Evolution of ore fluids in the Donggou giant porphyry Mo system, East Qinling, China, a new type of porphyry Mo deposit: Evidence from fluid inclusion and H–O isotope systematics

• The Donggou porphyry Mo deposit occurs in East Qinling continental collision orogen.
• The fluid-system forming the Donggou deposit was initially CO2-rich.
• It is unique for the external mineralization.
• It represents a new type of porphyry Mo deposits, termed collisional-type.

The Donggou Mo deposit in the eastern Qinling area, China, is a giant porphyry system discovered based on a targeting concept by using the tectonic model for collisional orogeny, metallogeny and fluid flow (CMF model). Mo mineralization is associated with the Donggou aluminous A-type granite porphyry and was formed during the Early Cretaceous in a tectonic regime of continental extension. The orebodies mainly occur as numerous veinlets in the host-rocks. Hydrothermal ore-forming processes include at least three stages, characterized by veinlets of (1) quartz + K-feldspar + minor molybdenite, (2) quartz + molybdenite ± beryl and (3) quartz + carbonate + fluorite.Three types of fluid inclusions (FIs) are distinguished in quartz and beryl in stages 1 and 2, i.e., aqueous (W-type), carbonic–aqueous (C-type) and solid-bearing (S-type), with only aqueous FIs observed in stage 3 minerals. S-type FIs contain variable daughter minerals including halite, chalcopyrite, calcite and an unidentified transparent crystal, but only halite can dissolve during heating. Halite-bearing S-type FIs are mainly homogenized by halite dissolution at 182–416 °C, corresponding to salinities of 30.9–49.2 wt.% NaCl equiv.; minor halite-bearing S-type FIs are homogenized to liquid at 190–360 °C via vapor disappearance, with salinities of 29.1–36.2 wt.% NaCl equiv. Other FIs in minerals of stages 1, 2 and 3 are homogenized at temperatures of 341–550 °C, 220–440 °C and 125–225 °C, with salinities of 8.0–18.3, 5.3–16.8 and 0.5–7.3 wt.% NaCl equiv., respectively. The estimated minimum trapping pressures are up to 141 MPa in stage 1 and up to 81 MPa in stage 2, respectively, corresponding to an initial mineralization depth of no less than 5 km. The quartz in veinlets yields δ18O values of 8.5–10.0‰, corresponding to δ18OH2O values of − 2.9 to 5.9‰, while the δDH2O values of fluid inclusions range from − 59 to − 82‰. These data suggest that the ore fluids forming the Donggou deposit changed from high-temperature, high-salinity, CO2-rich magmatic to low-temperature, low-salinity and CO2-poor meteoritic fluids via boiling and mixing, resembling those of other magmatic–hydrothermal systems in Qinling Orogen and Dabie Shan. This supports the notion that the porphyry systems generated in a post-collisional tectonic setting were initially CO2-rich, as indicated by abundant C-type and CO2-bearing S-type fluid inclusions.

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