Geology and ore-forming fluid evolution of the Aktogai giant porphyry Cu deposit, Kazakhstan. (1st October 2018) - Li, Changhao
Graphical abstract: Highlights: Ore-forming tonalite porphyry has high oxygen fugacity and sulfur contents. Three hydrothermal alteration stages are recognized. Decreasing of t emperature and pressure result in copper precipitation. Copper transported by brine at high to low temperature, but also by vapor at high temperature. Abstract: The Aktogai Group (more than 12.5Mt of Cu and 80t of Au) is located in the western part of the Central Asian Orogenic Belt (CAOB). The mineralization of the Aktogai deposit is associated with the Early Carboniferous tonalite porphyry which intruded into the Koldar complex. The ore-forming tonalite porphyry has higher oxygen fugacity and sulfur contents than those of wall rocks based on results of mineralogical chemistry analyses. Hydrothermal fluid activities result in the potassic, sericite(-chlorite) and propylitic alteration, and are associated with disseminated and/or vein mineralization. The alteration and mineralization include three stages. Potassic stage is characterized by hydrothermal K-feldspar, biotite, magnetite, "A" veins and disseminated mineralization. Sericite(-chlorite) stage that partly overprinted on potassic stage is characterized by sericite, chlorite, "C" veins and disseminated mineralization. Propylitic stage is characterized by chlorite, epidote and "C" veins, and develops in the tonalite porphyry and wall rocks. Five types of fluid inclusions have been recognized in the Aktogai deposit, including liquid-rich (L-type), vapor-rich (V-type), daughter mineral-bearing (S-type), CO2 -bearing (C-type) and minor SO2 -bearing fluid inclusions. In the potassic stage, ore-forming fluid belongs to H2 O-NaCl-CO2 system with four types fluid inclusions, homogeneous temperatures and salinities of fluid inclusions are 320 °C to 448 °C and 0.35 wt% to 51.08 wt%, respectively. In the sericite(-chlorite) stage, the fluid belongs to H2 O-NaCl-CO2 system with L-type, V-type and C-type fluid inclusions, homogeneous temperatures and salinities of fluid inclusions are 243 °C to 315 °C and 1.22 wt% to 13.27 wt%, respectively. In the propylitic stage, the fluid belongs to H2 O-NaCl system with L-type and V-type fluid inclusions, homogeneous temperatures and salinities of fluid inclusions are 211 °C to 328 °C and 0.35 wt% to 11.46 wt%, respectively. A relatively negative δ 34 S‰ values (−4.61‰ to −3.18‰) indicate that the sulfur derived from a high oxygen fugacity magma. Decreases of the temperature, pressure and oxygen fugacity of the ore-forming fluid result in metal precipitation in the Aktogai deposit.