!,stalllzsd NffillMasslve t-GEnularFto. 8. Cumulative frequency diagram showing proportions of

various pyrite-bearing facies within each concentrationinterval in Figure 6. Massive Fine and Granular Facies areprimary facies; recrystallized facies is secondary.

chalcopyrite in the Main and Satellite lenses showmaxima in the 0.5-1 ppmw range, whereas the maxi-mum for the 1 100 lens is in the 0.0il.l2 ppmw range(Fig. 10).

Primary chalcopyrite occurs as thin colloform layersor fine grains intergrown with pyrite, and thus was toofine-grained to be analyzed with the 50 pm-diameterbeam used for this study. Therefore, most of thechalcopyrite atalyzed consists of coarse grains insecondary veins. However, analysis of pyrite withcolloform bands or fine inclusions of chalcopyriteyielded higher concentrations of gold than associatedooclean" pyrite, indicating that gold is present inprimary chalcopyrite in higher amounts than in theassociated pydte. In addition, ion images of chalco-pyrite veinlets emanating from primary colloformchalcopyrite in spheroidal aggregates ofpyrite indicatethe presence of gold in primary chalcopyrite (Frg. 1l).

Pyrrhotite

Relatively few analyses ofpynhotite were done, allof which were on samples from the 1100 lens;pyrrhotite is not present in the other orebodies. The low

0.0r0.001

384 TIIE CANADIAN MINERALOGIST

3Eo

z

FzUz

DEFfH 0rm)Hc. 9. SMS depth-profiles tbrough samples of pyrite from

Mobrun. The peak in A1 is caused by an inclusion of elec-trum about 0.2 pm in diameter.

number of samples analyzed may account for theskewed distribution in gold content of p)ryrhotite,which has a maximum in the 0.03-0.06 ppmw range(Frg. 12).This maximum is lower than the maxima ingold content of pyrite in any of the orebodies, andslightly lower than the maximum for gold-content ofchalcopyrite in the I 100 lens.

Maln Lensn = ' 1 7

1100 Lsnsn = 2 8

Satelmo Lensn = 1 4

0.m 0.06 0.12 0.25 0.5 1 2 4 I

Au (ppmw)

Frc. 10. Frequency-distribution diagrams for gold content ofchalcopyrite as determined by SIMS.

DIscussIoN

The presence of significant amounts of gold inprimary pyrite and chalcopyrite establishes unequivo-cally that syzgenetic gold is present in the Mobrun ore-bodies. The fact that recrystallized pyrite contains lessgold than the associated primary pyrite indicates thatrecrystallization resulted in the release of gold. It ispossible that this release resulted from synvolcanichydrothermal recrystallization (Huston et al. 1993).Howeveq the presence of pressure-solution textures inpyrite and the occurrence of coarse euhedral pyrite onthe margins of tectonic veins and oriented overgrowthson granular pyrite indicate that at least some recrystal-lization accompanied metamorphism and deformation(Larocque 1993, Larocque et al.1993a).T\vs, mzta-morphic recrystallization resulted in the release ofrefractory goldfrom pyrire and its subsequent deposi-tion as electrum in tectonic veins.

Although electrum occurs in all three orebodies atMobrun, it is most abundant and coarse grained in the1 100 orebody. This is partly due to the higher degree ofrecrystallization in the 1100 orebody relative to theother lenses, resulting in the liberation of a greateramount of gold. The results of the ion-microprobeanalyses indicate that remobilized gold also may bedeposited as "invisible" gold in secondary chalco-pyrite. Unlike the frequency-distribution diagramsof gold in pyrite, which all have a maxima in the1-2 ppmw range, the histograms showing the goldcontent of chalcopyrite exhibit different maxima, withthe maximum for the 1100 lens bang an order ofmagnimle lower than the maxima for the Main andSatellite lenses. This observation suggests that remobi-lized gold may have been partitioned differently duringdeposition from the metamorphic fluids allecting thedifferent orebodies: in the Main and Satellite lenses,remobilized gold was deposited mainly in gold-richchalcopyrite associated with minor elecffum, whereasin the 1100 orebody, remobilized gold was depositedmainly as electrum in association with gold-poorchalcopyrite. The differences in host mineral and in

I I C{) L€ng

n = 1 5

<MDL .0+.06.0&.12.12-.25 .25-.5 .S1 1-2 24Au (ppmw)

Fro. 12. Frequency-distribution diagram for gold content ofpyrrhotite, as determined by SMS.

