Hydrothermal alteration and elemental mass changes of the Xiangyangping uranium deposit in the Miao’ershan ore field, South China

Abstract Hydrothermal alteration records the effects of fluid-rock interactions and can therefore be used to constrain metal mineralization. Although hydrothermal alteration is widely developed in the hydrothermal vein-type uranium deposit in South China, consideration of the elemental mass changes during alteration is rare. The Xiangyangping uranium deposit in the Miao’ershan uranium orefield is mainly hosted by the Douzhashan granite in South China. Observations made at this deposit are used to address hydrothermal alterations and their relationship with elemental mass change. The Xiangyangping deposit is characterized by seven types of alteration appearances, including K-feldspathization, K-mica/illitization, silicification, carbonatization, chloritization, pyritization, and hematitization. Based on the field characteristics, the altered samples in the Xiangyangping deposit were divided into the red-altered (type 1) and grey- altered (type 2) zones. Whole-rock geochemistry of the altered samples in this deposit indicated that there are varying element concentrations depending on the different altered zones: (1) the altered samples in type 1 had higher molar K/Al and lower (2Ca + Na + K)/Al ratios and the altered samples in type 2 displayed lower molar K/Al and lower (2Ca + Na + K)/Al ratios than the least-altered host granitoids; (2) the altered samples in type 1 showed Si, K, Al, P, and Rb gains and Na loss, whereas the samples in type 2 showed Si, K, Al, P, Rb, and Na losses. Alteration assessment results revealed that the samples in type 1 experience moderate K-H metasomatism and intensive Si-metasomatism, and the samples in type 2 underwent intensive K-H metasomatism and weak Si-metasomatism. The samples in type 1 were dominated by conversion of plagioclase to K-mica/illite and K-feldspar, while the abundant K-feldspar grains in the samples of type 2 were altered to K-mica/illite.