U-Pb Isotopic Dating of the Pitchblende Ore Mineralization at the Coles Hill, Pittsylvania, Virginia, USA

The study conducted U-Pb isotopic dating of the pitchblende mineralization of the Coles Hill uranium deposit in Pittsylvania County, VA. The ore body is found along the Chatham fault and within the Martinsville Igneous Complex. In attempt to constrain the time of uranium mineralization, samples were collected within the fractures to the west of the Chatham fault that separates the host rock from the Danville Basin. Fifteen samples were analyzed for uranium and lead concentrations, as well as their isotopic lead compositions. BSE imaging revealed that the milligram sized samples consisted of multiple phases mixed on a micron scale. Uranium and lead isotopic analyses revealed that some phase or phases contained highly radiogenic (206Pb/204Pb ratios between 2,000 and 3,000) while some phases or phases were dominated by common lead. Highly discordant Pb/U ages of the highly radiogenic samples seem to be due to recent loss of substantial amounts of uranium. Analyses of these on a Concordia plot seems to indicate that uranium loss was not in a single event but occurred over time and likely recently. Therefore it was not possible to directly determine the age of the uranium mineralization. The four samples dominated by lead lie closely at 1115 Ma Pb-Pb isochron. It is inferred from this that the common lead present was derived from Grenville age basement that underlies the Smith River Allochthon.