Nature, origin, and evolution of carbon-rich fluids in orogenic gold deposits: Insights from fluid inclusion and C-H-O isotope studies of the Tokuzbay gold deposit, Chinese Altai.

[Display omitted] • CL images of quartz are dark in Tokuzbay orogenic gold deposit. • PC, C-type FIs are trapped from initially carbon-rich metamorphic fluids. • Organic carbon in the sediments significantly contributed to the carbon in the fluids. • Carbon isotopes of carbon-rich water-poor fluids are light in orogenic gold deposits. Carbon-rich or carbonic fluids are identified in some orogenic-type gold deposits worldwide. Their nature, origin, evolution, and relations with gold mineralization are still poorly understood. The Tokuzbay large-size orogenic-type gold deposit in Chinese Altai is characterized by enormous CO 2 -rich fluid inclusions (FIs), providing an ideal case to investigate the genetic relationship between carbon-rich hydrothermal systems and orogenic-type gold mineralization. The mineralization process in the Tokuzbay includes four stages: (1) stage I quartz-magnetite veins; (2) stage II quartz-pyrite veins; (3) stage III quartz-polymetallic sulfide veins; and (4) stage IV quartz-carbonate veins. Based on the microscopic investigation, cathodoluminescence (CL) imaging, Raman spectrum analysis, and mircothermometric measurements, three types of FIs are identified in the veins, including the N 2 -bearing carbonic or CO 2 -rich inclusions (PC-type), the CO 2 -H 2 O inclusions (C-type), and the H 2 O-dominated or aqueous inclusions (W-type). The FIs in the stage I minerals are dominated by PC- and C-types, with minor amount of W-type, and occur as primary and pseudosecondary within quartz grains. These inclusions show a salinity of 5.0—9.5 wt% NaCl eqv. and homogenize to liquid phase during 272℃ to 399℃, indicating their entrapment from an initially homogeneous carbon-rich CO 2 -H 2 O-NaCl metamorphic fluid system. From early to late stages, the PC- and C-type FIs are getting infertile, while the W-type FIs become more abundant, indicating the escapes of CO 2 and compositional transition from CO 2 -rich to H 2 O-rich system. Moreover, the decreasing in temperature (272–399 ℃, 238—326 ℃, 173—267 ℃ and 135—182 ℃ from stages I to IV, respectively) and salinity (5.0—9.5, 2.6—9.2, 2.3—7.8 and 1.4—4.3 wt% NaCl eqv. from stages I to IV, respectively), together with the decrease of the δ18O values, suggests an input of meteoric water during mineralization. In the main mineralization stages (II and III), some FIs show divergent homogenization direction (liquid vs vapor) at similar homogeneous temperatures, yielding quite different salinities. For example, the coexisting W-type and C-type FIs yield similar homogenization temperatures, and the liquid-rich and vapor-rich FIs divergently homogenize to liquid and vapor at similar temperatures with different salinities. This indicates that the fluid effervescence drove phase separation and caused the CO 2 escape from the fluid system which promoted the gold precipitation at the ductile-to-brittle transition zone that revealed by the depth (8—11 km) calculated by the C-type FIs of the immiscible inclusion assemblages. The δ13C values of the CO 2 extracted from fluid inclusions range from –7.6 to –22.8‰, suggesting that the CO 2 might be derived from the hydrolysis of organic matter in the source sediments during metamorphism, rather than simply sourced from magma or the mantle. This understanding is supported by the widespread observation of the low δ13C values from most of orogenic-type gold deposits hosted in sedimentary rocks worldwide. [ABSTRACT FROM AUTHOR]