The Coles Hill Uranium Deposit, Virginia, USA: Geology, Geochemistry, Geochronology, and Genetic Model

Abstract The Coles Hill uranium deposit, with an indicated resource of about 130 Mlb U3O8, is the largest unmined deposit in United States. hosted Taconian (approx. 480–450 Ma) Martinsville igneous complex, which consists Ordovician Leatherwood Granite (granodiorite) and Silurian Rich Acres Formation (diorite). host rock was metamorphosed to orthogneiss during Alleghanian orogeny 325–260 Ma), when it also underwent dextral strike-slip movement along Brookneal shear zone. During Triassic, extensional tectonics led development Dan River Basin that lies east Hill. mineralized zone brittle structures footwall Triassic Chatham fault forms western edge basin. Within fracture zones, silicate uranium-bearing fluorapatite traces brannerite form veins breccia-fill chlorite, quartz, titanium oxide, pyrite, calcite. Uranium silicates coat replace primary titanite, zircon, ilmenite, sulfides. Sodium metasomatism preceded accompanied mineralization, pervasively altering forming albite from feldspar, depositing limpid rims on titanite oxide calcite, riebeckite. Various geothermometers indicate temperatures less than ~200°C mineralization. In situ U-Pb analyses Ti-oxide, apatite, Rb/Sr U/Pb isotope systematics whole-rock samples, resolve timing geologic processes affecting containing euhedral dated at 450 445 Ma, agreement previously obtained ages zircon complex. A regional metamorphic event 330 310 Ma formed anhedral some reequilibrated isotopes, interpreted have coincided shearing metamorphism, refractory minerals including uranothorite were recrystallized, liberated mixed locally hematite, clay, other fine-grained minerals. mineralization by a metasomatic episode between 250 200 reset Rb-Sr systems apatite are associated and, places, intimately intergrown dating This coincides rifting precursor breakup Pangea. orientation late-stage tectonic stylolites compatible their formation Late Early Jurassic basin inversion, postdating main stage effectively as Mesozoic. Based close spatial temporal association we propose carried uranyl-phosphate reduced coupled redox reactions ferrous iron sulfide rock, silicates. release calcium sodium alteration calcic feldspar initiated successive phosphate mineralizing fluids combined U-enriched fluorapatite. mineralogy, oxygen geochemistry, trace element characteristics gangue minerals, likely included connate and/or meteoric water sourced adjacent Basin. High heat flow related Mesozoic may driven these (P-Na-F-rich) through local aquifers into margin faults, transporting or mobilizing U zone, older basement rock.