RARE-EARTH ELEMENT GEOCHEMISTRY OF THE MOLYBDENUM-BEARING GRANITOIDS IN THE JINDUICHENG-HUANGLONGPU DISTRICT, SHAANXI PROVINCE, NORTHWEST CHINA

Located in the Luonan county, Shaanxi Province, northwest China, Jindu icheng, Shijiawan and Huanglongpu molybdenum deposits constitute the m ost important molybdenum mineralized district in China. Among these th ree deposits, the Jinduicheng and Shijiawan molybdenum deposits are co nnected spatially and genetically with granitoid porphyry (124 +/- 6 M a, K-Ar biotite), and consist of disseminated-veinlet ores. Geochemica l studies of rare earth elements (REE) furnish further evidence for un derstanding the rock- and ore-forming processes of these two porphyry molybdenum deposits and their related granitoid rocks. The REE distrib ution in molybdenum ore, granitoids and their Middle Proterozoic meta- volcanic wall rocks is discussed. The similarities between the REE sig natures 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 granitoi d magma derived mainly from crustal anatexis. The Shijiawan biotite mo nzogranite porphyry may be an apophysis of the Laoneushan granitoid ba tholith. Compared to the Shijiawan monzogranite porphyry, the Jinduich eng molybdenum-bearing granite porphyry is characterized by a high con tent of HREE, and depletion in LREE. The unique REE patterns indicates that the molybdenum-bearing granite porphyry was formed by thermograv itation diffusion of a granitoid magma. The slight depletion of REE ab undance in the altered granitoid porphyry and meta-volcanic wall rocks shows that leaching of REE occurred during breakdown of the primary m ineral assemblage, and crystallization of secondary minerals. The high REE content of molybdenum ore represented re-deposition of the mobili zed molybdenum and REE.