安徽茶亭铜金矿床赋矿石英闪长玢岩黑云母成分特征及成岩成矿指示意义

Compositional characteristics, petrogenesis and metallogenic significance of biotite from the ore-bearing quartz diorite porphyry in Chating Cu-Au deposit, Anhui Province

安徽宣城茶亭矿床位于长江中下游成矿带东南缘的南陵-宣城盆地内,是近年来该盆地内新发现的大型斑岩型Cu-Au矿床。石英闪长玢岩是茶亭矿床主要的赋矿岩石,文章从石英闪长玢岩的造岩矿物黑云母入手,利用EMPA及LA-ICP-MS等原位微区分析方法,对黑云母进行了详细的矿物化学分析,旨在查明赋矿石英闪长玢岩形成的物理化学条件,并探讨其成岩成矿意义。分析结果表明:茶亭赋矿石英闪长玢岩中黑云母具有高w(TiO_2)、低w(Al_2O_3)的特征,整体富Mg、贫Fe。黑云母温压计算结果表明,在不同深度上,石英闪长玢岩具有相似的结晶温度及压力,其结晶温度为746～773℃,压力为44～85 MPa,对应深度为1.7～3.3 km,平均为2.47 km,显示石英闪长玢岩侵位深度相对较浅。黑云母中高的氧化系数(0.613～0.864)、高Mg#(0.557～0.864),以及w(MgO)介于14.73%～15.86%,均显示石英闪长玢岩为壳幔混源,并且以幔源为主。黑云母成分显示石英闪长玢岩形成于较高氧逸度条件下。浅部岩体具有较高的X_(Mg)值,表明浅部岩体氧逸度较高,可能指示随着岩浆的不断演化,残余岩浆的氧逸度不断降低。同时,石英闪长玢岩黑云母相对一般斑岩型铜矿具有较高的w(Cu),并且高Cl、低F。这些特征均有利于茶亭Cu-Au矿床的形成。

The Chating Cu-Au deposit is located in the Nanling-Xuancheng basin,southeast of the Middle Lower Yangtze River Metallogenic belt,Anhui Province.It is the biggest Cu-Au deposit in this area.In this study,Electron Microprobe Analysis(EMPA)and Laser Ablation Inductively-Coupled Plasma Mass Spectrometry(LA-ICP-MS)techniques were used to investigate the major and minor element compositions of magmatic biotites,so as to indicate the physicochemical conditions of the ore-bearing quartz diorite porphyry.Biotites from quartz diorite porphyry have high w(TiO2)and low w(Al2O3)content.Biotite geothermobarometry shows that the calculated temperature and pressure of biotite are746~773℃and44~85MPa,corresponding to the depth of1.7~3.7km,2.47km on average.High oxidation coefficient(0.613~0.864),high Mg^#(0.557~0.864)and a variable range of w(MgO)content between14.73%~15.86%indicate that the quartz diorite porphyry was generated by the mixing of crustderived and mantle-derived magma.Biotites compositions indicate a high f(O2)value for the quartz diorite porphyry.The increasing trend of the XMg from deep to shallow part indicate a decreasing f(O2)Value with the evolution of the ore-bearing quartz diorite porphyry.Compared with typical porphyry deposits,biotites in Chating have higher w(Cu),w(Cl)and lower w(F)content which played an important role in the copper mineralization.