Canadian Mineralogist Vol. 17, pp, 463-468 (1979)

THEJOHNS - MANVIIIE PLATINUM- PALLADIUM PROSPECT, STITLWATERCOMPLEX, MONTANA, U.S.A.

H. K. CONN Exploration Department,Minittg Group, Johns-M atl ill e Corporntion, Denrer, Colorado80217, U.S.A.

AssrRAcr INtnooucttoN

Exploration initiated in 1967 has resulted in the A field exploration program initiated in 1967 discovery of a significant deposit of platinum-group by the Johns-ManvilleCorporation has resulted elements in the Stillwater complex. an early Precam- in the discovery of significant occurrencesof brian igneous intrusion in south-central Montana. platinum-group elements (PGE) in the Still- The mineralization occurs in a horizon of sulfide- water complex, an early Precambrianstratiform rich anorthositic (plagioclase plagioclase- rocks and igneousrocks which extend for a strike length of bronzite cumulates) within the Banded Znne of longitude 110'00/ W in south-centralMontana. the complex. The favorable horizon is essentially geological continuous ov€r a strike length of 39 km. In the This paper briefly describes the West Fork exploration adit, the av€rage grade is setting of the mineralization and the methods 14.7 e/t (: F.p.mJ Pt+Pd and 0J5Vo Cu*Ni of exploration that led to its discovery, across a width of 1.9 m and over a strike length The Stillwater complex comprises a differen- of 657 m. In another location, an average grade of tiated sequenceof layered mafic and tlltramafic 22.3 e/t Pt+Pd is indicated across a width of igneousrocks which extend for a strike length of 2.1 m and along a strike length of 5.5 km. The 48 km and have a maximum exposedthickness average Pt:Pd ratio is 3.5 in each instance. The of 7.4 km (Fig. l). The igneouslayering gener- presence of mineralization is reflected by coincident soil geochemical, induced polarization and magnetic ally strikes west-northwest and dips moderately anomalies in most cases. to steeply northward. The facing of the layers is also to the north. Along its lower (southern) margin the intrusion is in contact with older (> 3140 m. y.) hornfelsed metasedimentary SolrMerns rocks and younger intrusive quartz monzonite (275O m. y.) which bracket the age of em- placement of the Stillwater magma (Nunes & Un programme d'exploration lanc6 1967 a en Tilton 1971, Page 1977), The upper part of abouti i la d6couverte d'un important gisement de by m6taux du groupe du platine dans le complexe de the complex is overlain unconformably Stillwater. massif intrusif du Pr6cambrien inf6- Phanerozoic sedimentary rocks so that the rieur, situ6 dans la partie centrale m6ridionale du original vertical extent of the intrusion is not Montana. La min6ralisation se pr6sente dans un known. In addition to the aforementionedearly niveau d'anorthosites riches en sulfures (cumulats Precambrian quartz monzonite, the conrplex is de plagioclase ou de plagioclase * bronzite) dans intruded by late Precambian diabase and by la zone ruban6e du complexe. Ce niveau favorable Tertiary granitic rocks. The Stillwater complex persiste sur une distance de 39 km. Dans la galerie is cut by numerousnorth- or northeast-trending d'exploration West Fork, la teneur moyenne est transverse faults and west-northwest-trending de 14.7 (: p.p.m.) Pt Pd et o.l\Vo Ni s/t + cu + thrust and ramp faults, most of which prob- sur une largeur de 1.9 m et une distancede 657 m. orogeny A un autre endroit, on trouve une teneur moyenne ably developed during the Laramide (Jones de 223 e/t Pt + Pd sur une largeur de 2.1 m et el al. 196O). et une distance de 5.5 km. Aux deux endroits. le The nature of the igneous layered sequence rapport Pt:Pd moyen e,st de 3.5. Dans la plupart as described by Jones et al. (196O) is outlined des cas, des anomalies simultan6es de trois types: in Table 1. The subdivisionsummarized here (l) g6ochimique (dans le sol), (2) magn6tique, et differs somewhat from that of Hess (1960), (3) polarisation induite, signalent la pr€sence de la particularly with regard to the Banded and min6ralisation. Upper Zones; these have not been as thoroughly (Traduit par la Redaction) studied as the Basal and Ultramafic Zones. 463

