The Relationship Between Structural and Tectonic Evolution and Mineralization at the Coles Hill Uranium Deposit, Pittsylvania County, Virginia

The role of structure and tectonics in the formation of hydrothermal ore deposits and the localization of high-grade mineralization associated with fractures is well documented. In this study we have characterized the structural setting associated with uranium mineralization in the Coles Hill uranium deposit by relating the observed metamorphic and structural features (mylonitic foliation and fractures) to regional tectonic activity.

Drill cores and outcrops observed in this study show that NE/SW oriented fractures appear to be related to Mesozoic movement along the Chatham Fault. NW/SE oriented fractures cross cut and offset the NE/SW oriented fractures by1 to 2 cm and therefore post-date the NE/SW oriented fractures. NW/SE fracture orientations and parallel to the NW/SE regional cross faults and are suggested to relate to the formation of the cross faults during post Triassic basin inversion. Uranium mineralization is located within horizontal to shallowly dipping fractures suggesting uplift and erosion to form possible tension veins.

The cross faults with NW/SE orientations created pathways in which uranium bearing hydrothermal fluids could migrate from the Triassic basin shales westward into the adjacent highly fractured crystalline rocks, precipitating uranium due to oxidation-reduction reactions.