Mineralization characteristics and geochemical elements migration during alkali metasomatized hydrothermal process of Yunji deposit in Xiangshan uranium ore field.

Yunji deposit is the most typical alkali-metasomatized uranium deposit in Xiangshan volcanic-related uranium ore field in Jiangxi Province, and its ore bodies are mainly hosted within porphyroclastic lava. Exemplified by two typical uranium mineralization sections, we systematically studied the alteration zoning and mineral assemblage of alkali metasomatic uranium mineralization, as well as the elements migration regularity during hydrothermal process. Detailed petrographic observations indicated that the uranium mineralization is characterized by laterally horizontal zoning, which can be subdivided into mineralized central zone, near-ore altered zone, distant-ore altered zone and fresh wall rock outwardly, and the dominant alteration types include albitization, hematitization, carbonation, chloritization and apatitization. The occurrence of uranium is mainly composed of independent uranium minerals (brannerite and coffinite), isomorphism and adsorption. The result of standardized Isocon graphical method showed that, from the distant-ore zone to the central zone, the contents of CaO, Na2O, P2O5, MgO, TiO2, Fe2O3, U, HREE, Sr, Cr, Co, Ni, Zr and Hf gradually increase, of which CaO, P2O MgO, U and Sr are strongly enriched in the central zone, while SiO2, K2O, LREE, Ba, Rb and Cs decrease dramatically. All three altered zones are enriched in Na2O and extremely depleted in K2O, and not accompanied by enrichment in Th and Mo. The significantly increased Y/Ho, delta Eu and Fe2O3/(Fe2O3 + FeO) values in the mineralized zone suggest that the alkali metasomatic ore-forming fluid is characterized by a relatively high oxygen fugacity, which may be caused by mixing of multi-source fluids. The intense desiliconization, enrichment of Zr, and extremely high Na+/K+ and Zf/Hf values indicate that the ore forming fluid has a high alkalinity. Combined with petrography, elements migration characteristics and previous studies, it is believed that the oxygen fugacity, pH values and temperatures of ore-forming fluid might gradually decrease during the fluid evolution, and the uranium in the fluid might dominantly migrate in the form of uranyl carbonate complexes. The effervescence of CO2 could lead to the co-precipitation and enrichment of uranium minerals, and a large amount of apatite and carbonate, and finally resulted in the formation of high-grade uranium ore bodies. The uranyl phosphate complexes could also be one of the important forms of uranium migration in the ore-forming fluids of this deposit.