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MONTANA Bu.rNGs ila ilvrNcsToN\ ifF-AREA'lN lwvol,ilt,lb MrLEs,6V:t P-r-,€-r--la R3"," rg llm 16.5

g

06 t-\ e \t GronireFli[:9- Aeu it6 .Gobbro LOST dfN [o& Anoithosite MTN. I Bronzitite MTN member E (u rz t@Peridofite WESTFORK ADIT =o membe r ]N LM BLACKBUTTE Melomorphic Foult ( defined , opproximote) orocks ..** tili3; uoooo' Thrustfoult i""i".h:1'at";i'; Jn'a i, ri I -ffi (sowteethon upper plole) Frc. 1. Geological map of the Stillwater complex indicating position of ZOI mineralized horizon.

Concentrations of chromite in the Pr,errNur"r-GRoup-Er-eMsNrMrNrneuzATloN member of the Ultramafic Zone have been exploited to a limited extent in the past. Mag- PGE mineralization has been recognized in matic nickel-copper sulfide deposits occur in four settingswithin the Stillwater complex: (1) the Basal Zone of the intrusion (Page & Simon The first interestingPGE occurrencediscovered 1978). The platinum-group-elementmineraliza- as part of the Johns-Manville program is asso- tion outlined by the Johns-Manville Corporation ciated with the contact between the rocks of the is localized for the most part in the Banded complex and a younger intrusive granite stock Zone, with less important quantities occurring south of Picket Pin Mountain (Fig. .1). A bulk in the upper part of the bronzitite member of sample of this material assayed5.I g/t (- the Ultramafic Zone. p.p.m.) Pt, 3.4 g/t Pd, O.4glt Rh, O.24VoNi,

TABTEI. SEQUENCEOF TAYEREDIGNEOUS ROCKS IN THE STILLIIATERCOI,IPLEX.

Zone Member Thlckness Characterlstics

Upper Zone 2040 m Anorthoslte, , troctolite; lorer boundarymrked by appearanceof ol'ivine as cumulusmlneral

BandedZone 2255 Anorthosite, norite, gabbroi lower boundaryrnrked by appearanceol plaglo- clase as cumulusmlneral; rhythmlclayering

Bronzltl te 305 Bronzitite, essentially a single layer Ultramfic Zone Peridotlte 915 ,dunite,bronzltite,chromitjte: rhythmic layering

Basal Zone 60 Noritic rocks, m€dlun-grained,variable textural relations

rJones et al. (1960)

