Rare earth element geochemistry of the molybdenum-bearing granitoids in the Jinduicheng-Huanglongpu district, Shaanxi Province, northwest China

Abstract

Located in the Luonan county, Shaanxi Province, northwest China, Jinduicheng, Shijiawan and Huanglongpu molybdenum deposits constitute the most important molybdenum mineralized district in China. Among these three deposits, the Jinduicheng and Shijiawan molybdenum deposits are connected spatially and genetically with granitoid porphyry (124 ± 6 Ma, K-Ar biotite), and consist of disseminated-veinlet ores. Geochemical studies of rare earth elements (REE) furnish further evidence for understanding the rock- and ore-forming processes of these two porphyry molybdenum deposits and their related granitoid rocks. The REE distribution in molybdenum ore, granitoids and their Middle Proterozoic meta-volcanic wall rocks is discussed. The similarities between the REE signatures of the Shijiawan molybdenum-bearing monzogranite porphyry and the neighbouring Laoneushan monzogranite (130 ± 5 Ma, K-Ar biotite) show that they were produced at the same evolutional stage of granitoid magma derived mainly from crustal anatexis. The Shijiawan biotite monzogranite porphyry may be an apophysis of the Laoneushan granitoid batholith. Compared to the Shijiawan monzogranite porphyry, the Jinduicheng molybdenum-bearing granite porphyry is characterized by a high content of HREE, and depletion in LREE. The unique REE patterns indicates that the molybdenum-bearing granite porphyry was formed by thermogravitation diffusion of a granitoid magma. The slight depletion of REE abundance in the altered granitoid porphyry and meta-volcanic wall rocks shows that leaching of REE occurred during breakdown of the primary mineral assemblage, and crystallization of secondary minerals. The high REE content of molybdenum ore represented re-deposition of the mobilized molybdenum and REE.