Magmatic-hydrothermal fluid evolution and genesis of the Machangqing porphyry Cu-Mo deposit, western Yunnan, China: Constraints from fluid inclusion and H-O-S isotopes

Abstract

The Machangqing Cu-Mo deposit is situated in the in western Yunnan (southwest China), which is tectonically located in the NW-trending Jinshajiang-Ailaoshan fold belt. The Cu-Mo vein mineralization is developed in altered wall rocks, associated with the Machangqing granite porphyry. Wall rock alterations include propylitic, phyllic, and potassic, and the alteration/mineralization veining comprises four major stages (I to IV): (I) pre-ore barren quartz; (II) early-ore quartz-sulfide (chalcopyrite + pyrite); (III) late-ore quartz-polymetallic (chalcopyrite + molybdenite + pyrite) sulfide; (IV) post-ore quartz-calcite. Four types of fluid inclusions (FIs) were recognized: liquid-rich two-phase (L-type), vapor-rich two-phase (V-type), hypersaline (H-type), and trace pure vapor (P-type). FI microthermometry suggests that the pre-ore (stage I) fluids were of relatively high temperature (375–510 °C) and salinity (48.2–62.8 wt% NaCl eqv.), with lithostatic pressure of 208–508 bars (eqv. 0.6–1.6 km depth); Fluid temperature may have decreased from syn-ore stage II (405 °C) to stage III (250 °C) fluids, with hydrostatic pressure of ∼100 bars (eqv. 0.9 km depth). Fluids obtained during the post-ore phase (stage IV) exhibited the minimum temperature (below 234 °C) and salinity (ranging from 1.1 to 3.2 wt% NaCl eqv.), indicating meteoric water incursion. We suggest that the stage I alteration occurred under alkaline and highly-oxidized fluid conditions (pH = 6.4–7.7; logfO₂ = -25.4 to –23.8), whereas stage II and III occurred under acidic and less oxidizing conditions (pH = 3.5–4.2; logfO₂ = -30.1 to −27.2). The ore fluid source may have transitioned from magmatic to meteoric, as evidenced by hydrogen–oxygen isotopes, whilst sulfur isotope compositions indicate a deep-seated magmatic source. We suggest that the Cu-Mo ore deposition at Machangqing deposit was triggered by decompression boiling, meteoric water incursion, and the consequent redox change at shallow (∼1 km) depth.