桂北加里东期越城岭岩体和都庞岭西体黑云母矿物化学特征对比及对成岩成矿指示

Comparison of mineral chemistry of biotite between Caledonian Yuechengling and western Dupangling granites in northern Guangxi and its implication for petrogenesis and mineralization

加里东期越城岭岩体和都庞岭西体同处于南岭西段W-Sn-Cu多金属成矿带,但二者成矿能力不同。越城岭岩体以铁质黑云母为主,形成于相对低温(平均531℃)和高压(平均277 MPa)环境,具有贫镁(MgO平均8.05%、X_(Mg)=0.39),富铁(FeOT平均22.4%、I_(Fe)=0.61)、铝(Al_(2)O_(3)平均16.8%)等特征;都庞岭西体以镁质黑云母为主,形成于高温(平均687℃)和低压(平均194 MPa)环境,具有富镁(MgO平均11.9%、X_(Mg)=0.56),贫铁(FeOT平均17.0%、I_(Fe)=0.45)、铝(Al_(2)O_(3)平均15.5%)等特征。综合岩相学和前人全岩分析结果表明,越城岭岩体具有S型花岗岩特征,其岩浆可能主要来自上地壳变质泥岩的部分熔融,而都庞岭西体具有I-S过渡型花岗岩的特征,其岩浆可能主要来自壳幔混合源区岩浆岩的部分熔融。越城岭岩体具有变质泥岩源区岩浆岩属性、低氧逸度(log f_(O_(2))=-23.6)、富氟(F平均1.10%、IV(F)=1.62、IV(F/Cl)=6.00,log(f_(H_(2)O)/fHF)^(fluid)=4.78)、高铁指数(XFe=0.58)及高结晶分异程度(log(f_(H_(2)O)/f_(HCl))^(fluid)=5.10,log(fHF/f_(HCl))^(fluid)=0.10)等特征,指示其具有较好的钨及一定的铜成矿潜力;而都庞岭西体具有壳幔混合源区岩浆岩属性、较低的氧逸度(log f_(O_(2))=-17.3)、富氯(Cl平均0.04%,log(fHF/f_(HCl))^(fluid)=-0.36)、中等铁指数(XFe=0.41)及较低的结晶分异度(log(f_(H_(2)O)/f_(HCl))^(fluid)=4.41,log(fHF/f_(HCl))^(fluid)=-0.36)等特征,指示其局部可能有一定的铁成矿潜力。综合分析认为,岩石成因、源区性质以及成岩过程的物化条件是制约越城岭岩体和都庞岭西体不同类型金属成矿能力的重要因素。

The Caledonian Yuechengling and western Dupangling granites,both located in the western Nanling Range of W-Sn-Cu metallogenic belt,but have different mineralizations.The Yuechengling biotite is mainly composed of iron biotite formed at a relatively low crystallization temperature(avg.,531℃)and high pressure(avg.,277 MPa),showing relatively poor features of magnesium(MgO 8.05%,X_(Mg)=0.39),but relatively rich in iron(FeOT 22.4%,I_(Fe)=0.61)and aluminum(Al_(2)O_(3)16.8%),whereas the western Dupangling biotite is primary magnesium biotite formed at a higher crystallization temperature(avg.,687℃)and lower pressure(avg.,194 MPa),showing features of a relatively rich magnesium(MgO 11.9%,X_(Mg)=0.56),but poor in iron(FeOT 17.0%,I_(Fe)=0.45)and aluminum(Al_(2)O_(3)15.5%).Together with petrography observations and the published whole rock data,it indicates that the Yuechengling granites are S-type granite,probably formed via partial melting of metapelites in the upper continental crust contaminated,and the western Dupangling granites are more like IS type granite formed via partial melting of crust-mantle mixed source.Further research indicates that the Yuechengling granites probably show magmatic features in favor of tungsten and a certain copper ore-forming,such as crust-source,lower oxygen fugacity(log f_(O_(2))=23.6),high fluorin(F 1.10%,IV(F)=1.62,IV(F/Cl)=6.00,log(f_(H_(2)O)/fHF)^(fluid)=4.78),high iron coefficient(XFe=0.58)and high magma fractionation(log(f_(H_(2)O)/f_(HCl))^(fluid)=5.10,log(fHF/f_(HCl))^(fluid)=0.10),and the western Dupangling granites probably have characteristics of crustmantle mixed source,high oxygen fugacity(log f_(O_(2))=17.3),high chlorine(Cl 0.04%,log(fHF/f_(HCl))^(fluid)=0.36),moderate iron coefficient(XFe=0.41)and low magmatic fractionation(log(f_(H_(2)O)/f_(HCl))^(fluid)=4.41,log(fHF/f_(HCl))^(fluid)=0.36),which are only conducive for iron ore-forming in small scale.Therefore,petrogenesis and magmatic conditions may be the important factors restricting the metallogenetic ability of different types of metals in Yuechengling and western Dupanglingxi granites.