BENEATH THE STILLWATER COMPLEX: PETROLOGY AND GEOCHEMISTRY OF QUARTZ-PLAGIOCLASE-BEARING CORDIERITE (OR GARNET) - ORTHOPYROXENE ± SPINEL HORNFELS, MOUNTAIN VIEW AREA, MONTANA

This study focuses on a 163 feet (50 m) section of diamond drill hole core (Core No. DDH 383-334) collected from the contact metamorphic aureole in the Mouat nickel-copper prospect in the Mountain View area of Stillwater County, MT. The rocks investigated are predominantly quartz-plagioclase-bearing cordierite – orthopyroxene ± spinel hornfels with trace amounts of biotite, Fe-Cu-Ni sulfides and Fe-Ti oxides. In particular, the drill core was investigated to characterize the mineral assemblages and their textures, search for evidence of partial melting, and to determine mineral and whole-rock geochemistry to provide an understanding of the petrogenesis of the hornfels close to the contact with the Stillwater Complex. Ten samples from a collection of 24 core depths were selected for detailed petrography, electron microprobe and trace and rare earth element XRF and ICP-MS analyses. Although most of the samples have the assemblage cordierite-orthopyroxene±biotite-plagioclase-quartz-ilmenite, one sample contains garnet (Alm _73-77 Pyr _20–17 Sps ₃ Grs ₄) in addition to cordierite. Three samples contain roughly prismatic-shaped cordierite-hercynite symplectites surrounded by a narrow mantle void of orthopyroxene, plagioclase and biotite. Few samples show evidence of in situ partial melting in the form of microscopic veinlets or lenticular leucosomes consisting of subhedral to euhedral plagioclase, cordierite±orthopyroxene with interstitial quartz. Average T-P conditions of hornfels genesis is 725 – 800°C and 4 kbar. Geochemistry suggests that the alkali-poor, intermediate SiO₂ (46-60) rocks are most likely metamorphosed mafic graywackes and ferromagnesian-rich pelites.