Fluid evolution of the Chalukou giant Mo deposit in the northern Great Xing'an Range, NE China

Abstract

The Chalukou giant Mo deposit in the Heilongjiang Province, northeastern China, is a porphyry deposit hosted in an intermediate-felsic complex surrounded by Mesozoic volcano–sedimentary rocks. The mineralization process is composed of four stages, including quartz + K-feldspar (Stage I), quartz + molybdenite (Stage II), pyrite + chalcopyrite + quartz ± other sulphides (Stage III) and carbonate ± fluorite ± quartz (Stage IV). The mineralization is generally associated with intense K-feldspar-, fluorite-, phyllic- and propylitic alteration. Primary fluid inclusions (FIs) in quartz include four compositional types, i.e. pure carbonic (PC-type), aqueous-carbonic (C-type), daughter mineral-bearing (S-type) and aqueous (W-type) inclusions. Halite, sylvite and hematite are recognized as the daughter minerals in Stage I S-type FIs, whereas molybdenite and chalcopyrite occur as daughter minerals in Stage II S-type FIs. High-salinity and high pressure (>220 MPa) FIs exist in Stage I quartz veins, characterized by homogenization through halite dissolution at temperatures of 324 to 517 °C. The paucity of coexisting vapour-rich FIs with similar homogenization temperatures at this stage indicates that the initial S-type inclusions have directly exsolved from the magma rather than boiling off of a low-salinity vapour. Stage I quartz has captured the C- and W-type FIs, which have totally homogenized at 270–530 °C with salinities of 1.6–17.0 wt.% NaCl equiv. At Stage II, the coexistence of all FI types were only observed at pressures of 150–218 MPa and temperatures of 352–375 °C, with two salinity clusters of 0.9–16.6 wt.% NaCl equiv. and 37–56 wt.% NaCl equiv. Stage III quartz contains W-type FIs with homogenization temperatures of 158–365 °C, salinities of 0.5–9.0 wt.% NaCl equiv., and minimum pressures of 12–116 MPa; whilst Stage IV fluorite or calcite only contains W-type FIs with homogenization temperatures of 121–287 °C, salinities of 0.5–5.3 wt.% NaCl equiv., and minimum pressures of 10–98 MPa. The estimated trapping pressure from Stages II to III suggests an alternating lithostatic–hydrostatic fluid-system caused by fluid boiling. Ore fluids at the Chalukou Mo deposit may have been evolved from a CO2-rich, high-salinity, and high-oxygen fugacity (fO2) magma system, to a CO2-poor, low-salinity, and low-fO2 epithermal system. Two key points may have contributed to the formation of the Chalukou giant Mo deposit: The magmatic origin and fluid boiling that has resulted in decompression and rapid precipitation of metals. Copyright © 2014 John Wiley & Sons, Ltd.