VEINS, FRACTURES AND PARAGENESIS, COLES HILL URANIUM DEPOSIT, PITTSYLVANIA COUNTY, VIRGINIA

The Coles Hill deposit is the largest unmined uranium deposit in the United States. The uranium is hosted in veins and fractures in Na-metasomatized crystalline basement (Leatherwood Granite and Rich Ares Gabbro)in the footwall of the Triassic Chatham fault. The fractures include linear, discontinuous domains of microbreccia, zones of jigsaw breccia and irregular, anastomosing fractures that may encompass a large part of a thin section. The microbreccias are characterized by recognizable fragments of host rock, especially feldspar, set in a nearly opaque, apparently isotropic fine-grained matrix. In other cases, the fractures are filled by minerals, most commonly chlorite. The fractures and microbreccias are interpreted as dilational features. Veins both cut and are cut by the dilational fractures. Vein assemblages are complex, but typical constituents are chlorite, calcite, albite, and apatite and, less commonly, reibeckite, zeolites, quartz and opaques. Neoblastic albite, U-bearing apatite, and quartz occur as subhedra growing into space later filled by calcite. Veins consisting of chlorite, chlorite-apatite, felted apatite euhedra, and fine-grained albite are common. Stylolites appear to postdate all veins. These sinuous dissolution features contain concentrations of chlorite and opaque and semi-opaque minerals, including U minerals. Uranium occurs as a component of relatively early formed U-rich apatite, but most ore occurs as concentrations of U-oxides and silicates. Some of the U minerals appear to have formed early in the paragenetic sequence (e.g. along vein margins) but some of the high-grade ore is associated with low-T phases including chalcedony and clay minerals as well as conspicuous hematite staining. It is not clear whether this represents a secondary concentration, a down-T re-equilibration of highly altered and oxidized ore, a primary low-T ore deposition event or some combination of the three.