Stillwater Complex

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.

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 complex was emplaced into ~3.3 Ga metasedimentary rocks at depths of 6 to 7 km (Page, 1977; Mogk and Mueller, 1990; McCallum, 1996). 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 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 of basal norite and basal bronzitite zones totalling 50 to 150 m in thickness, followed by harzburgite and upper bronzitite zones which together are 800 to 1200 m thick. These are overlain by a 4500 m thick banded series of plagioclase rich cumulates. The banded series is divided into six megacycles from bottom to top of  i). norite and gabbro,  ii). norite, gabbro,  iii). anorthosite, troctolite-gabbro,  iv). anorthosite-troctolite, troctolite-gabbro  v). anorthosite, troctolite-gabbro, and  vi). gabbro, pigeonite gabbro. Two thin troctolite-anorthosite bands are found at the top of cycle I and cycle II respectively.   The second hosts the J-M Reef.

The principal mineralised unit is the 1 to 3 m thick J-M Reef (some 400 to 450 m above the base of the banded series described above) which 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.

For detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1982.    
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.
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
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
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
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.
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
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 ArculusR,  2005 - Origin of chromitites in layered intrusions: Evidence from chromite-hosted melt inclusions from the Stillwater Complex: in    Geology   v33 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.
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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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