DEFORMATION MECHANISMS IN QUARTZ AND THEIR IMPLICATION FOR THE CONDITIONS OF DEFORMATION AT THE DEEP UNDERGROUND SCIENCE AND ENGINEERING LABORATORY (HOMESTAKE MINE) LEAD, SOUTH DAKOTA, USA

Lattice preferred orientation (LPO) patterns of quartz in quartz-rich rocks 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. Mapping of prograde index minerals demonstrated that the surface trace of a garnet isograd intersects the deposit area on the western limb of the Lead Anticline. The isograd correlates with 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.

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. Preliminary EBSD analyses on several samples suggest the presence of observable LPOs. Further LPO observations may suggest deformation mechanisms congruent with metamorphic conditions described in previous studies, confirming the presence of a steep metamorphic field gradient across the anticline. Additionally, observed LPOs may suggest the presence of previously undescribed ductile faulting.