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 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 degrees 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 degrees 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 degrees C, 220-440 degrees C and 125-225 degrees 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 delta O-18 values of 85-10.0%0, corresponding to delta(OH2O)-O-18 values of -2.9 to 5.9%., while the delta D-H2O 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. (C) 2014 Elsevier B.V. All rights reserved.