3s54$sf ;z

1

8oo5g4l L 2

o

9 r3 A

t s2

1

> 4o5 g

Iz[ 1

0

TI{E METAMORPHIC REMOBILZAIION OF GOLD

FIG. 11. Ion images for Cu (op) and Au (botiom) in chalcopyrite veinlet shown inFigure 38. Scales at left correspond to secondary-ion counts.

385

degree of concentration of remobilized gold in the dif-ferent orebodies rnay reflect differences in the sulfuractivity of tle remobilizing fluids, as the concentrationof sulfide-held gold in equilibrium with free goldincreases with increasing activity of sulfur in the fluid(Barton 1970). The stratigraphically lower part of the1100 orebody is characterized by abundant pyrrhotite(stable under low activity of sulfur), whereas the ironsulfide in the Main and Satellite lenses is pyrite (stableunder high activity of sulfrn). The composition of the

remobilizing fluid would have been influenced by itsinteraction with primary ore-mineral assemblages.Relatively gold-poor pyrrhotite would not have con-tributed much gold to the fluid, but would have influ-enced the activity of sulfur in the fluid, thereby increas-ing the stability of free gold relative to sulfide-heldgold. Thus, differences befween primary mineralassemblages of the orebodies rnay have exercisedsignificant control on the partitioning of gold amongsecondary phases in each of the orebodies.

386 THE CANADIAN MINERAIOGIST

Our results are signif,cant for a number of reasons.First, although the intensity of metamorphism atMobrun is relatively low, metamorphic remobilizationof gold has been demonstrated. The amount of goldreleased and the distances oftransport were not large inthe Main and Satellite lenses; in the 1100 lens, redisni-bution occuned on the scale ofthe orebody (Larocqueet al. 1993a). However, mztamorphic recrystallizationhas been important in increasing the amount ofcyanide-recoverable gold in the deposit. Moreover, thefact that metamorphic remobilization has occurred atMobrun suggests that some sulfide-rich gold deposits(e.9., Bousquet Dumagami) may have resulted fromintense metamorphic reworking of primary gold-richbase-metal sulfide deposits like Mobrun (Iourignyet al. 1989, Larocque 1993, Larocque & Hodgson1993).

CoNcLUsIoNs

(1) The presence of measurable quantities of gold inprimary facies of mineralization establish that syn-genetic gold is present in the Mobrun orebodies.(2) "Invisible" gold in pyrite is present in submicro-scopic inclusions ofelecfium and as fine colloid-size orsffucturally bound gold.(3) Metamorphic recrystallization resulted in therelease of "invisible" gold from pyrite, and was impor-tant in upgrading refractory gold in pyrite to a moreeasily recoverable form (electrum).(4) Partitioning of remobilized gold between elecfrumand gold-bearing chalcopyrite may have been con-trolled by interaction of the metamorphic fluid withprimary ore-assemblages.

ACKNOwlIDCBVBlnS

We are gratefrrl for the support of geological andmine personnel at Ressources Audrey Inc. and MineMobrun. Our research has benefitted from discussionswith Michel Bouchard of Ressources Audrey Inc.,G6rald Riverin of Mi:rnova Inc.. and Jim Franllin andMark Hannington of the Geological Survey of Canada.At CANIvIET, Vic Charrand provided capable techni-cal assistance with ion-microprobe analysis. AtQueen's University, Martin Burtt drafted some figures,and Tom Pearce kindly provided access to a photo-graphic microscope. The manuscript was preparedwhile the first author was a guest scientist in Earth andEnvironmental Sciences Division, Group L, at [.osAlamos National Laboratory New Mexico. Logisticalsupport provided by EES-I is greatly appreciated. Thefirst author thanks Fraser Gofffor his sponsorship andJim Stimac for providing access to a photographicmicroscope. We thank Eric Montoya (EES-l) and RuthBigio GES-4) for computer-aided drafting of figures.We are indebted to Tim,. Barrett, an anonymous review-er, Associate Editor Frank Hawthorne aud Editor

Robert Martin for comments that improved the manu-script. Financial support was provided by RessourcesAudrey Inc., Minnova Inc., and the Natural Sciencesand Engineering Research Council of Canada througha Collaborative Research and Development Grantawarded to CJH.

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Received June 10, 1993, revised rnanuscript accepted.January 4, 1994.