High-U hydrothermal apatite with complex UPb 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, UPb 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) UPb data for mineralized whole rock samples, and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) UPb isotope data for apatite in both unmineralized and U-mineralized host rocks.

Massive deficits in radiogenic Pb preclude reliable UPb “chemical ages” calculated from EPMA data obtained from coffinite. In contrast, LA-ICPMS data for secondary apatite in unmineralized rocks indicate low-U concentrations (10⁰–10² ppm), “normal” (consistent with models of terrestrial Pb isotopic evolution) initial Pb isotope compositions, and UPb age estimates of ~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 10²–10³ ppm but up to 2.4 wt% U) and large excesses of ²⁰⁶Pb (²⁰⁷Pb/²⁰⁶Pb < 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 ²⁰⁶Pb-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 ²⁰⁶Pb in apatite is derived from decay products of ²²²Rn lost from coffinite and mobilized by Na-, P-, and U-enriched metasomatic fluids during the main mineralizing event at ~230 Ma. Ore-stage alteration did not uniformly reset the UPb systematics in host rocks precluding a well-constrained whole-rock isochron age. However, whole-rock isotope data imply U mobility at ~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.

具有复杂 U Pb 系统的高 U 热液磷灰石在空间上与 Coles Hill 矿床的矿化密切相关，Coles Hill 矿床是美国已知的最大的未开采铀矿床。该矿床位于弗吉尼亚州中南部的 450 至 430 年 Ma-old Martinsville Intrusive Complex 的交代岩石中。由于开放系统氡损失，无法直接对变质铀矿石矿物（主要是铜镍矿）进行测年。相反，研究了共生磷灰石中的U Pb 同位素作为评估矿化年龄的一种手段。在这里，我们报告了棺材的原位电子探针微量分析 (EPMA)、同位素稀释热电离质谱 (ID-TIMS) U矿化全岩样品的 Pb 数据，以及未矿化和 U 矿化主岩中磷灰石的激光烧蚀电感耦合等离子体质谱 (LA-ICPMS) U Pb 同位素数据。

放射成因 Pb 的大量缺陷排除了可靠的 U Pb“化学年龄”，这些数据是根据从棺材获得的 EPMA 数据计算得出的。相比之下，未矿化岩石中次生磷灰石的 LA-ICPMS 数据表明低 U 浓度（10 ⁰ –10 ²  ppm）、“正常”（与陆地 Pb 同位素演化模型一致）初始 Pb 同位素组成和 U Pb 年龄估计约 330 Ma，这与先前为该矿床所在的区域古生代剪切带提出的日期一致。与棺材相关的矿石阶段磷灰石具有高 U 浓度（通常为 10 ² –10 ³  ppm，但高达 2.4 wt% U）和大量过量的²⁰⁶ Pb ( ²⁰⁷ Pb/ ²⁰⁶Pb < 0.01) 不受原位 U 衰变的支持。数据显示，初始 Pb 具有可变的同位素组成，包括来自主岩的“正常”Pb 和由次生交代流体引入的²⁰⁶ Pb 富集 Pb。对 Pb 源的复杂演化和混合的评估对热液磷灰石的U Pb 定年具有更广泛的意义。

磷灰石中过量的²⁰⁶ Pb 来自于230 Ma 左右的主要矿化事件期间从棺材中损失的²²² Rn 的衰变产物，并被富含 Na、P 和 U 的交代流体迁移。矿阶段蚀变并没有统一重置主岩中的 U Pb 系统，从而排除了受良好约束的全岩等时线年龄。然而，全岩同位素数据暗示 U 在~200-220 Ma 的迁移率，并支持矿化最后阶段的三叠纪时代。结果还表明，含有高达几个重量百分比的铀的磷灰石能够将 U 及其衰变产物保留数亿年；在评估这种矿物作为长期储存放射性废物的潜在基质时的一个重要考虑因素。