Stillwater Complex - Stillwater, East Boulder

Montana, USA

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The Stillwater Complex is a major layered 2700 Ma ultramafic-gabbroic intrusion exposed over a distance of 45 km in the Beartooth Mountains of Montana, USA.

Platinum Group Elements are exploited from Stillwater and East Boulder, which are shallow to intermediate level underground mines. Stillwater is located ~135 km SW of Billings and 7.5 km SSW of Nye, in Sweet Grass County, whilst East Boulder is ~50 km south of the town of Big Timber, and 20 km WNW of the Stillwater Mine.
(#Location: Stillwater - 45° 23' 21"N, 109° 52' 32"W; East Boulder - 45° 30' 12"N, 110° 5' 7"W).

The 200 km2 Stillwater Complex comprises an up to 6.5 km thick, relatively steeply dipping, sheet of layered mafic-ultramafic rocks and an associated pre- and syn-marginal suite of sills and dykes (Hess, 1960; Jackson, 1961; McCallum, 1996). The igneous layering has an average dip of ~65° NNE to locally overturned in the area of the Stillwater River Valley. The complex was emplaced into ~3.3 Ga hornfelsed metasedimentary rocks at depths of 6 to 7 km (Page, 1977; McCallum, 1996; Mogk and Mueller, 1990). It was intruded by mafic dykes at 2.6, 2.4 and 1.6 Ga (Nunes and Tilton, 1971; Baadsgaard and Mueller, 1973) and subjected to regional metamorphism at ~1.7 Ga, locally producing greenschist facies assemblages (Nunes and Tilton, 1971; Page, 1977). The complex was subsequently eroded to variable depths prior to the middle Cambrian and unconformably covered by Palaeozoic and Mesozoic sedimentary rocks. Thrusting and uplift during the Cretaceous to Eocene Laramide Orogeny tilted the complex and exposed a cross-sectional view at the surface (Jenkins et al., 2020).

The upper portion of the intrusion has been eroded along a pre-Middle Cambrian unconformity, and is overlain by Palaeozoic and Mesozoic sedimentary rocks. Regional Laramide thrusting, faulting and uplift has resulted in the present-day northeasterly dip of the layered complex. Altered cumulate xenoliths derived from the Stillwater Complex are found in Late Cretaceous plutonic rocks 8 to 12 km north of the outcropping intrusion in the Beartooth Mountains, suggesting the exposed intrusion is part of a much larger body (Brozdowski, 1985).

The Complex is composed, from the base upwards, of (after Wall et al., 2018):
Basal Series, which was deposited at ~2712 Ma, forms an irregular sheet-like mass made up of early-formed cumulate rocks totalling 60 to 400 m in thickness. It is dominated by orthopyroxenite with subordinate norite and sulphide-bearing assemblages and contains xenoliths of cordierite-pyroxene hornfels (Page, 1979; McCallum, 1996). Weak magmatic nickel-copper sulphide mineralisation occurs at the base of the Basal Zone of the intrusion (Page and Simon 1978). Cu-Ni grades decrease both upward from, and below the contact with the underlying metasedimentary rocks (Attanasi and Bawiec, 1987). Based on sparse drilling and sampling, Attanasi and Bawiec (1987) estimated potential of up to 20 Mt @ 0.5% Cu+Ni in selected tested areas in the western half of the complex. A suite of rocks with five different chemical composition occur as discontinuous sills and dykes of dolerite, mafic norite and massive sulphide that intrude metasedimentary rocks beneath the complex (Zientek, 1983; Helz, 1985).
Ultramafic Series, which comprises cycles of harzburgite (a peridotite) and bronzitite (an orthopyroxenite) with conformable layers of chromitite in the lower parts of each cycle, which together are 840 are 2000 m thick. The series is divided into a lower Peridotite and an upper Bronzitite zone reflecting the dominent mineral. The Peridotite zone contains ~20 cyclic units, where a complete unit consists of peridotite-harzburgite-bronzitite, with or without chromitite seams. There are 11 main chromitites, A to K. The cyclicity and absence of cryptic variation is interpreted to reflect influxes of magma and venting of fractionated magma from the chamber. Two distinct age groups are recognized in the Ultramafic series: i). the lowermost peridotite zone, up to and including the G chromitite, crystallised at 2710 Ma from magmas emplaced below, and ii). the overlying, olded, uppermost peridotite and bronzitite zones that crystallised earlier at 2711 Ma.
Banded Series, which was deposited at ~2709 Ma, and varies from 1935 to 4468 m in thickness. It is dominated by plagioclase-rich rocks (norite, gabbronorite, troctolite, anorthosite) and has been divided into a Lower, Middle and Upper Banded Series. The Lower and Upper Series are primarily composed of norite and gabbronorite while the Middle Series is predominantly anorthosite, troctolite and olivine gabbro. Cryptic variation in the Lower and Upper Banded Series is consistent with crystal fractionation and accumulation. In contrast, plagioclase in the Middle Banded Series has a uniform composition throughout, interpreted to be the result of a sorting mechanism in which plagioclase crystals were suspended in a convecting magma for extended periods. A generalised sequence through the Banded Series, from the base comprises:
  - Lower Banded Series - divided into the Norite zone I; Gabbronorite zone I; Olivine-bearing zone I; Norite zone II; Gabbronorite zone II; and Olivine-bearing zone II. The J-M Reef is hosted within Olivine-bearing zone I, some 400 to 450 m above the base of the banded series.
  - Middle Banded Series - is ~1800m thick and contains two thick anorthosite zones, Anorthosite-I and Anorthosite-II at its top and bottom, separated by Olivine-bearing zone III and Olivine-bearing zone IV, the latter of which contains a thinner Anorthosite zone in its lower sections (Haskin & Salpas, 1992; McCallum, 1996).
  - Upper Banded Series - is composed of two zones, the lower relatively thin Olivine-bearing zone V and Gabbronorite zone III. • Late-stage granophyres and at least one phase of post-emplacement mafic dykes which also crystallised at ~2709 Ma. The granophyres, which occur throughout the Banded series (Czamanske et al., 1991) are composed of white to pink veins that are 1 to 12 cm thick and up to 100 m long. They are almost exclusively composed of quartz and sodic plagioclase, with 30 cm splays of tremolitic to actinolitic amphibole. They are typically oriented at high angles to layering, with sharp linear contacts and narrow tapering terminations suggesting they were emplaced after consolidation of the host mafic rocks.

