Geology, fluid inclusion and H–O–C isotope geochemistry of the Doranasai gold deposit, Chinese Altai: implications for ore genesis

The Central Asian Orogenic Belt experienced a complex subduction accretion–collision history, forming the Irtysh gold belt. The Doranasai deposit is a typical gold deposit in the Irtysh gold belt, but its genesis is still unclear. The Doranasai gold deposit occurs in the east wing of the Aksay syncline. The orebodies are controlled by the NS-trending fault. The main ore types are quartz vein type and altered rock type. Wall rock alteration is represented by albitization, pyritization, sericitization, silicification, chloritization and carbonation. The ore-forming process includes three stages: albite–quartz–pyrite stage, quartz–polymetallic sulfide stage and barren quartz–carbonate stage. The fluid inclusions in the early- and middle-stage quartz are mainly aqueous inclusions and CO2–H2O inclusions, while only aqueous inclusions are developed in the late-stage quartz. The homogenization temperature of fluid inclusions has evolved from up to 300 °C in the early stage, 180–280 °C in the middle stage to lower than 200 °C in the late-stage, and the salinities have evolved from 4.2–9.6 wt% NaClequiv. in the early stage and 4.3–10.9 wt% NaClequiv. in the middle stage to 2.1–6.9 wt% NaClequiv. in the late stage. The coexistence of different types of inclusions in middle-stage quartz indicates that the fluid is not uniformly trapped, and fluid phase separation is the main mechanism of gold precipitation. The isotopic compositions of quartz from the Doranasai gold deposit show some variation but are generally comparable to those of other orogenic gold deposits. Fluids trapped in early-stage quartz have a δ18Owater range of 12.5–7.6‰, δD of − 101‰ and $$\delta^{13} {\text{C}}_{{{\text{CO}}_{2} }}$$ of − 12.9‰, and the isotopic compositions of fluids trapped in middle-stage quartz have δ18Owater values of 11.4‰ to 4.4‰, δD of − 112‰ to − 102‰ and $$\delta^{13} {\text{C}}_{{{\text{CO}}_{2} }}$$ of − 11.7‰ to − 9.5‰. In contrast, isotopic compositions of fluids trapped in late-stage quartz have δ18Owater values of 7.6‰ to 1.8‰, δD of − 108‰ to − 90‰ and $$\delta^{13} {\text{C}}_{{{\text{CO}}_{2} }}$$ of − 12.4‰ to − 11.1‰. The early-stage fluids are probably derived from metamorphic decarbonation of the sedimentary host rock at depth, leading to the precipitation of barren quartz veins. In the middle stage, a decrease of the regional pressure and temperature may lead to the incorporation of meteoric water into the ore-forming system. Late-stage fluids trapped by calcite veins show isotopic compositions similar to meteoric water, indicating the cessation of hydrothermal fluid circulation. The geological and fluid inclusion characteristics of the Doranasai gold deposit are consistent with those of the orogenic gold deposit, indicating that it is an orogenic gold deposit formed in the collisional setting.