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O.O3%Cu and SVo Crfig (2) PGE mineraliza- proportion of the palladium is also present in tion occurs in coarse-grained bronzitite at the solid solution in pentlandite (E.W. Nuffield, upp€r contact of the bronzitite member of the pers. comm. 1974, 1975; L.J. Cabri, p€rs. comm. Ultramafic Zone. The 'Janet 5,0' zone, an ex- 1974). Estimated grade in the ZOI exposed in ample of such mineralization, occurs about the exploration adit is 14.7 g/t Pt+Pd with 1 km NE of Iron Mountain (Fig. 1). The Pt:Pd = 3.5 and O.lSVo Cu*Ni over a strike surface area of the showing is approximately length of 657 m and a true width of 1.9 m. In 30 x 150 m and the averagegrade is O.24% another part of the ZDI extrrlored by shallow Ni*Cu and 2.7 g/t Pt*Pd with Pt:Pd from diamond drilling, a grade of. 22.3 g/t Pt+Pd I to 2. Dr. L.J. Cabri (pers. comm. 1974) with Pt:Pd = 3.5 is indicated over a strike found that the principal opaque in a length of 5.5 km and a true width of 2.1 m. bulk sample from this zone are pyrrhotite, pent- A layer of 'troctolite' with up to TOVocumulus landite and pyrite with lesser chalcopyrite and olivine, 20Vo intercumulus and the minor galena, sphalerite, violarite, arsenopyrite, balance intercumulus plagioclase, nor'mally oc- magnetite, gold, merenskyite/kotulskite, mon- curs below the anorthositic rocks that contain cheite, sperrylite, graphite and Ag{b. A signi- the mineralization. As the olivine in the trocto- ficant proportion of the platinum-group minerals lite is generally serpentinized,the rock is highly (PGM) occurs as inclusions in pyrrhotite. Pal- magnetic and servesas a useful marker horizon ladiu,m is also found in solid solution in pent- in magnetometer surveys. Iandite. (3) A large lens of bronzitite, possibly a cognate xenolith, occurs within the Banded Zone and contains mineralization similar to that Expron.lrroN MsrHons at the upper contact of the bronzitite member, with the addition of Pt-Rh and Pt-Fe alloys The decision was taken in 1962 to investigate (Cabri & Laflamme 1974), braggite and Pd the feasibility of an exploration program aimed arsenides including stillwaterite. This has been 'Janet at discovering deposits of the platinum-group designated the 55W' zone; grades of 7.2 elements. The Stillwater complex was selected g/t PtfPd were obtained across widths of as a favorable area for exploration on the basis 1.5 to 3 m. The mineralizedzones are discon- of (a) its similarity to the well-known Bushveld (4) tinuous. The most important PGE - complex in South Africa, (b) reference by ization found to date occurs in a sulfide-rich Howland et al. (7936) to an occurrenceof PGE horizon in the Banded Tr:lne some 335 to 425 in what is known now as the Banded Zone, m above the upper contact of the bronzitite and (c) the reported occurrence of sulfide hori- member. The horizon is essentially continuous. zons, also in the Banded Zone (Hess 1960). with an indicated strike length of 38.6 km and The field program was initiated in 1967 and has a true width typically on the order of 1.8 m. involved an expenditure, exclusive of salaries, The mineralization in the zone of interest (ZOI) of about 7 million dollars through 1977. The occurs mainly in dark-colored anorthositic rocks chronology of exploration activities is sum- characterized by grey cumulus plagioclase and marized in Table 2, A variety of exploration oikocrysts of ortho- and clinopyroxene. Sulfide methods has been employed, including tradi- and associatedminerals occur principally as tional prospecting, geological .mapping, soil and finely disseminatedgrains but also as globules silt geochemistry surveys, magnetometer sur- up to several cm in diameter and as minute veys, induced polarization (I.P.) surveys, very interstitial veins. The most abundant sulfide low-frequency electromagnetic survey ( VLF minerals are pentlandite,chalcopyrite and pyr- EM), trenching, diamond drilling and under' rhotite. Sphalerite,galena, millerite and marca- ground exploration. site occur in minor amounts whereas covellite. One of the principal reasons for the success native copper, stibnite, argentian tetrahedrite, of the exploration program was the develop- magnetite, graphite, gold and palladian gold are ment in 1968 of sensitivetechniques for the observed even less commonly. The principal analysis of Pt and Pd in soils by Bondar{legg PGM are, in order of decreasingabundance, Company of Ottawa. Reconnaissancesilt and braggite, vysotskite, moncheite and Pt-Fe alloy, soil geochemical surveys resulted in the identi- with minor or rare amounts of cooperite, still- fication of numerous anomalies related to waterite, arsenopalladinite, palladobismutharse- sourcesin the Banded Zane. For example, in nide, palladoarsenide,kotulskite, zvyagintsevite, the area west of the East Boulder River, back- Pd-tellurides, merenskyite and sperrylite (L.J. ground concentrations in the soils were less Cabri, pers. comm. 1974, 1976). A significant than 30 ppb Pt and 10 ppb Pd whereas ano-

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malous stations gave maximum values of. 265 assayed15.4 g/t Pt+Pd with Pt:Pd = 2.5, are ppb Pt and 193 ppb Pd. Theseregional anomalies summarized in Table 3. were in general followed up with more detailed The results of the geochemical and I.P. sur- 'Janet soil sampling, time-domain I.P. and magneto- veys over the 96W zone, part of the meter surveyi. In one case,the regional anomaly zone of interest (ZOI), are illustrated in Figure was found to result from mineralized boulders 2. The superimposed soil chemistry and I.P. (i.e,, glacial'floaf) which in turn were traced chargeability anomalies clearly outline the mine- 800 m up-ice to their source in what became ralized zone in the bedrock. In general, back- designated as the 'Janet 50' zone. The results ground I.P. chargeabilities are on the order of of a detailed soil geochemistry traverse over I to 3 milliseconds whereas anomalies are 3.5 the7.6 m wide C-602 mineralized zone. which to 8 milliseconds.

