URANIUM METALLOGENESIS IN SOUTH CHINA

Vein-type uranium deposits are epigenetic concentrations of uranium minerals, typically pitchblende, in open spaces, such as fractures, fissures, shear zones and breccias, in igneous, sedimentary and metamorphic rocks (Ruzicka, 1993). This type of uranium deposit occurs worldwide and includes deposits such as those in Hercynian granites of the La Crouzille district of the Massif Central, France, in the Upper Proterozoic weakly metamorphosed pelitic rocks in the Pribram district, Czechoslovakia, and in volcanic rocks in North America.

South China is rich in vein-type hydrothermal uranium deposits which have provided the major sources of uranium for the country in the past several decades. These deposits are hosted in granitic, volcanic, and carbonaceous and siliceous pelitic sedimentary rocks. Many previous studies have focused on individual deposits and provided abundant geological and geochemical data in the Chinese literature. This work reviews previously published data, and proposed a model that links the formation of these deposits to the crustal extension.

The uranium deposits are spatially associated with extensional structures and/or mantle-derived mafic dikes. Both the uranium deposits and mafic dikes are Cretaceous to Tertiary in age, temporally coincident with the crustal extension. Carbon isotopic analyses of calcite deposited in the main-stage mineralization in the veins from twelve representative uranium deposits yield δ¹³C values of ore-forming fluids mainly from -4 to -8‰, which are permissive of a mantle origin for the CO₂ in the ore-forming fluids. A mantle origin is consistent with the association of the deposits with mafic dikes and the ³He/⁴He ratios of ore-forming fluids (0.1-2.0Ra for several uranium deposits). Isotopic compositions of H and O demonstrate that water in the ore-forming fluids is predominantly meteoric in origin. Ore-forming temperatures ranged from 150 to 250°C.

Uranium-rich crustal rocks in South China may have been the sources for the uranium. Crustal extension and associated mafic magmatism are considered to have heated the rocks and allowed CO₂ to migrate upward and to mix with CO₂-poor meteoric water. The CO₂-rich hydrothermal fluids mobilized uranium from the source rocks and then the uranium was deposited in various host rocks to form the uranium deposits.