CRYSTALLOGRAPHIC FABRICS OF QUARTZ IN THE ELLISON FORMATION: INSIGHTS ON DEFORMATION MECHANISMS AND METAMORPHIC CONDITIONS AT THE DEEP UNDERGROUND SCIENCE AND ENGINEERING LABORATORY (DUSEL) AT HOMESTAKE, SD

Quartz lattice preferred orientation (LPO) patterns have been used to study plastic deformation in the crust. Electron backscatter diffraction (EBSD) with the scanning electron microscope is a powerful method for measuring LPOs. The Homestake gold deposit in the Black Hills of South Dakota is hosted by a package of Proterozoic marine metasediments and metavolcanics, including the Homestake formation and superjacent Ellison formation. The surface trace of a garnet isograd intersects the deposit area on the western limb of the Lead Anticline. This isograd marks the transition zone from greenschist facies in the western anticlinorium to amphibolite facies in the east; the isograd and transition zone strike approximately N 20º W, have near vertical dips and parallel a major high-strain shear zone (left-lateral motion, east side up). Previous studies used garnet-biotite geothermometry and oxygen isotope data from samples across the deposit to define a steep (150 °C/km) metamorphic field gradient across the anticlinorium. We have obtained quantitative quartz LPO data from Ellison formation quartzites in order to relate LPOs to observed deformation and fabric overprinting relationships. Ten samples were taken along a 3 km W-E transect across the anticline. Samples were described as sericite-biotite-quartz phyllites/schists or as impure (e.g. biotite, muscovite, etc.) quartzites. Structural data describe NW to NE striking foliations with steep to moderately NE to SE dipping mineral aggregate lineations in the planes of foliation. Observations generally concur with previous descriptions of structural domains in the study area. Petrographic observations reveal complex microstructures that vary across the study area, reflecting original sedimentary textures and varying degrees of overprinting of deformation events. EBSD analyses of several samples have provided LPO patterns that may suggest deformation mechanisms consistent with metamorphic conditions described in previous studies, and may confirm the presence of a steep metamorphic field gradient across the anticline.