PETROGRAPHIC ANALYSIS OF THE ROCHFORD FORMATION, SOUTH DAKOTA, USA

The Rochford Formation shares many parallels with the famous Homestake Formation of South Dakota. The stratigraphic settings of both formations are strikingly similar. The metamorphosed sequence of the Rochford District includes the thick Rapid Creek greenstone, overlain by pelitic to semipelitic metamudstones, the Nahant Formation, an iron rich silicate-carbonate dominated chemical metasediment and chert, the Rochford Formation, and an overlying unit of metamudstones, the Poverty Gulch Formation. Both Sequences were formed as part of rift basins, possibly related to the breakup of the supercontinent Kenorland. Despite the similar stratigraphy these units are not correlative and geochronologic studies constrain the deposition of the Homestake Formation at 2012-1974 Ma and the maximum age of deposition of the Rochford Formation to 1887-1883 Ma, resulting in a separation is space and time of ~25 km and ~80-130 Ma. The accretion of the Yavapai terrane and the suturing of the Wyoming and Superior cratons induced large scale regional metamorphism that altered the mineralogy of these banded iron formations during the Black Hills and Trans-Hudson orogenies, circa 1775-1715 Ma. Modern exposure of the core of the Black Hills, SD is part of the eastern most extent of the Laramide uplift. The Rochford Formation and Homestake Formation are extensively studied for their economic value as a host for gold. However, little petrographic analysis has been carried out on the Rochford Formation. A variety of analytical techniques were used to classify the Rochford Formation, including optical mineralogy, X-ray diffraction, and Scanning Electron Microscopy. Analysis has uncovered a potential growth pattern for the grunerite and quartz lenses in the sample.