TABLE2. SUMIIIARYOF THE EXPLORATIONHISTORY OF THE JOHNS-MANVILLEPt-Pd PROSPECT

Year Expenditure Act,ivl ty Results 1952-66 - Literature search selection of Stillwater complexas favorable target area on basls of similarity to Bush- veld conplex and reported presenceof PGE 1967 $ 2,500 Prellminary sanpling of knowndisseninated Lowto negligible assays, no significant sulfidernineralization,soilsampling anonalies 1968 19,000 Reconnaissancerbpping, prospecting& soil Discoveryof mineralization at contact of sanpling south of Plcket Pin ilountain gabbrcic rocks with youngergranite Regionalstream geochemJstry Anonaliesrelated to a source ln the banded zone Jn the Iron Creekarea 1969 77'000 Reconnaissancesoll geochenistry,mapplng, Nurproussrnall, erratlc soil anonnlies in magneticsurveys, prospectlng the lron Creekarea I 970 81,000 0verburdentrenching of soll geochernical Discoveryof sulflde-rich c.-9. bronzltite anor6li es bouldersin till Regionalsoll geochemicalsurveys Numerousanomalies' especlally west of East Boulder.River I 971 80,000 Boulder tracing to locate source of sulflde- Sourcearea of boulders near upper contact rlch boulders found ln 1970 of upper bronzltlte, nlneralized outcrop found during line cutting ln preparation for l.P. survey of Janet 50 zone Mapping,trenchlng, sanpling of Janet 50 zone Reglonalsoil geochemicalsurvey of western flu[Erousanorn]ies part of complex 1972 237,000 lhpping, trenchlng, sanpling & dfilling of Nu[Eroussulfide-rlch zoneslocated ln Janet 50 dlscovery area c.-9. bronzJtite I.P. survey of 'ranet 50 ar€a Anoraly (70 m) related to graphite-rich brcnzi tl te Detailed nagnetic, geochemical& I.P. surveys Nunerousgeochemlcal/geophysical anonalies of area found to be anonElousdurlng recon- naissancesurveys (,lanet 55l,lzone) Drilllng and trenchlng of compositegeochem- Zoneof dJscontinuoussulfide mlnerallza- ical/geophyslcal anomaliesin Janet 55!l zone tlon Jn c.-9. bronzltite outllned t973 568,000 Tr€nchlngof superlmposednagnetic/soil geo- Exposureof 2-3%dlsseninated sulfide in chemical/I.P. anornlies in Janet 961,1zone pegmatoldalbronzite troctollte Drilllng of ragnetic/soil geochemical/I.P. Dlscoveryof main mlneralized bandZ0I anonnlies near Janet 961{zone (zone of interest) ln louer norite unlt 1974 I,004,000 Reconnalssance& detalled geochemical Strlke length of Z0I estirnted to be surveys, rEpplng & prospectlng to evaluate 41.8 km extent of Z0I along strike Bulk samplingof Z0I for rEtallurgical Satlsfactory recoverles of Cu, Ni, Pt testing and Pd indicated CormEncer€nt,of exDloration adit 1975 2,527,000 Contlnuationof undergroundexploratlon Adit driven 855 m' 365 m of raises & crosscuts completed. Undergrounddlamond drilllng totEled 745m. Surface exploration, deepdrill lng Z0I dellneated further 1976 1,530,000 Undeigroundexploration terninated l'letallurglcal benchtests Recoveryo1 659 P1+Pd' 68U Ni, 752 Cu 1977 855,000 DrJlllng and surface exploratlon of Z0I Conflnnationof the presenceof a hlgh- grade zone 5.5 km long x 2.1 m wlde, averaging22.3 glt Pt+Pd

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TABLE3. GEOCHEMICALTRAVERSE ACROSS C-602 MINEMLIZEDZONE

\\ Station Distance ppmCu ppmNi ppb Pt ppmPD C-600 0 m 25 123 20 40 \ c-60t 30 80 250 1l0 375 '1670 c-602 60 I 68 480 ?575 'l c-603 90 f 9 375 ?20 425 c-604 120 43 160 30