The 1 to 3 m thick J-M Reef, hosted within the Olivine-bearing zone I of the Lower Banded Series, is the principal mineralised unit. It has been traced over a length of 40 km.   One 5.5 km segment of this reef averages 24 g/t Pt+Pd over a 2.1 m thickness. In the Stillwater mine area it averages 1.75 m in thickness.
  The host Olivine-bearing zone I cumulates comprise repeated cyclic sequences, from bottom to top, of coarse-grained to pegmatoidal peridotite, troctolite and anorthosite (McCallum et al., 1980; Todd et al., 1982). The peridotites vary from irregular layers to pod-shaped lenses of olivine cumulate with very large poikilitic clinopyroxene and orthopyroxene, intercumulus plagioclase, and phlogopite and minor chromite. The olivine of the troctolites varies from amoeboidal textured to small, isolated rounded crystals, most of which has been altered, most commonly to serpentine and magnetite. Overall, the quantity of olivine in each cycle decreases upward. The anorthosites in each cycle usually includes intercumulus to poikilitic pyroxenes as much as 10 cm across. Locally this sequence of olivine-bearing rocks to anorthosite has been seen to be repeated as many as 10 times (Leroy, 1985).

In the Stillwater mine area the J-M Reef is found within a reef package which corresponds roughly to the fifth olivine-bearing sequence in Olivine-bearing zone I (Todd et al., 1982). Within the package, the olivine-bearing cumulates include a complex sequence of troctolite, anorthosite, peridotite/dunite and norite of varying thickness that characterise the lower part of the reef package (Corson et al., 2002). The contact between these olivine-bearing layers and the underlying noritic and gabbronoritic cumulates defines the footwall of the reef package, the bottom of which is usually a coarse-grained olivine-rich peridotite with large post-cumulus pyroxene. This basal peridotite usually overlies gabbronorite (Corson et al., 2002). However, in the Stillwater mine, this footwall contact is discordant with the underlying rocks, with the entire Gabbronorite zone I absent, and the reef package resting on Norite zone I (Turner et al., 1985; Corson et al., 2002; Zientek et al., 2002). The top of the reef package is not related to any specific lithology, but corresponds to an igneous textural discontinuity between the coarse-grained to pegmatoidal cumulates of the reef package and the finer-grained hanging-wall cumulates. The texture in the upper reef package, below this upper boundary commonly includes pegmatoidal pyroxenes, atoll-textured olivine, like those observed in association with the chromitites of the Peridotite zone, lower in the complex, with spidery pyroxene overgrowths on anhedral olivine (Jenkins and Mungall, 2018). Another common texture immediately below the hanging wall comprises a rounded, buckshot-textured olivine, poikilitic pyroxene and fine-grained intercumulus pyroxenes (Corson et al., 2002). These buckshot-textured units are commonly 15 to 30 cm thick and typically include a buckshot textured dunite that grades up into a troctolite followed by a leucotroctolite. The buckshot olivine appears as rounded (serpentinised) black spots or as grey talc-altered spots when in contact with chlorite-altered plagioclase. They are laterally continuous over tens to hundreds of metres with sharp contacts with the underlying anorthosite. The stratigraphic thickness between the hanging-wall textural discontinuity of the reef package and the base of the first buckshot olivine unit varies from a few centimetres to tens of metres (Jenkins et al., 2020).

