GEOLOGY AND GEOCHEMISTRY OF THE COLES HILL URANIUM DEPOSIT

The Coles Hill U deposit in central Pittsylvania County, Virginia is localized along the normal Chatham fault that separates late Proterozoic to Paleozoic crystalline rocks of the Smith River Allochthon to the west from Triassic sedimentary rocks of the Danville Basin to the east. Mineralization occurs exclusively as fracture-fill in crystalline rock of the footwall, bounded below by a 432 Ma biotite gneiss. The host rocks are the 442Ma Leatherwood Granite and the 430Ma Rich Acres gabbro, which are both part of the Ordovician Martinsville Igneous Complex (MIC). Both are ductilely deformed in the vicinity of the fault, such that the granite is best described as a gneiss and the gabbro as an amphibolite. The amphibolite appears to intrude the gneiss. The early ductile deformation is overprinted by a brittle fracturing event that hosts the ore. Although mineralization occurs in both granite and gabbro, the richest ore is associated with the amphibolitized gabbro, possibly reflecting reduction of soluble U⁺⁶ to insoluble U⁺⁴ during contemporaneous oxidation of ferrous iron to iron oxides in the mafic rock. The earliest alteration event associated with the fracturing is pervasive Na metasomatism that caused albitization of the host plagioclase and potassium feldspar, crystallization of neoblastic albite in the fractures, chloritization of biotite, and local formation of reibeckite. In the metasomatized rocks, Na₂O may exceed 10wt.% and K₂O is typically <1%, compared with 4 and 5%, respectively, in unmetasomatized MIC granitic rocks elsewhere in the region. Albitization was followed by growth of fracture-hosted, euhedral neoblastic apatite. The metasomatized rocks often contain several weight % P₂O₅. U mineralization is coeval with the latter stages of apatite crystallization. Some U is hosted within apatite, but most occurs as coffinite or nearly amorphous “pitchblende” type ore. Although all economic U mineralization occurs exclusively in the Na-P metasomatized rocks, not all of the metasomatized rocks host U, especially if the rock is not highly fractured. The age of the epigenetic U mineralization is currently unresolved, although the upper age limit is established by the age of the host rock and possible U sources include the crystalline basement and black shales within the Triassic basin.