Geology and ore fluid geochemistry of the Jinduicheng porphyry molybdenum deposit, East Qinling, China

Jinduicheng deposit is a giant Mesozoic porphyry Mo system deposit in the East Qinling molybdenum belt, Shaanxi Province, China. The mineralization is associated with the I-type Jinduicheng granite porphyry. Both the porphyry stock and country rocks underwent intense hydrothermal alteration. The alteration, with increasing distance from the parent intrusion, changes from silicification, through potassic and phyllic assemblages, carbonation, to propylitic assemblages. Molybdenite, the dominant ore mineral,, occurs in veinlets, most of which are hosted by the altered country rocks, with less than 25% of the ore in the porphyry body. The hydrothermal system comprises four stages, including pre-ore quartz and K-feldspar; two ore stages of quartz, K-feldspar, molybdenite, and Pb- And Zn-bearing sulfides; and post-ore quartz and carbonate. Six main types of primary fluid inclusions are present in hydrothermal quartz, including two-phase aqueous, one-phase aqueous, three-phase CO2-bearing, CO2-dominated fluid inclusions, gas inclusions, and melt inclusions. The homogenization temperatures of fluid inclusions range from 210 to 290 degrees C in the pre-ore stage, 150-310 degrees C in ore stage I, 150-360 degrees C in the ore stage II, and 195-325 C in the post-ore stage quartz. Estimated salinities of the ore-forming fluids range from 6.9 to 13.5, 4.3 to 123, 6.2 to 12.4, and 3.4 to 9.9 wt.% NaCI equiv. in stages 1-4, respectively. The delta S-34 values of pyrite in the two ore stages range from 2.8 parts per thousand to 4.3 parts per thousand, whereas the delta S-34 values of molybdenite range from 2.9 parts per thousand to 6.2 parts per thousand. The data suggest both magmatic and crustal sources of sulfur. The delta D and 6180 values for the hydrothermal fluids are -57.2 parts per thousand, to -84.4 parts per thousand and 8.0 parts per thousand to -3.2 parts per thousand, respectively. The fluid inclusion and stable data indicate that the pre-ore hydrothermal fluids were mostly of magmatic origin, but the fluids responsible for ore deposition were mixed magmatic and meteoric, and eventually meteoric water dominated the system in the post-ore stage. (C) 2013 Elsevier Ltd. All rights reserved.