Metamorphic Petrography and Geochemistry of a Garnetiferous Gneiss in the Tobacco Root Mountains, Southwestern Montana

Abstract

Both our sample and the Australian garnet gneiss are made primarily of banded iron/manganese garnets, quartz, with lesser magnetite, and trace monzite, ilmenite, grunerite, and biotite. Our sample, the Australian gneiss, and the ICMS’ BIF all have high Fe content and elevated Mn content found in no other units in the ICMS. The Australian garnet gneiss has been interpreted as being a metamorphosed exhalative hydrothermal iron deposit related to volcanic massive sulphides. The ICMS sediments have also been interpreted as coming from a back arc marine environment, with evidence of hydrothermal alteration. Considering the previous information, it is most likely that the garnet gneiss of our study represents a 3.13-2.85 Ga exhalative iron formation genetically related to the other banded iron formations of the ICMS. Variations in the composition of mixing seawater and hydrothermal fluids, Eh-pH conditions, and the contamination of outside sediment, caused the variations in composition between our sample the other BIF. The orogen at 2.45 Ga and the Big Sky Orogeny at 1.78-1.72 caused the rock in this study to be metamorphosed to the granulite facies, with the iron oxides, Mn-silicates, and clays reacting to form garnet.