Hydrothermal evolution and mineralization of the Pulang porphyry Cu-Au deposit in the Sanjiang Tethys, Southwest China: Constraints from fluid inclusions and D-O-S isotopes

Abstract

Pulang is the largest porphyry Cu-Au deposit that formed in the Late Triassic and is located in the eastern Tethys. Its ore-forming fluids and mineral assemblages show some reducing characteristics. Magma and fluid are two key parameters in porphyry mineralization. Studies have shown that syn-mineralization quartz monzonite porphyries have higher oxygen fugacities, which are conducive to mineralization. However, this does not answer the question of why the deposit has reducing characteristics, which should be positively related to fluid activity. Based on detailed geologic observation, hydrothermal evolution and ore-forming process have been divided into three successive periods, namely, pre-mineralization, syn-mineralization (subdivided into three stages, including early-, inter-, and late-mineralization, corresponding to stage 1, 2, and 3 veins), and post-mineralization, in the Pulang deposit. The corresponding fluid inclusions and sulfur isotopic compositions of sulfides are studied on this basis. The results show that the salinity ranges of the stage 1 and stage 2 veins are larger, which indicates that fluid immiscibility may exist. Pyrrhotite is developed in some of the stage 2 veins, and their fluid inclusions contain CH4. However, CH4 was not found in fluid inclusions in veins that do not contain pyrrhotite. The homogenization temperatures and salinities of the fluid inclusions decrease gradually, and the hydrogen and oxygen isotopes gradually approach the meteoric water line overall, from premineralization to postmineralization. Furthermore, sulfur isotopes of pyrrhotite are the highest, indicating a possible additional source of sedimentary sulfur. That is, the ore-forming fluids were dominated by magmatic water, and meteoric water was added in the late stage as the fluid evolved. The trend of the Heishuitang fault is consistent with the extension of the main ore body in the mining area. Because the strata is affected by faults, the strata acts as a good channel for meteoric water. Meteoric water was added to the magmatic fluid after extraction and carried organic matter from the Tumugou Formation and along the cracks. The fluid was mixed, and the ore-forming elements were effectively precipitated in a reducing environment. Therefore, pyrrhotite and CH4 occur in some veins. That may be why the Pulang deposit is giant.