Metallogenic Fluid Characteristics In Ganshahe Rare Earth Deposit In Gansu Province

The Ganshahe rare earth deposit is the fourth largest rare earth deposit in China,the proved reserves are 600,000 tons.This paper uses the research methods of inclusions,stable isotope composition and other methods to study the geological characteristics of ore deposits and ore-forming fluids.The results show that:the rare earth ore body is mainly produced in the Nephrite slate and Nephrite porphyry.The ore minerals in the ore are mainly fluorocarbon,molybdenum,chalcopyrite,porphyrite,sphalerite and galena,and gangue minerals are mainly calcite,quartz,fluorite and barite.The ore-forming processes of this deposit can be divided into magmatic,magma-hydrothermal,hydrothermal and epigene metallogentic periods.Of them,the magma-hydrothermal metallogenic period is the dominant metallogenic period.There are three major types of inclusions in the deposit,namely melt inclusions,fluid-melt inclusions and fluid inclusions.inclusion assemblages show the characteristics of fluid immiscibility.The homogenization temperature of melt inclusions at magmatic period is 780?.The homogenization temperatures of fluid-melt inclusions and fluid inclusions at magma-hydrothermal period range from609?625?and from 191?439?,respectively.Accroding to coexisting CO₂ inclusions and H₂O inclusions,the mineralization temperature at magma-hydrothermal period range 274?314?,the pressure of mineralization between 68?95 MPa,corresponding to 2.6?3.6 km of ore-forming depth.At hydrothermal period,the homogeneous temperature of ore fluids range from 129?225?.Fluid temperature reveals a reduced trend from magma period to hydrothermal period.Mineralization mainly occurred in the magma-hydrothermal period and the mineralization temperature between274?625?.Therefore,this deposit belongs to a middle-high temperature and medium deep magma-hydrothermal transitional deposit.C?H and O isotope results show that the ore-forming fluid was mainly derived from magmatic water with input of meteoric water.During the ore-forming process,the variation of fluid inclusion assemblages from melt inclusions,fluid-melt and fluid inclusions to fluid inclusions suggest that ore-forming fluids have undergone an evolution from magma,magma+hydrothermal fluid,to hydrothermal fluid.A large number of fluid-melt inclusions recoded at the magma-hydrothermal metallogenic period,and H₂O-rich inclusions and CO₂-rich CO₂-H₂O inclusions co-existed closely.The two fluid inclusion assemblages show two immiscibility occurred in the mineralization,namely,early magma-hydrothermal fluid miscibility resulted in the formation of fluorite and REE-fluoro carbonate,whereas CO₂-H₂O immiscibility caused sulfide precipitation.