Abstract:

The Shijinpo gold deposit is an important middle-size lode gold deposit in the Beishan orogenic belt, northwestern China. The gold mineralization is hosted in the early Devonian granite and consists primarily of auriferous quartz veins that are controlled by the EW-trending faults. Wall-rock alteration is dominated by sericite+quartz+sulfide assemblages in close proximity of gold veins. The pyrite is the predominant sulfide mineral, variably coexisting with minor amounts of galena, sphalerite and chalcopyrite. Laser incremental heating ⁴⁰Ar/³⁹Ar analysis of hydrothermal sericite yields a plateau age of （364.6±3） Ma (2σ), which suggests that the gold mineralization took place in the late Devonian period. The comprehensive fluid inclusion study shows that there are three major types of fluid inclusions in the early stage: liquid-rich, gas-rich aqueous and CO₂-bearing fluid inclusions, with intermediate homogenization temperature (280-325℃), and low salinity (7.86%-9.21% NaCl eqv). The fluid boiling is evidenced by the co-existence of three types of fluid inclusions with similar homogenization temperatures. There is only liquid-rich aqueous fluid inclusions in the late stage,with hqmogenization temperatures and salinities varying from 160 ℃ to 230 ℃,9.47% to 11.10%Nacl equivalent. Fluid pressures declined from ca.113 MPa to ca.11 MPa through the early stage to the late stage, and the ore-forming fluids are assumed to undergo boiling in a transitional setting (from compression to extension). Oxygen and hydrogen isotopic contents in the ore-forming fluid range from 1.39‰ to 3.39‰ and -71‰ to -99‰, respectively, suggesting that the ore-forming fluid was mainly derived from the metamorphic water, and mixed by the meteoric water in the late stage. The measured δ³⁴S values in sulfide minerals range from 4.44‰ to 11.33‰. Both the Precambrian metamorphic rock and granitic wall-rock might be the sulfur sources. Based on the paper and the regional geologic data, we conclude that the Shijinpo gold deposit belongs to the orogenic-type gold deposit.

Key words: Shijinpo, gold deposits, ⁴⁰Ar/³⁹Ar dating, ore-forming fluids, ore genesis