矿床地球化学国家重点实验室知识库

The Xiangshan U deposit, the largest hydrothermal U deposit in China, is hosted in late Jurassic felsic volcanic rocks although the U mineralization post dates the volcanics by at least 20 Ma. The mineralization coincides with intrusion of local mantle-derived mafic dykes formed during Cretaceous crustal extension in South China. Ore-forming fluids are rich in CO2, and U in the fluid is thought to have been dissolved in the form of UO2 (CO3)2 2− and UO2 (CO3) 3 4− complexes. This paper provides He and Ar isotope data of fluid inclusions in pyrites and C isotope data of calcites associated with U mineralization (pitchblende) in the Xiangshan U deposit. He isotopic compositions range between 0.1 and 2.0Ra (where Ra is the 3 He/4 He ratio of air= 1.39 × 10−6 ) and correlates with 40Ar/36Ar; although there is potential for significant 3 He production via 6 Li(n,α) 3 H(β)3 He reactions in a U deposit (due to abundant neutrons), nucleogenic production cannot account for either the 3 He concentration in these fluids, nor the correlations between He and Ar isotopic compositions. It is more likely that the high 3 He/4 He ratios represent trapped mantle-derived gases. A mantle origin for the volatiles of Xiangshan is consistent with the δ13C values of calcites, which vary from −3.5‰ to −7.7‰, overlapping the range of mantle CO2. The He, Ar and CO2 characteristics of the ore-forming fluids responsible for the deposit are consistent with mixing between 3 He- and CO2-rich mantle-derived fluids and CO2-poor meteoric fluids. These fluids were likely produced during Cretaceous extension and dyke intrusion which permitted mantle-derived CO2 to migrate upward and remobilize U from the acid volcanic source rocks, resulting in the formation of the U deposit. Subsequent decay of U within the fluid inclusions has reduced the 3 He/4 He ratio, and variations in U/3 He result in the range in 3 He/4 He observed with U/3 He ratios in the range 5–17× 103 likely corresponding to U concentrations in the fluids b0.2 ppm.