INVESTIGATION AND 3D MODELLING OF PRECAMBRIAN DEFORMATION EVENTS AND GOLD MINERALIZATION IN KEYSTONE MINING DISTRICT, SOUTH DAKOTA

The Black Hills Uplift is a 100-km-long, 60-km-wide doubly-plunging anticline formed during the Laramide Orogeny, ~ 60-65 Ma. It constitutes one of only a few remnants of the Trans-Hudson orogenic belt, and its core consists of Archean to Early Proterozoic (>2.5 to 1.7 Ga) phyllite, schist, granite, iron-formation, and metavolcanics, flanked by Phanerozoic sedimentary rocks. The Black Hills region has been well-known for significant gold production since the early 1800’s (in particular, 40 million ounces of gold at the Homestake mine until 2002). However, structural controls on gold mineralization are not fully understood. A detailed study of complex structural patterns associated with Proterozoic deformation events and the Laramide Orogeny can help us better understand the kinematics of gold mineralization in the region. The Keystone Mining District of SD is one of the unique locations where historical gold production came from possible Proterozoic mineralization associated with iron formation, faults, and shear zones, displaying similarities with deposits at Homestake. The focus of this study is the Holy Terror property, historically one of the major gold producing regions of the Keystone District. The property includes major mine sites such as the Keystone, Holy Terror, Bismarck, and Bullion mines. Precambrian deformation of the region includes at least two major folding episodes followed by the intrusion of the Harney Peak Granite, which caused localized deformation and metamorphism. Evidence suggests that regional-scale faulting accompanied the second stage of Precambrian folding. Previous research hypothesizes that gold mineralization in the Holy Terror region is primarily affiliated with brittle deformation, and perhaps occurs in association with late Proterozoic or later re-activation of Precambrian faults. This study aims to document temporal and spatial relationships between ductile and brittle deformational structures along fault zones between the Holy Terror, Keystone, and Bismarck shafts. A three-dimensional model based on detailed field mapping, core logging, and kinematic analysis of major structures will show the primary structural controls on ore mineralization. Temporal and spatial relationships between deformation and gold mineralization will be explained in detail.