GEOCHRONOLOGY OF THE COLES HILL URANIUM DEPOSIT AND PORTIONS OF THE BROOKNEAL SHEAR ZONE, SOUTHEAST VIRGINIA

Coles Hill, the largest unmined uranium deposit in the United States, is hosted in the Late Ordovician Martinsville igneous complex adjacent to the Pennsylvanian (?) Brookneal shear zone and the Triassic Chatham fault, along the NW boundary of the Dan River Basin near Chatham, VA. To better understand the genesis of the deposit, we present the results of new geochronological analyses that focus on in situ U-Pb methods by laser ablation on titanite and apatite, and Rb-Sr and Pb-Pb isochron methods on whole-rocks and mineral separates. This work is challenged by the complex mineral paragenesis and the small grain sizes of ore and paragenetically related gangue minerals.

Three distinct events were identified at Coles Hill: (1) a primary crystallization event at ~450 Ma that formed euhedral titanite in the unaltered host rock, concordant with previous zircon dates from the Martinsville igneous complex, (2) an intermediate event at ~320 Ma that formed anhedral titanite and is coincident with previously dated movement of the Brookneal shear zone and, (3) the youngest event that formed titanite alteration products (TiO₂) (~200 Ma) and introduced Na-rich fluids that metasomatically altered host rocks (whole-rock Rb-Sr errorchrons between 200 and 220 Ma). The youngest event is attributed to late stage modification of the Dan River Basin that included emplacement of regional diabase dikes (Central Atlantic magmatic province magmatism). Whole-rock Pb-Pb analyses yield a poorly defined errorchron of ~300 Ma, possibly recording open-system behavior or a mixture of Pb from multiple sources. Felted masses of high-U (> 1000 ppm) apatite are intergrown with bands of uranium silicate minerals and are found with aggregations of coffinite. Apatite was a primary dating target to determine the age of mineralization. The lack of high-U apatite standards has limited the success of this approach. Nonetheless, preliminary results indicate that apatite formed coincidently with Na-metasomatism and alteration of titanite to TiO₂.