Fluid evolution and mineralization mechanism of the East Kounrad porphyry Mo-W deposit in the Balkhash metallogenic belt, Central Kazakhstan

Abstract

The East Kounrad Mo-W deposit is located in the eastern part of the Balkhash metallogenic belt, Central Kazakhstan. The intrusion in the East Kounrad deposit is a Permain complex that includes medium- and fine-grained granites. Both the primary magma and hydrothermal fluids of the East Kounrad deposit are characterized by high fluorine and high oxygen fugacity, which would be important for the formation of high-grade large Mo deposit. Four mineralization stages are identified in this study. Stage I quartz–wolframite ± molybdenite ± fluorite veins, which were associated with intensively greisen alteration, were formed from fluids with high temperature (340–460 °C), high pressure (250–400 bars), high/low salinity (54.5 wt% and 3.4–10.2 wt%, respectively), and high oxygen fugacity with a fluid system of H2O-NaCl-CO2. Stage II quartz–molybdenite ± pyrite veins, which were accompanied by K-feldspars–muscovite–fluorite alteration, were formed from fluids with medium-high temperature (260–400 °C), medium pressure (100–250 bars), high/low salinity (39.1–58.4 wt% and 2.4–11.7 wt%, respectively), and relatively low oxygen fugacity. After the main stage of W and Mo mineralization, stage III quartz–pyrite ± molybdenite veins, associated with quartz–sericite alteration, were formed from fluids with medium-low temperature (160–340 °C), low pressure (less than 100 bars), various salinity (4.5–46.2 wt%) and high oxygen fugacity. Stage IV quartz–polymetallic sulfide veins, related to argillic alteration, were formed under temperature of 120–220 °C and salinity of 1.7–8.1 wt%. Remarkably, fluid boiling is the most important precipitation mechanism of molybdenite and wolframite. In addition, the decreasing oxygen fugacity and temperature also play a vital role in triggering deposition of molybdenite. The Pb isotopic composition of sulfides shows that the ore-forming materials at East Kounrad are dominantly derived from the lower crust.