GEOCHRONOLOGICAL CONSTRAINTS ON THE ORIGIN OF THE COLES HILL URANIUM DEPOSIT, VIRGINIA

The Coles Hill uranium deposit, located in south central Virginia, is the largest unmined uranium deposit in the United States, with estimates that range from 25 million pounds U₃O₈ (cutoff grade = 0.15% U₃O₈) to 109 million pounds U₃O₈ (cutoff grade = 0.025% U₃O₈). A rigorous genetic model has not yet been developed for the Coles Hill deposit, and much uncertainty exists concerning the age of mineralization and surrounding rocks. The deposit is hosted in a highly brecciated and altered meta-igneous unit that is thought to be correlative with the heterogeneous Leatherwood granite or its equivalent which is part of the Smith River Allochthon. The mineralized zone is within the zone of associated shearing immediately to the west of the Chatham Fault, which separates the host rock and deposit from the Triassic Danville sedimentary basin. Uncertainties in the age of mineralization and the host rock results in multiple potential sources for the uranium, including black shales in the adjacent Triassic basin, or the igneous and metamorphic host rocks. Because Triassic rift-related sedimentary basins are common and often bordered by crystalline rocks similar to those at Coles Hill throughout much of the eastern Piedmont, exploration for other similar deposits in this region requires an understanding of the role (if any) of the sedimentary and adjacent igneous and metamorphic rocks in the genesis of the deposit.

As part of a larger study to develop a genetic model for the Coles Hill deposit, we have established the age of the host rock at Coles Hill. Continuing work will further constrain the age of mineralization. Preliminary zircon U/Pb SHRIMP data for two compositionally similar samples of the Leatherwood granite collected from the immediate vicinity of the deposit yield similar but statistically distinct ages of 455.2 ± 2.3 Ma and 441.4 ± 2.4 Ma. A third more vein-rich sample of the Leatherwood granite did not yield a coherent age group, but individual zircons yield ages that range from 458 to 416 Ma. For all samples, no apparent age distinction exists among different zircon populations. These new ages provide the chronologic framework that will allow ongoing work to examine if the Leatherwood granite is the uranium source for the deposit.