Mineralogy and Geochemistry of Garnet Rhyolites, Black Hills, South Dakota

Abstract

This study is looking at the mineralogy of the garnet bearing rhyolites to determine the origin of the garnets and other associated minerals. Thin sections are being examined using SEM/EDS. Initial results suggest the garnets are igneous phenocrysts, they are free of inclusions of metamorphic minerals, and are associated with ferro-knebelite (Mn-olivine), quartz, fluorine-rich biotite, anorthoclase, plagioclase (~An 30) and thorium-rich monazites. Continued study of this suite of rocks should allow us to determine more about the igneous history of these rhyolite intrusions.