Metallogenic mechanism and geochemical characteristics of porphyry type uranium deposits, Xiangshan ore field

Most of the uranium deposits in the northern Xiangshan ore field is related with super hypergene granitic porphyry, so they can be classified as porphyry type uranium deposit. Based on analysis of geochemical features of REE, trace elements and carbon and oxygen isotopes in the Hengjian-Gangshangying and Shazhou deposits, sources of uranium metallogenic mechanism are discussed. The study shows that volcanic rock series are homologous, and the product of melting crust. Volcanic magma experiences crystallization of high fractionation during upwelling. Uranium of metallogenic substance and high field strength elements are preferentially enriched in magmatic hydrothermal solution in the latest stage to provide sources for uranium metallogenesis. Interaction between magmatic hydrothermal solution and rocks result in widespread alteration in pre-mineralization, which developed into metallogenic fluid with plentiful volatiles such as F, Cl and CO2 etc. High enrichment of HREE, Pb, Zn and Mo in ore indicates that metallogenic temperature is 200∼250 degree C. Feature of carbon and oxygen isotopes shows that mantle substances involved uranium metallogenesis. Degas of CO2 is one of release mechanisms of metallogenic substances. Favorable places for release of metallogenic substances are internal and external contact zones of granitic porphyry, front and upward fracture zone of porphyry intrusion and contact of different rocks. (authors)