The J-M reef is defined as the PGE-rich layer hosted within the reef package. Sulphide mineralisation is largely stratabound, confined to the reef package. The most abundant sulphide minerals are pentlandite, chalcopyrite and pyrrhotite. Sphalerite, galena, millerite and marcasite occur in minor amounts whereas covellite. native copper, stibnite, argentian tetrahedrite, magnetite, graphite, gold and palladian gold are observed even less commonly. The principal PGMs are, in order of decreasing abundance, braggite, vysotskite, moncheite and Pt-Fe alloy, with minor or rare amounts of cooperite, stillwaterite, arsenopalladinite, palladobismutharsenide, palladoarsenide, kotulskite, zvyagintsevite, Pd-tellurides, merenskyite and sperrylite (Conn, 1979). Copper-nickel grades of up to 0.15% combined has been outlined in sections of the J-M Reef over widths of 1.9 m (Conn, 1979).

The dominant texture of the reef is interstitial, ~3 vol.%, disseminated sulphide mineralisation. In rare cases, however, isolated pockets of net-textured to massive sulphides with extremely large platinum group mineral (PGM) grains may also occur. In general, the crystal size of the cumulus minerals in the reef host rocks control the size and texture of the sulphide mineralisation, e.g., fine-grained cumulates host fine-grained disseminated sulphide, whilst pegmatoidal textured cumulates can host pods of massive sulphide. The reef PGEs are in solid solution or spatially associated with base metal sulphides, predominantly pentlandite, pyrrhotite and chalcopyrite, and minor pyrite, with reef pentlandite containing up to 5 wt.% Pd (Barnes and Naldrett, 1985; Godel and Barnes, 2008), accounting for most of the economic Pd mineralisation of the reef. The remainder is within discrete PGMs and alloys such as bismuthides, tellurides, arsenides, antimonides and sulphides (Jenkins et al., 2020 and references cited therein).

On the mine scale, the PGE-rich sulphide mineralisation in and around the reef package is not uniform, and is not confined to any cumulate lithology in the reef package. While most of the sulphide minerals in the Stillwater mine occur in Olivine-bearing zone I, multiple lenses of sulphides can also occur in the melanogabbronorite unit, whilst thin stratiform layers of sulphide may also occur in the norite and leuconorite units. Whilst the quantity and location of sulphide minerals and the ore tenors of the J-M reef can vary on a scale of metres to hundreds of metres, the distinct textural change between the reef package and the overlying cumulates remains, even where sulphides are locally absent. On the regional scale, the reef is generally a planar body. However, there are several instances where this planar nature is disrupted. On a small scale, rare, irregular, pod-shaped clouds of reef sulphides, known as 'ballrooms' occur and can reach >20 m in thickness. These tend to be associated with areas of the Stillwater mine where Norite zone I is the footwall to the reef package. A larger irregularity, known as the 'Dow Meadow Depression' occurs in the western part of the Stillwater mine. This depression is a 610 m wide basin-like zone and hinge point where the dip of the reef mineralisation changes by ~10° on either side of the depression, with associated late-stage ~2.2 Ga felsic intrusions. This hinge is interpreted to possibly represent the location of a feeder to the Banded Series cumulates that has been obliterated by later intrusions (Ching, 2017). This depression zone is also spatially associated with removal of the footwall stratigraphy below the reef package e.g., the thickness of Gabbronorite zone I varies between 0 and 60 m in the area of the depression as opposed to 240 m west of the depression (Jenkins et al., 2020 and references cited therein).

The J-M Reef was discovered by the Johns-Manville Corporation in 1973. The Stillwater mine has been in operation since 1986, and produces ~11.8 tonnes per annum of platinum and palladium in concentrate. East Boulder has been in operation since 2002, and currently produces 7.2 tonnes per annum of platinum and palladium in concentrate.

