AGE OF HOST ROCKS AT THE COLES HILL URANIUM DEPOSIT, PITTSYLVANIA COUNTY, VIRGINIA, BASED ON ZIRCON U-Pb GEOCHRONOLOGY

Abstract

New U-Pb zircon SHRIMP geochronology confirms that the Coles Hill uranium deposit in Pittsylvania County, Virginia, is hosted within the Late Ordovician to Silurian Martinsville Intrusive Complex. The meta-igneous host rocks at Coles Hill consist of two units of the Martinsville Intrusive Complex: the felsic Leatherwood Granite and the mafic Rich Acres Formation. Two samples of unmineralized Leatherwood Granite orthogneiss yield 206Pb/238U ages between 444.5 ± 2.5 and 447.5 ± 1.9 Ma. A third sample of unmineralized Leatherwood Granite orthogneiss shows a wider range in 206Pb/238U ages, possibly due to Pb loss, and a 206Pb/207Pb age of 452 ± 18 Ma. Unmineralized Rich Acres Formation amphibolite that cuts the Leatherwood gives a mean 206Pb/238U age (426.2 ± 7.0 Ma), slightly younger than the Leatherwood age. Samples of mineralized orthogneiss and mineralized amphibolite give similar 206Pb/207Pb ages of 419 ± 19 and 426 ± 21 Ma, respectively. A biotite gneiss unit that underlies the mineralized zone yields a 206Pb/207Pb age of 415 ± 21 Ma, indicating that it is part of the Martinsville Intrusive Complex and not a member of the early Cambrian Fork Mountain Schist, as has been previously reported. A genetic model for the Coles Hill uranium deposit has not yet been developed, although age constraints indicate that mineralization is either late or postmagmatic, and this is consistent with the epigenetic, fracture-controlled nature of the mineralization. Results obtained here do not preclude either the igneous host rocks (or similar rocks at depth) or the sedimentary units in the adjacent Triassic basin as possible sources for the uranium.