甘肃干沙鄂博稀土矿床熔体和流体包裹体对成矿的制约

Melt and Fluid Inclusions and Their Constraints on Ore-Forming Conditions of Ganshaebo Rare Earth Deposit,Gansu Province,China

甘肃天祝干沙鄂博稀土矿床产于霓辉正长岩和霓辉正长斑岩中,矿体形态呈不规则脉状、透镜状和板状。成矿过程可分为岩浆期、岩浆-热液期、热液期和表生期,其中岩浆-热液期为主要成矿期。本矿床中的包裹体有熔体包裹体、流体-熔体包裹体、H2O包裹体、CO2包裹体、CO2-H2O包裹体、含子矿物H2O包裹体和含子矿物CO2-H2O包裹体7类,并以富含流体-熔体包裹体、CO2-H2O包裹体为显著特征。包裹体组合从熔体包裹体→流体-熔体包裹体、H2O包裹体、CO2包裹体和CO2-H2O包裹体→H2O包裹体的变化,反映本矿床的形成经历了从岩浆→岩浆+热液→热液的演化过程。岩浆期熔体包裹体均一温度为780℃;岩浆-热液期均一温度为191700℃,盐度为5.26%22.24%,属中低盐度,成矿压力为6895 MPa,相应的成矿深度为2.63.6 km;热液期均一温度为129225℃,盐度为0.35%7.73%,为低盐度。从岩浆期到岩浆-热液期再到热液期,温度逐渐降低,矿化作用主要发生在岩浆-热液期,属中高温、中深成岩浆-热液过渡型矿床。

Ganshaebo rare earth deposit in Gansu Province,China is hosted by aegirine-augite syenite and aegirine syenite prophyry. The orebodies occur as irregular veins,lenses or layers. The ore-forming processes can be divided into four distinct periods,e.g.,magmatic,magmatic-hydrothermal,hydrothermal and supergene. The main period of mineralization is of magmatic-hydrothermal. Inclusions in the deposit can be divided into seven categories,melt inclusions,fluid-melt inclusions,H2 O inclusions,CO2 inclusions,CO2-H2 O inclusions,daughter-bearing H2 O inclusions and daughter-bearing CO2-H2 O inclusions. Variation of inclusion types from melt inclusion to fluid-melt inclusion + fluid inclusion and then to fluid inclusion indicates that the ore-forming medium varies from magma to magma + hydrothermal,and then to hydrothermal solution. The homogenization temperature of melt inclusions in magmatic period is 780 ℃. During the magmatic-hydrothermal period,homogenization temperatures vary from 700 ℃to 191 ℃ with salinity varying from 5. 25% to 22. 14% Na Cleqvand ore-forming pressure varying from 68 MPa to 95 MPa that corresponding to a depth ranging from 2. 6 km to 3. 6 km. Homogenization temperatures of the hydrothermal period vary from 129 ℃ to 225 ℃ with salinity varying from 0. 35% to 7. 73% Na Cleqv. The temperature gradually decreases from magmatic period to magmatic-hydrothermal period and to hydrothermal period. Mineralization hadmainly occurred during the magmatic-hydrothermal period. The deposit belongs to mesozonal to hypozonal,and medium to high temperature magma-hydrothermal transitional deposit.