Ore Reserves and Mineral Resources at Stillwater and East Boulder at 31 December, 2019 (Sibanye Stillwater Mineral Reserve and Mineral Resources Statement, 2020) were:
  Mineral Resources:
    Measured - 5.6 Mt @ 19.5 g/t Pt+Pd;
    Indicated - 31.3 Mt @ 17.9 g/t Pt+Pd;
    Inferred - 48.1 Mt @ 17.3 g/t Pt+Pd;
    TOTAL - 85.0 Mt @ 17.6 g/t Pt+Pd for ~1496 tonnes of contained Pt+Pd.
  Ore Reserves:
    Proved - 3.8 Mt @ 19.2 g/t Pt+Pd;
    Probable - 23.0 Mt @ 19.5 g/t Pt+Pd;
    TOTAL - 26.8 Mt @ 19.4 g/t Pt+Pd for ~520 tonnes of contained Pt+Pd.
East Boulder
  Mineral Resources:
    Measured - 3.8 Mt @ 14.6 g/t Pt+Pd;
    Indicated - 26.2 Mt @ 14.8 g/t Pt+Pd;
    Inferred - 37.9 Mt @ 15.3 g/t Pt+Pd;
    TOTAL - 67.9 Mt @ 15.1 g/t Pt+Pd for ~1025 tonnes of contained Pt+Pd.
  Ore Reserves:
    Proved - 2.7 Mt @ 14.4 g/t Pt+Pd;
    Probable - 18.9 Mt @ 14.7 g/t Pt+Pd;
    TOTAL - 21.5 Mt @ 14.7 g/t Pt+Pd for ~316 tonnes of contained Pt+Pd.
  Combined TOTAL 152.9 Mt @ 16.5 g/t Pt+Pd for ~2522 tonnes of contained Pt+Pd.
NOTE: Mineral Resources are inclusive of Ore Reserves.

For detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2020.     Record last updated: 19/4/2021
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Aird, H.M., Ferguson, K.M., Lehrer, M.L. and Boudreau, A.E.,  2017 - A study of the trace sulfide mineral assemblages in the Stillwater Complex, Montana, USA: in    Mineralium Deposita   v.52, pp. 361-382.
Attanasi, E.D. and Bawiec, W.J.,  1987 - A Resource Assessment of Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana: in    Contributions on ore deposits in the early magmatic environment; B,   US Geological Survey bulletin 1674-B, 36p.
Barnes S J, Naldrett A J  1985 - Geochemistry of the J-M (Howland) reef of the Stillwater complex, Minneapolis adit area. 1. Sulfide geochemistry and Sulfide-Olivine equilibrium: in    Econ. Geol.   v80 pp 627-645
Barnes, S.-J., Page, P. and Zientek, M.L.  2020 - The Lower Banded series of the Stillwater Complex, Montana: whole-rock lithophile, chalcophile, and platinum-group element distributions: in    Mineralium Deposita   v.55, pp. 163-186.
Barnes, S.-J., Page, P., Prichard, H.M., Zientek, M.L. and Fisher, P.C.,  2016 - Chalcophile and platinum-group element distribution in the Ultramafic series of the Stillwater Complex, MT, USA - implications for processes enriching chromite layers in Os, Ir, Ru, and Rh: in    Mineralium Deposita   v.51, pp. 25-47.
Boudreau A E, McCallum I S  1986 - Investigations of the Stillwater complex: III. The Picket Pin Pt/Pd deposit: in    Econ. Geol.   v81 pp 1953-1975
Conn, H.K.,  1979 - The Johns-Manville Platinum-Palladium Prospect, Stillwater Complex, Montana, U.S.A.: in    Canadian Mineralogist   v.17, pp. 463-468.
Ernst R.E., Liikane D.A., Jowitt S.M., Buchan K.L. and Blanchard, J.A.,  2019 - A new plumbing system framework for mantle plume-related continental Large Igneous Provinces and their mafic-ultramafic intrusions: in    Journal of Volcanology and Geothermal Research,   v.384, pp. 75-84. doi.org/10.1016/j.jvolgeores.2019.07.007.
Godel B and Barnes S-J,  2008 - Image Analysis and Composition of Platinum-Group Minerals in the J-M Reef, Stillwater Complex : in    Econ. Geol.   v.103 pp. 637-651
Godel B, Barnes S-J and Maier W D,   2006 - 3-D Distribution of Sulphide Minerals in the Merensky Reef (Bushveld Complex, South Africa) and the J-M Reef (Stillwater Complex, USA) and their Relationship to Microstructures Using X-Ray Computed Tomography : in    J. of Petrology   v47 pp 1853-1872
Irvine T N, Keith D W, Todd S G  1983 - The J-M Platinum-Palladium Reef of the Stillwater Complex, Montana: II Origin by double-diffusive convective magma mixing and implications for the Bushveld Complex: in    Econ. Geol.   v78 pp 1287-1334
Jenkins, M.C., Mungall, J.E., Zientek, M.L., Holick, P. and Butak, K.,  2020 - The Nature and Composition of the J-M Reef, Stillwater Complex, Montana, USA: in    Econ. Geol.   v.115, pp. 1799-1826.
Lambert D D, Simmons E C  1988 - Magma evolution in the Stillwater Complex, Montana: II. Rare earth element evidence for the formation of the J-M Reef: in    Econ. Geol.   v83 pp 1109-1126
Mansur, E.T., Barnes, S.-J. and Duran, C.J.,  2021 - An overview of chalcophile element contents of pyrrhotite, pentlandite, chalcopyrite, and pyrite from magmatic Ni-Cu-PGE sulfide deposits: in    Mineralium Deposita   v.56, pp. 179-204.
McCallum, I.S.,  1996 - The Stillwater Complex: in    Developments in Petrology   v.15, pp. 441-483.
Meurer, W.P. and Meure, M.E.S.,  2005 - Using apatite to dispel the trapped liquid concept and to understand the loss of interstitial liquid by compaction in mafic cumulates: an example from the Stillwater Complex, Montana: in    Contrib. to Mineralogy & Petrology   v.151, pp. 187-201.
Mungall, J.E., Jenkins, M.C., Robb, S.J., Yao, Z. and Brenan, J.M.,  2020 - Upgrading of magmatic sulfides, revisited: in    Econ. Geol.   v.115, pp. 1827-1833.
Naldrett A J,  2004 - Deposits in layered intrusions (extract) - Stillwater Complex: in Naldrett A J, 2004 Magmatic Sulfide Deposits: Geology, Geochemistry and Exploration, Springer, Heidelberg,    pp 523-531
Polovina J S, Hudson D M, Jones R E,  2004 - Petrographic and geochemical characteristics of postmagmatic hydrothermal alteration and mineralization in the J-M Reef, Stillwater Complex, Montana - **CURRENTLY UNAVAILABLE**: in    The Canadian Mineralogist   v42 pp 261-277
Raedeke L D, Vian R W  1986 - A three-dimensional view of mineralization in the Stillwater J-M Reef: in    Econ. Geol.   v81 pp 1187-1195
Ripley, E.M., Wernette, B.W., Ayre, A., Li, C., Smith, J.M., Underwood, B.S. and Keays, R.R.,  2017 - Multiple S isotope studies of the Stillwater Complex and country rocks: An assessment of the role of crustal S in the origin of PGE enrichment found in the J-M Reef and related rocks: in    Geochimica et Cosmochimica Acta   v.214, pp. 226-245.
Song, X., Wang, Y. and Chen, L.,  2011 - Magmatic Ni-Cu-(PGE) deposits in magma plumbing systems: Features, formation and exploration: in    Geoscience Frontiers   v.2, pp. 375-384.
Spandler, C., Mavrogenes, J. and Arculus, R.,  2005 - Origin of chromitites in layered intrusions: Evidence from chromite-hosted melt inclusions from the Stillwater Complex: in    Geology   v.33, pp. 893-896.
Todd S G, Keith D W, Le Roy L W  1982 - The J-M Platinum-Palladium Reef of the Stillwater Complex, Montana: I. Stratigraphy and petrology: in    Econ. Geol.   v77 pp1454-1480
Wall, C.J. and Scoates, J.S.,  2016 - High-Precision U-Pb Zircon-Baddeleyite Dating of the J-M Reef Platinum Group Element Deposit in the Stillwater Complex, Montana (USA): in    Econ. Geol.   v.111, pp. 771-782.
Wall, C.J., Scoates, J.S., Weis, D., Friedman, R.M., Amini, M. and Meurer, W.P.,  2018 - The Stillwater Complex: Integrating Zircon Geochronological and Geochemical Constraints on the Age, Emplacement History and Crystallization of a Large, Open-System Layered Intrusion: in    J. of Petrology   v.59, pp. 153-190. doi: 10.1093/petrology/egy024.
Zientek M L, Ripley E M  1990 - Sulfur isotope studies of the Stillwater complex and associated rocks, Montana: in    Econ. Geol.   v85 pp 376-391

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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