MINERAL PARAGENESIS OF THE COLES HILL URANIUM DEPOSIT, PITTSYLVANIA COUNTY, VA

The Coles Hill uranium deposit is hosted in the Ordovician Leatherwood Granite (granodiorite) and Silurian Rich Acres Formation (gabbro), both part of the informal Martinsville igneous complex. Uranium enrichment within the deposit area is spatially associated with sodium metasomatized rocks in which original feldspars were altered to albite. Iron released during albitization produced red ferric oxide staining. Uranium silicates are found 1) with calcite intercalated in sheets of biotite and chlorite 2) replacing or on the surface of paragenetically early, possibly primary, accessory sulfide minerals (pyrite, jordisite, chalcopyrite, sphalerite, and cobaltite), 3) on the surface of zircon, 4) within titanium oxide that likely formed from titanite, and 5) between crystals of apatite that is zoned with varying uranium contents (3,000 to 4,000 ppm uranium determined by microprobe). The paragenesis of the uranium and associated minerals is difficult to resolve because of the complex alteration history of these rocks. Early-formed uranium-bearing minerals identified by SEM-EDS include uranothorite, uraninite, and uranium silicates of indefinite mineralogy. Additional uranium-enrichment was noted in other early formed phases including fluorapatite, titanite, zircon, and monazite. Biotite is altered to chlorite, but multiple episodes of chloritization are indicated, and thus the relative timing is uncertain. Later-formed uranium minerals are coffinite, brannerite, and an unknown uranium silicate that contains variable amounts of Th, Fe, Zr, Y, P, and Ca. In some areas, uranium in early-formed silicates is clearly distributed across tens to hundreds of micrometers and redeposited as secondary uranium silicates that are intimately associated with paragenetically later minerals. Distinct phases of quartz, chlorite, smectite, pyrite, potassium feldspar, and calcite post-date all uranium minerals. These latest phases are associated with etching and dissolution of coffinite. Galena is spatially associated with uranium minerals leading to the conclusion that contained lead is likely radiogenic in origin.