Use Of High-U Hydrothermal Apatite Containing Excess Pb-206 To Constrain The Age Of Uranium Mineralization At The Coles Hill Deposit, Virginia, Usa

High-U hydrothermal apatite with complex U-Pb systematics is closely spatially associated with mineralization at the Coles Hill deposit, the largest unmined uranium deposit known in the United States. The deposit is hosted in metasomatized rocks of the 450- to 430-Ma-old Martinsville Intrusive Complex in south-central Virginia. Direct dating of metamict uranium-ore minerals, mostly coffinite, is not possible due to open-system radon loss. Instead, U-Pb isotopes in cogenetic apatite were investigated as a means of evaluating the age of mineralization. Here we report in situ electron probe microanalyses (EPMA) of coffinite, isotope-dilution thermal-ionization mass spectrometry (ID-TIMS) U-Pb data for mineralized whole rock samples, and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb isotope data for apatite in both unmineralized and U-mineralized host rocks.Massive deficits in radiogenic Pb preclude reliable U-Pb "chemical ages" calculated from EPMA data obtained from coffinite. In contrast, LA-ICPMS data for secondary apatite in unmineralized rocks indicate low-U concentrations (10(6)-10(2) ppm), "normal" (consistent with models of terrestrial Pb isotopic evolution) initial Pb isotope compositions, and U-Pb age estimates of similar to 330 Ma, which is consistent with dates previously proposed for the regional Paleozoic shear zone that hosts the deposit. Ore-stage apatite associated with coffinite has high-U concentrations (typically 10(2)-10(3) ppm but up to 2.4 wt% U) and large excesses of Pb-206 (Pb-207/Pb-206 < 0.01) unsupported by in situ U decay. Data show that initial Pb had variable isotopic compositions including both "normal" Pb derived from host rocks and Pb-206-enriched Pb introduced by secondary metasomatic fluids. Evaluation of the complex evolution and mixing of Pb sources has broader implications for UPb dating of hydrothermal apatite.Excess Pb-206 in apatite is derived from decay products of Rn-222 lost from coffinite and mobilized by Na-, P-, and U-enriched metasomatic fluids during the main mineralizing event at similar to 230 Ma. Ore-stage alteration did not uniformly reset the U-Pb systematics in host rocks precluding a well-constrained whole-rock isochron age. However, whole-rock isotope data imply U mobility at similar to 200-220 Ma and support a Triassic age for the final stages of mineralization. Results also indicate that apatite with up to several weight percent uranium is able to retain U and its decay products for hundreds of millions of years; an important consideration when assessing this mineral as a potential matrix for long-term storage of radioactive waste.