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 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/17/2/463/3446067/463.pdf by guest on 26 September 2021 464 THE CANADIAN MINERALOGIST 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 Norite.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 peridotite 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, gabbro, 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 Harzburgite,dunite,bronzltite,chromitjte: rhythmic layering Basal Zone 60 Noritic rocks, m€dlun-grained,variable textural relations rJones et al. (1960) Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/17/2/463/3446067/463.pdf by guest on 26 September 2021 PGE PROSPECT IN THE STILLWATER COMPLEX 465 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 minerals 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 pyroxene 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 mineral- 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.