Fluid inclusion characteristics of the Mawu gold deposit in Gansu Province and their geological significance

Abstract: Located in the western part of the Min-Li metallogenic belt within the West Qinling Mountains, the Mawu gold deposit is a medium Carlin-type disseminated gold deposit discovered in recent years. The orebody is controlled by the NW-trending faults and hosted in the Devonian phyllite and limestone. Two main types of fluid inclusions, i.e., CO2-H2O inclusions and aqueous fluid inclusions, are trapped in the gangue minerals. The homogenization temperatures range from 135 °C to 389 °C, while the salinities range from 4.4% to 15.9%, indicating medium-low temperature and medium-low salinity. The trapping pressures are 34.6 MPa to 219.8 MPa. The gold mineralization occurred at the depths of 1.3 km to 8.1 km. Fluid inclusion analyses show that the gas phase is mainly composed of H2O and N2, with minor CO2 and O2, while the liquid phase is mainly composed of Ca2+, Na+, Cl? and SO42?. The ore-forming fluid belongs to the N2-H2O-Ca2+-Cl? system. The hydrogen and oxygen isotopic compositions of fluid inclusions show that δ18OH2O values range from 5.3‰ to 10.5‰, while δD values range from -123‰ to -93‰. The ore-forming fluid came from the geothermal brine. An analysis of the fluid evolution shows that the variation of the stability of the fluid system caused by the structural transition and the boiling action played vital roles in the precipitation of gold.