The Stillwater Complex is an Archean layered mafic-ultramafic intrusion located in southwestern
Montana. The complex hosts a platinum-group element ore deposit — the J-M Reef. Recent geophysical data collected by the USGS from an area to the north of the Stillwater complex suggests that the complex continues at depth under cover. The northern exposure of Precambrian rock is not the complex but some enigmatic amphibolites. During the 2013 and 2014 field seasons, eight samples were collected from this Precambrian (?) lower-amphibolite to greenschist-facies metamorphic rock unit located approximately 2 kilometers north of the complex. Vail (1955) described the unit as an amphibolite schist composed primarily of amphibole and clinozoisite with minor amounts of quartz, calcite, chlorite, and epidote. He proposed three possible origins for the amphibolite schist: (1) metamorphosed floor rocks from below the Complex, (2) metamorphosed roof rocks from above the Complex, or (3) metamorphosed Complex rocks. Geraghty (2013) described the rock unit as a fine-grained, laminated, schistose mylonite, which may be the northern extension of the Madison mylonite zone (Erslev and Sutter, 1990). Electron microprobe analyses of coexisting amphibole (actinolite to actinolitic hornblende) and plagioclase (albite, An98) were used to constrain the temperature conditions at various pressures for three samples (P at 3, 5, and 6 kb; T = 417-441°C, respectively using Holland and Blundy, 1994). These conditions are consistent with greenschist facies metamorphism. Bulk rock major, trace and rare earth element (REE) geochemistry of the samples were used to test the origin hypotheses proposed by Vail (1955). Our data were compared to existing geochemical data from floor rocks below the complex (hornfels, mafic dikes) and from rocks within the complex (cumulates). No known roof rocks that might be comparable in age to the samples and located above the complex have been identified. Rare earth element spider diagrams suggest that the samples do not appear to be cumulates, suggesting that Vail’s (1955) hypothesis (3) is not valid. However, the REE patterns do resemble those of Precambrian dike rocks cutting the complex and adjacent Archean rocks. Our results provide a possible link to the origin of these enigmatic metamorphic rocks.
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Jenkins, M. Christopher; Thomson, Jennifer A.; and Zientek, Michael L., "Origin of Enigmatic Rocks Located North of the Stillwater Complex, Montana" (2015). Geology Student Works. Paper 2.