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Abstract: J-M Reef of the Stillwater Complex
Wyoming Geological Association: 1982 Luncheon Meetings Casper, Wyoming: Abstracts of Papers
This study concerns a 120 m stratigraphic section of the Stillwater Complex containing the main zone of Ptenrichment. This zone, called the J-M reef, is visibly distinguished by small concentrations of sulfides through a 2 m thickness. It occurs in the plagioclase-rich, banded zone of the intrusion, about 440 m above the ultramafic zone. The investigated section comprises more than 35 cumulative layers, among them 8 principal olivine-rich layers and 12 anorthositic layers with alternate norite and two-pyroxene gabbro. More than 15 cyclic units might be defined, but at least 3 different crystallization orders are required to account for the cumulate sequences, and none of these orders are the same as that evident in the ultramafic zone.
Certain stratigraphic changes appear, however, to be critically associated with the formation of the J-M reef. About 50 m below the reef, olivine has its first appreciable occurrence above the ultramafic zone, anorthositic rocks first appear in major amounts, whole-rock sulfide contents increase significantly, and whole-rock Fe/Mg ratios erratically increase. Within and above the reef, sulfides become still more abundant, and olivine is distinctly richer in Fe and Mn. A fundamental change in the magma composition is indicated, and the presence of slump structures and of ultramaficcumulate xenoliths 50-75 m below the reef suggests that new magma was added to the intrusion. The repetition of layers and the precipitation of the Pt-bearing sulfides both appear reasonably explained by magma mixing in a double-diffusive system.
Acknowledgments and Associated Footnotes
1 Stan Todd: Manville
© Wyoming Geological Association, 2015