The Hongniu-Hongshan Cu skarn deposit (77.8 Mt at 1.8% Cu) is located in the central part of the Zhongdian porphyry and skarn Cu belt in southwestern China. Skarn and orebodies occur mainly between the different units of the Upper Triassic Qugasi Formation or within altered limestone adjacent to Late Cretaceous intrusions (78–76 Ma). Three main paragenetic stages of skarn formation and ore deposition have been recognized on the basis of petrographic observations: (1) pre-ore-stage hornfels with diopside (Di87–72Hd12–7), small-scale endoskarn with reddish grossular (Adr22–57Gr78–43), diopside (Di83–92Hd7–15), vesuvianite, and abundant exoskarn with red-brown andradite (Adr75–98Gr2–22), sahlite (Di28–41Hd58–71), and wollastonite; (2) syn-ore-stage retrograde minerals, sulfides (pyrite, chalcopyrite, pyrrhotite, molybdenite, galena, and sphalerite), quartz, and calcite; and (3) post-ore-stage calcite veins. Sulfur isotope values of sulfides are relatively high, with an average δ34S = 4.9‰ (n = 40), suggesting that the ore-forming fluid was magmatic and that the sulfides precipitated from a relatively reducing ore fluid.

The coexistence of silicate melt and primary fluid inclusions in quartz phenocrysts of the mineralization-related quartz monzonite porphyry indicates the simultaneous entrapment of fluid and melt, and records the process of the aqueous fluid exsolving from the crystallizing melt. The initial single-phase fluid has a salinity of 8.8 to 12.7 wt % NaCl equiv and homogenization temperatures of 566° to 650°C, corresponding to pressures of 680 to 940 bar and lithostatic depth of 2.5 to 3.5 km. The primary fluid inclusions in the pre-ore-stage garnet and pyroxene composed of coeval vapor-rich (V type) and halite-bearing (S-I and S-II types containing sylvite) inclusions (32–>79 total wt % salts) share similar homogenization temperatures (450°–550°C), indicative of the occurrence of fluid unmixing under lithostatic pressures of ~550 to 780 bar (>2.0-km depth). Primary fluid inclusions trapped in syn-ore quartz, calcite, and epidote show the common development of S-type inclusions (~37.3 wt % NaCl equiv) with coexisting V-type, liquid-rich (L type), and CO2-bearing (C-I type) inclusions, all of which have homogenization temperatures of 300° to 400°C and trapping pressures of 100 to 400 bar (~1.5-km depth). Brine inclusions homogenized by halite dissolution after vapor disappearance in both the pre- and syn-ore stages are interpreted to have been trapped under overpressured conditions (>1,520 bar).

Oxygen isotope analyses were conducted on garnet, wollastonite, epidote, quartz, and calcite. The pre-ore-stage garnet and wollastonite have δ18Ofluid values of 5.6 to 8.1‰, whereas the syn-ore-stage epidote, quartz, and calcite have more variable δ18Ofluid values in the range of 3.9 to 17.5‰. The δ18Ofluid values of the post-ore-stage vein calcite (15.2–21.3‰) are much higher than both the pre- and syn-ore stages. The vapor phase of inclusions contains H2S, CH4, and C2H6 in the syn-ore stages. All these observations reveal that (1) the formation of the Cu skarn deposit was dominated by a magmatic hydrothermal system, (2) multiple fluid pulses contributed to the formation of the pre- and syn-ore-stage skarn minerals and sulfides, and (3) the increase in pH due to the neutralization of the acidic fluid could be the main factor controlling the large-scale ore deposition in Hongniu-Hongshan.

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