Occurrence and enrichment mechanism of uranium in the Yankong phosphorite-type uranium deposit, Northern Guizhou

The Yangtze Platform is one of the most important tectonic units holding sedimentary-type phosphorite deposits in the world. The orebodies of phosphorite deposits are mainly hosted in the Upper Sinian-Lower Cambrian strata in the Yangtze Platform. Among them, the Yankong deposit in the northern Guizhou are rich in U, REE, and V，and is a representative phosphorite-type uranium deposit in China. In order to identify the occurrence and enrichment mechanism of uranium of the deposit, in this paper, we have conducted studies of mineralogy, elemental geochemistry and organic carbon contents of relevant samples from the deposit. Results of the EMPA WDS analysis and EDS Mapping show that uranium was mainly enriched in apatite in the form of isomorphic substitution, partly occurred as micron-sized independent uranium minerals, and partly adsorbed by clay minerals and organic matters. The phosphorite samples have Y/Ho values ranging from 55 to 66, obvious Ce-negative and Y-positive anomalies in the chondrite-normalized REE distribution patterns, indicating that the ore-forming materials in the Yankong deposit were mainly originated from the seawater authigenic sedimentation. The phosphorites are enriched in redox-sensitive elements, with average contents of 492×10–6, 19.1×10–6 and 4611×10–6 for U, Mo and V, respectively, and average value of 1338, 156 and 330 for the UEF, MoEF and VEF, respectively, suggesting that the P-U mineralization of the Yankong deposit occurred in a reduced environment. Compared with the contemporary phosphorite deposits in the region, it is suggested that the relatively reduced sedimentary environment and the relatively low sedimentary rate could be the important factors for the significant enrichment of REE and U in the Yankong deposit.