Fluid inclusion study of the Tangjiaping Mo deposit, Dabie Shan, Henan Province: implications for the nature of the porphyry systems of post-collisional tectonic settings

The Tangjiaping porphyry Mo deposit in Shangcheng County is located in the Dabie continental collisional orogenic belt. The hydrothermal alteration and mineralization processes can be divided into three stages: K-feldspar + quartz + pyrite + magnetite + molybdenite in the early stage, quartz + pyrite + molybdenite ± chalcopyrite in the middle stage, and quartz + carbonate ± pyrite veins or carbonate veinlets in the late stage. The middle stage involved important Mo mineralization. Four compositional populations of fluid inclusions (FIs) occur in hydrothermal quartz formed in the early and middle stages, namely: pure CO2, CO2–H2O, daughter mineral-bearing, and NaCl–H2O. The late-stage quartz-carbonate ± pyrite veinlets contain only NaCl–H2O FIs. Homogenization temperatures of the early-stage FIs are mainly above 375°C, with salinities up to 62.10 wt.% NaCl equiv. Haematite daughter minerals, which probably represent an oxidizing environment, together with halite, sylvite, and chalcopyrite, are present in the FIs of this stage. Most of the middle-stage FIs homogenized between 235 and 335°C, with fluid salinities ranging from 1.06 to 45.87 wt.% NaCl equiv. In middle-stage quartz, besides halite and sylvite daughter minerals, abundant chalcopyrite and jamesonite are also present, reflecting a reducing environment. The daughter mineral-bearing FIs coexist with vapour- and liquid-rich FIs with contrasting salinities, and they homogenized in divergent ways at similar temperatures. This feature strongly suggests that fluid boiling occurred during the middle stage, which is well accepted as an important mechanism for the precipitation of ore-forming materials. FIs in late-stage minerals totally homogenized in the range 115–195°C, with low salinities ranging from 1.91 to 9.98 wt.% NaCl equiv. Combined with the published H–O isotope data, we propose that the initial ore fluids were magmatic in origin and were characterized by high temperature, high salinity, high oxygen fugacity, high levels of metallic elements, and high levels of CO2. In the middle stage, the fluids boiled and resulted in CO2 release, oxygen fugacity decrease, and rapid precipitation of ore-forming materials. The late-stage fluids, characterized by low temperatures, low salinities, and low contents of CO2, might have been sourced from meteoric water. Daughter mineral-bearing CO2–H2O FIs, especially multiple species daughter mineral-bearing FIs, are regarded as indicators of porphyry ore systems generated in continental collisional settings. This understanding is validated by recent studies of other porphyry ore systems in Chinese collision orogens.