Petrography and genetic history of coffinite and uraninite from the Liueryiqi granite-hosted uranium deposit, SE China

Coffinite and uraninite banded aggregates from the Liueryiqi granite-hosted uranium deposit, SE China, form a unique “bull's-eye” texture. These aggregates consist of concentric bands of uraninite and coffinite that range in thickness from 1 to 10 μm. Electron microprobe analyses indicate that coffinite and uraninite contain Ca and Al impurities. Altered uraninite, feldspar, quartz or pyrite grains form the center of these aggregates. Other minerals associated with the “bull's-eye” aggregates are marcasite, sphalerite, galena, chalcopyrite, bornite, clausthalite, microcrystalline quartz, fluorite and calcite. The aggregates occur within highly altered and brecciated zones of a two-mica leucogranite. The mineral assemblage associated with these aggregates is consistent with low-temperature fluids (∼125 to 200 °C), pH=6.0 to 8.5, Eh=−0.640 to −0.704 V, and dissolved silica contents of 10−3.5 to 10−2.7 mol/l (19 to 120 ppm as SiO2). At depth, CO2 concentrations of mineralizing fluids percolating through the breccia and fractured zones of the altered host granites likely ranged from 3.5 to 2.6 mol% at temperatures between 126 and 178 °C and a lithostatic pressure of 500 to 800 bars. Uranium was likely transported as a uranyl dicarbonate ion, which is the dominant dissolved uranium carbonate species in the pH range 6.0 to 8.5. Periodic changes in dissolved silica content in the mineralizing fluids resulted in the formation of the coffinite-uraninite intergrowth-banded aggregates. The precipitating mechanism resulting in the precipitation of the reduced uranium phases was likely the oxidation of reduced S2− and Se2−, which were supplied by the mineralizing fluids and were in equilibrium with uranyl ions.