COMPARATIVE ANALYSIS OF GOLD-BEARING MINERAL ASSEMBLAGES FROM THE WHARF MINE AND ROCHFORD AREAS OF SOUTHWESTERN SOUTH DAKOTA

Several lithologic units in the middle and northern Black Hills of South Dakota are known gold-bearing units that have been variably mined for many years. The Wharf mine region west of Lead, SD is characterized by Precambrian metamorphic rocks which are overlain by Cambrian sediments and intruded by multiple Tertiary-age dikes, sills, and irregular stocks. Gold mineralization in the Wharf mine region occurs in both the sedimentary and igneous rocks, but is primarily in the Deadwood Formation, which comprises sandstones, various conglomerates, siltstones, and carbonates. Ore is most commonly associated with replacement deposits adjacent to NE-trending fractures, and a monzonite sill near the top of the Deadwood Fm. Farther south, the Nahant and Poverty Gulch units are primarily phyllites and schists, and the Rochford Formation is a garnetiferous banded iron formation (BIF) where exposed in the field area around Rochford, South Dakota. This research focuses on the mineralogical and geochemical characterization, and comparison of samples collected from all three units near Rochford and from the Wharf Mine area. Minerals were first identified in hand sample and then using composite images taken using a Leica MZ16 camera with photomontage software. These images were also used as maps for more detailed mineralogical identification. Sample splits were crushed and powdered in an alumina shatterbox for analysis by powder X-Ray Diffraction, and fused into glass disks for X-Ray Fluorescence. Raman spectroscopy was employed to identify mineral assemblages from thin section slabs and hand samples. Almandine, biotite-annite, and quartz were positively identified in most metapelitic samples, along with accessory phases such as hematite and graphite. Almandine and Fe-rich orthoamphiboles are common in the BIF. Commercial fire assays were obtained to determine precious metal concentrations. This research was designed to not only gain insight into the metamorphic grade and mineral characterization of lithologic units around Rochford and the Wharf Mine, but also to model types of analyses and mineralogical characterization of samples employed by exploration geologists in the minerals industry.