Metallogeny of the Zoujiashan uranium deposit in the Mesozoic Xiangshan volcanic-intrusive complex, southeast China: Insights from chemical compositions of hydrothermal apatite and metal elements of individual fluid inclusions

Abstract

The Zoujiashan U deposit in the Mesozoic Xiangshan volcanic-intrusive complex is one of the largest volcanic-related type U deposits in China. Chemical composition of hydrothermal apatite and metal contents in individual fluid inclusions of quartz and fluorite are investigated in order to understand the precipitation mechanism of U and to further constrain the origin of U and ore-forming fluids of the Zoujiashan deposit. Illitization, hematitization, and U mineralization successively occurred within rhyolite and porphyritic lava. Three types of hydrothermal apatite are identified. Apatite Type 1 (Ap1) and apatite Type 2 (Ap2) are both euhedral-subhedral crystals found in hematitization zone, while only Ap2 shows evident irregular chemical zoning along its rims. Apatite Type 3 (Ap3) occurs as anhedral aggregates closely associated with U-minerals in U ore veins, implying synchronous precipitation of Ap3 and U ores. Comparable chemical compositions of Ap1 and cores of Ap2, including similar high Cl content, indicate both of them were precipitated from a Cl-rich fluid that induced hematitization. The elevated F contents of Ap3 and rims of Ap2 implies that a F-rich fluid was involved and altered Ap2 during the main stage of U deposition. Ap1 and cores of Ap2 have lower Mn contents than Ap3 and rims of Ap2, indicating an oxidized character for the Cl-rich fluid and a reducing character for the F-rich fluid.

Metal contents of fluid inclusions in fluorite and quartz occurring in U ores suggest that two end-members of fluids, including a U-rich and a U-poor, were involved in the main stage of U mineralization. Mixing between these two fluids led to the deposition of U- and Th-minerals through oxidation-reduction reactions and destabilization of Th-F complexes, respectively. The U-rich fluid is depleted in Th and Sr compared with the U-poor fluid. Based on comparison of chemical compositions of apatite and fluid inclusions, we suggest that the Cl-rich fluid precipitating Ap1 and Ap2 corresponds to the U-rich end-member fluid, while the F-rich fluid precipitating Ap3 represents a mixture between the two end-members. The high U (12.7–58.5 ppm) in the U-rich end-member fluid is in agreement with a relatively high oxygen fugacity and a surface-derived origin. The U-poor end-member fluid has lower Rb/Cs than the U-rich end-member and the unaltered host rocks, indicating the U-poor fluid was derived from a mantle wedge that experienced addition of Cs by fluids generated from the devolatilization of the subducted paleo-Pacific slab. Show more