Geological Characteristics And Deep Prognosis Of The Mawu Gold Deposit In Gansu, China

Located in the western part of the Min Li metallogenic belt in the West QinlingMountains, the Mawu gold deposit is a medium-sized Carlin-like type disseminatedgold deposit discovered in recent years. The orebody is controlled by theNW trending faults and hosted in the Devonian phyllite and limestone. The Mawugold deposit has a small variety of minerals with pyrite as the dominant mineral andminor chalcopyrite, arsenopyrite, sphalerite, galena and so on. The gangue mineralsare mainly quartz and calcite.Two main types of fluid inclusions, CO2H2O inclusions and aqueous fluidinclusions are trapped in the gangue minerals. The homogenization temperatureranges from135°C to389°C while the salinity ranges from4.4%to15.9%,indicating a medium low temperature and a medium low salinity. The trappingpressure is34.6MPa to101.5MPa. The gold mineralization formed at depths of1.3km to3.8km. Fluid inclusions analyses show that the gas phase is mainly composedof H2O, N2, with minor CO2, O2, while the liquid phase are mainly composed of Ca2+,Na+, Cl and SO42. The ore forming fluid belongs to the N2H2O Ca2+Cl system.The trace element signature indicates that the metallogenic materials are closelyrelated to the wall rocks while magmatism also plays an important role. The hydrogenand oxygen isotopes of fluid inclusions show that the δ18OH2Ovalue ranges from5.3‰to10.5‰while the δD value ranges from123‰to93‰. The ore forming fluidcame from the geothermal brine. The W/R ratio ranging from0.02to0.2indicatesintense water rock exchange reaction between the ore forming fluid and wall rocks.The δ13CPDBvalues of the hydrothermal calcite are from8.37‰to1.10‰, showingthat carbon is mainly derived from magmatic (mantle) source. The measuredδ34SV CDTvalue of the samples collected in the gold deposit (6.0‰to11.9‰) variesin a very limited range, suggesting that the sulfur may be leached from the strata.Lead isotope is characterized by the source of upper crust and shows signature ofcrust mantle mixing.In the mineralization process, the physicochemical conditions changed in the ore forming fluid system due to boiling and water rock reaction, which reduced thestability of gold complexes, causing the precipitation of ore forming materials infavorable position. The deep prognosis shows that the high average gold gradecorresponds with high P type pyrite thermoelectric coefficient value and highthermoelectric coefficient dispersion value. Furthermore, the probability ofdiscovering rich ore section is greater in the deep position than in the shallow. Themain orebody extends much deeper than currently discovered.