Abstract:

The Baimianshi uranium ore district is an important volcanic-hosted uranium district in South China. The uranium ore bodies occur in basalts and sandstones in the Baimianshi volcanic-sedimentary basin. The basement of the basin is composed of the Baimianshi granite and the Xunwu metamorphic rocks. Previous studies indicated that the Baimianshi granite provided uranium for the mineralization. In this study, LA-ICP-MS U-Pb dating of zircons from the Baimianshi granite was made showing an emplacement age of (241±7) Ma. The Baimianshi granite is a medium-fine grained two-mica granite, characterized by high SiO2 contents (71.99%-73.82%), with high A/CNK ratios (1.15-1.28). It is enriched in Rb, Th, Ta and depleted in Sr, Ba, Ti and Eu, and characterized by variable REE contents with LREE enrichment and obvious Eu depletion. The Baimianshi granite has relatively high initial 87Sr/86Sr ratios (0.71162—0.71613) and low εNd(t) values (-13.9 — -12.2). Two-stage Nd isotopic model ages vary from 2.01 Ga to 2.15 Ga. Its magmatic zircons have low εHf(t) values (-15.1—-11.9), with Hf model ages of from 2.01 Ga to 2.21 Ga, which are similar to the Nd model ages. Petrography and geochemistry indicate that the Baimianshi granite belongs to strongly peraluminous S-type. Combined with geochronology and isotope compositions, it is thus considered that the Baimianshi granite might be derived from partial melting of the metamorphic rocks similar to the Taoxi Group. The geochemical characteristics of the Baimianshi granite are similar to those of other uranium-bearing granites in South China. Multi-stage metamorphism and re-melting resulted in uranium enrichment in granites. Later hydrothermal fluid activity associated with crustal extension might have leached U from the granite to form the Baimianshi U deposit.

Key words: uranium-bearing granite, geochemistry, petrogenesis, Baimianshi U deposit, southern Jiangxi Province