湖北铜绿山矿床石英闪长岩的矿物学及Sr-Nd-Pb同位素特征

Mineralogy and Sr-Nd-Pb isotopic compositions of quartz diorite in Tonglushan deposit,Hubei Province

铜绿山是长江中下游鄂东南矿集区最重要的、大型夕卡岩型Cu-Fe(Au)矿床。本文对该矿区中与成矿密切的石英闪长岩进行了详细的矿物成分、地球化学及Sr-Nd-Pb同位素研究。结果表明:岩石中斜长石主要为更长石(An=21～31);角闪石贫Ti(<0.2),高Mg/(Mg+Fe)(>0.5),属于富镁角闪石;而黑云母为镁质黑云母。岩石的地球化学具有高硅(58.86%～67.71%),富碱(Na2O+K2O=5.67%～9.63%),富集轻稀土元素(LREE)和大离子亲石元素(LILE),并强亏损元素Nb、Ta、Ti等特征。岩石的(87Sr/86Sr)i为0.7055～0.7069,εNd(t)为-7.65～-3.44;(206Pb/204Pb)i=17.66～18.00,(207Pb/204Pb)i=15.49～15.56,(208Pb/204Pb)i=37.73～38.19。矿物成分、地球化学和同位素特征说明,铜绿山岩体与阳新岩体为同源岩浆的产物,源区为深度大于40km的富集地幔,经下地壳的混染及分离结晶作用形成。岩浆熔体形成的温度应大于889℃。角闪石和黑云母的温度计估算岩浆结晶温度分别为650～800℃和500～630℃,黑云母开始结晶温度略低于角闪石结晶结束温度,压力为1.49kbar,对应侵位深度约4.9km。岩浆具有利于Cu、Fe、Au等成矿元素进入熔体的条件,可能与板块俯冲作用相关。

Tonglushan skarn Cu-Fe (Au) deposit is located in the westernmost Daye district of MYLRB. A geochemical and Sr-Nd-Pb isotopic studies were carried out for the Tonglushan quartz diorite,which is closely related to the deposit. The compositions of plagioclases show they are oligoclase (An=21~31). Amphiboles belong to magnesihornblend,have characteristics of low Ti (<0.2) and high Mg/(Mg+Fe) (>0.5). MF values of biotites vary from 0.58 to 0.66,suggesting biotites are Mg-riched. The rocks are characterized by SiO2 and alkaline compositions between 58.86% and 67.71%,and from 5.67% to 9.63%,respectively,enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE),but relative depletion in Nb、Ta、Ti elements. They have initial Nd (εNd(t)=-7.65 ^-3.44),Sr ((87Sr/86Sr)i=0.7055~0.7069) and Pb ((206Pb/204Pb)i=17.66~18.00,(207Pb/204Pb)i=15.49~15.56,(208Pb/204Pb)i=37.73~38.19). Comparative analysis demonstrates that Tonglushan and Yangxin intrusions originated from homologous magma. The magma was from the enrich mantle of about 40km deep and at about 889℃ temperature. Amphiboles and biotite thermometers estimate magma crystallized at 650~800℃ and 500~630℃ temperatures,under 1.49kbar pressure,corresponds to the emplacement depth of about 4.9km. Magma has a high oxygen fugacity (fo2) condition,in favor of Cu,Fe and Au ore-forming elements into melt,may be related to plate subduction.