24 Structural Geology & Tectonics

Friday, 11 April 2014
8:00 AM - 12:00 PM
Solitude

STRUCTURAL SETTINGS OF THE COLES HILL URANIUM PROPERTY, CHATHAM, VA

Paper number: 24-5
Program: Technical Sessions
Time: 9:20 AM – 9:40 AM

Authors

Abstract

The Coles Hill uranium property located in Pittsylvania County, Virginia contains 132 MM lbs U3O8 in place (assuming a cutoff grade of 0.025 %) and is the largest undeveloped uranium deposit in the United States. The uranium is hosted in hydrothermal fractures in Leatherwood mylonitic granite that has undergone Na –K metasomatism and silica depletion, and Rich Acres gabbro amphibolite – these units form a structural trap. The Chatham fault separates the Danville Triassic basin to the east from the crystalline rocks of the Smith River allochthon to the west. The Chatham fault may have been reactivated in the Mesozoic. The uranium is bounded by the Chatham listric normal fault that forms the footwall and by unmineralized Leatherwood biotite gneiss, which is down section from the mylonitic granite, with a foliation of 30° SE. The Chatham fault is recognized in drill core from the southern part of the propertyin the form of a few feet of unconsolidated gouge with cataclasite that occurs in both the Leatherwood granite and Danville Triassic basin sedimentary rocks. This implies that the cataclasite was formed during or post Mesozoic. The Chatham fault at the northern extent of the property is further defined by rounded granite boulders on the Triassic basin side in Whitethorn Creek.

En-echelon isoclinal, recumbent folds, overturned to the northwest have axes that are sub-parallel to the Chatham fault. This series of folds is consistent with thrusting from the southeast. A secondary open fold system has axes that are perpendicular to the recumbent folds, and these represent zones of weakness that controlled the emplacement of later diabase dikes. This series of folds may indicate transpression from the northeast and southwest. The sub-parallel nature and surface continuity of the Leatherwood mylonitic granite and biotite gneiss are recognized in outcrop where the contact with the Fork Mountain schist shows hematite alteration. The uranium mineralization in the southern property does not continue to plunge at depth but rather folds back toward the surface. On the northern end of the southern property, mineralization extends to the surface at the discovery outcrop. Uranium mineralization in the northern property continues the same fold system at depth as is observed on the surface.