Local background a8 123 AF za

)l to be essentially continuous along 39 km of strike length and is typically 1.8 to 2.1 m thick. I The grade of mineralization exposed in the exploration adit averages 14.7 glt Pt*Pd and 0.l5Vo Cu*Ni across 1.9 m and over a strike \ "r. Iength of 657 m. In another locality, an average \{t grade of 22.3 g/t. Pt+Pd is indicated by dia- mond drilling across a width of. 2.1 m and along a strike length of 5.5 km. The average Pt:Pd ratio is 3.5. These grades are 2 to 4 Frc. 2. Geochemicaland induced polarizationano- malies associatedwith Janet 96W mineralized times those typical of the Merensky ReEf in zone. Dashed contours indicate ppb Pt - ppb the Bushveld complex of South Africa. Pd in soil. Solid contoursindicate induced polar- ization chargeabilitiesin milliseconds. A,CKNoWLEDGBMENTS The results of surface exploration activities were sufficiently encouraging to warrant initia- Major credit for the success of this project tion of underground exploration in 1974. An goes to the J-M field staff under the capable adit was driven into the north wall of the Wesr direction of E.L. Mann. Early work on the Fork canyon (Fig. 1), the most accessibleand project was carried out by S.G. Ellingwood, geographically favorable location known at the C.S. Stadler and, later, over the years by a time. This was undertaken in order to (a) es- large number of capable field geologi$ts, as- tablish the width, grade and contintrity of the sisted by students and field men. D.L. Monroe ZOl, (b) test mining and stoping methods, and supervised the adit mining program and rnetal- (c) mine a large bulk sample (1lO tons) for lurgical studies. Norman J Page, U.S. Geologi- metallurgical testing. These results were favor- cal Survey, and members of the local U.S.F.S. able. gave valuable assistance and cooperation, as Metallurgical bench tests demonstrated that did Louis J. Cabri, CANMET. Vanous con- an acceptable sulfide concentrate averaging 2 tractors and consultantsprovided effective serv- 'ore' to 3Vo of. the milled could be produced by ices, including assaying,deep drilling programs, f-lotation. Recoverieswere in excessof 85% f.or helicopter transport, mining methods and me- Pt and Pd, 68Vofor Ni and 75%of.or Cu. Gold, tallurgical processes and analyses. The per- silver and PGE other than platinum or palladium mission of Johns-Manville Corporation to amount to approxi.mately 2.3Vo of the total publish these data is appreciated. PGE content of the sulfide concentrate.Electric furnace melting produced a low-grade matte RrreneNcBs with only minor loss of economic metals to the slag. Si'milarly, hydrometallurgical refining of Cesnr,L.J. & Lenr-arrvrs,J.H.G. (1974): Rhodium, gold the matte yielded saleable copper and nickel platinum and alloys from the Stillwater Can. Mineral. 12, 399-403. precipitates, and acceptable products for PGE complex. refinery feed with only minor losses. Hrss, H.H. (1960): Stillwater igneous complex, Montana: a quantitative mineralogical study. Geol. Soc,Amer. Mem. 80. CoNCLUSIoN Howr-eNn, A.L., Prorrrs, J.W. & SerrapsoN,E. (1936): The Stillwater igneouscomplex and as- The ZOI horizon in the Banded Zone of the sociated occurrences of nickel and platinum Stillwater complex constitutes an important de- metals. Montana Bur. Mines Geol. Misc. Cont. posit of platinum-group metals. T\eZOl appears a.

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JoNrs, W.R., PEopLEs,J.W. & Howr,aNo. A.L. PecB, N.J (1977): Stillwater complex, Montana: (1960): Igneous and tectonic structuresof the rock succession, metamorphism and structure Stillwater complex, Montana. U.S. Geol. Surv. of the complex and adjacent rocks. U.S, Geol. Bull, L071-H,28I-335. Surv, Prof. Pap;999. & Sruotrt, F.O. (1978): Differentiation of (1971): Nunrs, P.D. & TnroN, G.R. Uranium- the sulfides in the Basal Tnne of. the Stillwater lead agesof minerals from the Stillwater igneous complex, Montana. l. Res. U,S. Geol. Surv, 6' complex and associatedrocks, Montana. Geol. 473-482. Soc, Amer. Bull. 82,2231-2249. Received October 1978, revised mantncript ac- ceoted December 1978,

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