Geology, Geochemistry, and Genesis of the Tongcun Reduced Porphyry Mo (Cu) Deposit, NW Zhejiang Province, China

The Tongcun Mo (Cu) deposit in Kaihua city of Zhejiang Province, eastern China, occurs in and adjacent to the Songjiazhuang granodiorite porphyry and is a medium‐sized and important porphyry type ore deposit. Two irregular Mo (Cu) orebodies consist of various types of hydrothermal veinlets. Intensive hydrothermal alteration contains skarnization, chloritization, carbonatization, silicification and sericitization. Based on mineral assemblages and crosscutting relationships, the ore‐forming processes are divided into five stages, i.e., the early stage of garnet + epidote ± chlorite associated with skarnization and K‐feldspar + quartz ± molybdenite veins associated with potassic‐silicic alteration, the quartz‐sulfides stage of quartz + molybdenite ± chalcopyrite ± pyrite veins, the carbonatization stage of calcite veinlets or stockworks, the sericite + chalcopyrite ± pyrite stage, and the late calcite + quartz stage. Only the quartz‐bearing samples in the early stage and in the quartz‐sulfides stage are suitable for fluid inclusions (FIs) study. Four types of FIs were observed, including 1) CO₂‐CH₄ single phase FIs, 2) CO₂‐bearing two‐ or three‐phase FIs, 3) Aqueous two‐phase FIs, and 4) Aqueous single phase FIs. FIs of the early stages are predominantly CO₂‐ and CH₄‐rich FIs of the CO₂‐CH₄‐H₂O‐NaCl system, whereas minerals in the quartz‐sulfides stage contain CO₂‐rich FIs of the CO₂‐H₂O‐NaCl system and liquid‐rich FIs of the H₂O‐NaCl system. For the CO₂‐CH₄ single phase FIs of the early mineralization stage, the homogenization temperatures of the CO₂ phase range from 15.4 °C to 25.3 °C (to liquid), and the fluid density varies from 0.7 g/cm³ to 0.8 g/cm³; for two‐ or three‐phase FIs of the CO₂‐CH₄‐H₂O‐NaCl system, the homogenization temperatures, salinities and densities range from 312°C to 412°C, 7.7 wt% NaCl eqv. to 10.9 wt% NaCl eqv., and 0.9 g/cm³ to 1.0 g/cm³, respectively. For CO₂‐H₂O‐NaCl two‐ or three‐phase FIs of the quartz‐sulfides stage, the homogenization temperatures and salinities range from 255°C to 418°C, 4.8 wt% NaCl eqv. to 12.4 wt% NaCl eqv., respectively; for H₂O‐NaCl two‐phase FIs, the homogenization temperatures range from 230 °C to 368 °C, salinities from 11.7 wt% NaCl eqv. to 16.9 wt% NaCl eqv., and densities from 0.7 g/cm³ to 1.0 g/cm³. Microthermometric measurements and Laser Raman spectroscopy analyses indicate that CO₂ and CH₄ contents and reducibility (indicated by the presence of CH₄) of the fluid inclusions trapped in quartz‐sulfides stage minerals are lower than those in the early stage. Twelve molybdenite separates yield a Re‐Os isochron age of 163 ± 2.4 Ma, which is consistent with the emplacement age of the Tongcun, Songjiazhuang, Dayutang and Huangbaikeng granodiorite porphyries. The δ¹⁸O_SMOW values of fluids calculated from quartz of the quartz‐sulfides stage range from 5.6‰ to 8.6‰, and the δD_SMOW values of fluid inclusions in quartz of this stage range from – 71.8‰ to 88.9‰, indicating a primary magmatic fluid source. δ³⁴S_V‐CDT values of sulfides range from +1.6‰ to +3.8‰, which indicate that the sulfur in the ores was sourced from magmatic origins. Phase separation is inferred to have occurred from the early stage to the quartz‐sulfides stage and resulted in ore mineral precipitation. The characteristics of alteration and mineralization, fluid inclusion, sulfur and hydrogen‐oxygen isotope data, and molybdenite Re‐Os ages all suggest that the Tongcun Mo (Cu) deposit is likely to be a reduced porphyry Mo (Cu) deposit associated with the granodiorite porphyry in the Tongcun area.