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

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美国弗吉尼亚州 Coles Hill 铀矿床：地质学、地球化学、地质年代学和成因模型

Coles Hill 铀矿床，指示资源约为 130 Mlb 的 U ₃ O ₈，是美国最大的未开采铀矿床。该矿床位于 Taconian（约 480-450 Ma）Martinsville 火成岩杂岩中，由奥陶系皮革木花岗岩（花岗闪长岩）和志留系 Rich Acres 组（闪长岩）组成。在阿勒格尼造山运动（约 325-260 Ma）期间，母岩变质为正片麻岩，当时它还沿着布鲁克尼尔剪切带经历右旋走滑运动。在三叠纪，伸展构造导致了位于科尔斯山以东的丹河流域的发展。矿化带位于形成盆地西部边缘的三叠纪查塔姆断层下盘的脆性结构中。在脆性断裂带内，硅酸铀和含铀的氟磷灰石形成脉络，而角砾岩则充满绿泥石、石英、二氧化钛、黄铁矿和方解石。硅酸铀也覆盖和替代原生钛矿、锆石、钛铁矿和硫化物。钠交代作用先于并伴随铀矿化，普遍改变母岩并由原生长石形成钠长石，在火成长石上沉积清澈的钠长石边缘，将榍石转变为氧化钛和方解石，并形成里贝克石。各种地温计显示矿化过程中温度低于~200°C。钛矿、氧化钛和磷灰石的原位 U-Pb 分析，以及全岩样品的 Rb/Sr 和 U/Pb 同位素系统学，解决了影响 Coles Hill 的地质过程的时间。含有原生自面体榍石的主体莱瑟伍德花岗岩的年代为 450 至 445 Ma，这与先前从马丁斯维尔火成岩中的锆石中获得的年龄一致。330 至 310 Ma 的区域变质事件形成了自面体榍石和一些磷灰石，重新平衡了全岩 Rb/Sr 和 U-Pb 同位素，并被解释为与沿 Brookneal 剪切带的运动相吻合。在剪切和变质作用过程中，钛铁矿、锆石、铀钍铁矿等原生难熔含铀矿物发生重结晶，铀被释放出来，并与赤铁矿、粘土等细粒矿物局部混合。铀矿化伴随着 250 至 200 Ma 之间的交代事件，重置了 Rb-Sr 和 U-Pb 同位素系统，并形成了相关的氧化钛和磷灰石，在某些地方，与铀硅酸盐测年矿化密切共生。这一事件恰逢形成丹河流域的裂谷，是泛大陆解体的前兆。晚三叠世至早侏罗世盆地反转期间晚期构造针线石的取向与其形成相一致，晚于铀矿化的主要阶段，有效地确定了中生代矿化的年代。基于铀与磷灰石在空间和时间上的紧密联系，我们提出铀以磷酸铀酰复合物的形式携带。铀通过与母岩中的亚铁和硫化物矿物发生耦合氧化还原反应而局部还原，形成铀硅酸盐。在主岩中原生钙长石和榍石的钠交代蚀变过程中钙的释放引发了连续的反应，在这些反应中，矿化液中的铀和磷酸盐与钙结合形成富铀氟磷灰石。根据矿床矿物学、氧同位素地球化学和铀硅酸盐和脉石矿物的微量元素特征，主要的矿化流体可能包括来自邻近丹河流域的原生水和/或大气水。与中生代裂谷相关的高热流可能驱使这些（富含 P-Na-F）流体通过局部含水层并进入盆地边缘断层，将铀从盆地运出或从布鲁克尼尔剪切带中先前形成的 U 矿物中移动铀，或来自富含 U 的较老的基底岩石。