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

Abstract

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 δ18OSMOW values of fluids calculated from quartz of the quartz‐sulfides stage range from 5.6‰ to 8.6‰, and the δDSMOW values of fluid inclusions in quartz of this stage range from – 71.8‰ to 88.9‰, indicating a primary magmatic fluid source. δ34SV‐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.