湘南宝山矿床花岗闪长斑岩中黑云母的矿物学特征及其指示意义

Mineral chemistry of biotites in Baoshan granodiorite_porphyry, southern Hunan Province: Implications for petrogenesis and mineralization

湖南宝山矿床处于坪宝矿带的北端,是湘南地区最大的铜多金属矿床。为了进一步探讨矿区内花岗质岩石的形成条件及成矿潜力,文章在详细的野外地质和岩相学观察的基础上,对与成矿密切相关的花岗闪长斑岩中的黑云母进行了详细的矿物化学分析。电子探针分析结果表明:宝山花岗闪长斑岩中的黑云母为铁质黑云母和镁质黑云母,其中,Ti介于0.18~0.30,且Mg/(Mg+Fe2+)值介于0.42~0.58,属于典型的岩浆成因黑云母;黑云母的氧化系数为0.16~0.26,w(Mg O)为8.17%~11.72%,平均9.3%,MF值范围为0.38~0.50,指示其岩体属于壳幔混源型的I型花岗岩;岩体中以黑云母的全铝含量计算的结晶压力为97~174 MPa,相应的结晶深度为3.67~6.57 km,平均深度为5.12 km。其log f(O2)变化范围为-14.5^-12.8,表明黑云母是在较高氧逸度条件下结晶形成的,有利于铜矿的形成。

Located in the north of the famous Huangshaping-Baoshan ore belt,the Baoshan Cu-Mo-Pb-Zn-Ag deposit associated genetically with granodiorite porphyry is the largest Cu deposit in southern Hunan Province.In order to further discuss the emplacement physical-chemical conditions of the granodiorite porphyry and its metallogenic potential,the authors investigated the petrographic and mineral chemistry of the biotites in Baoshan granodiorite porphyry.Electron microprobe analysis （EPMA） shows that the biotites belong to Fe-biotites and Mg-bioties.The Ti contents and the Mg/（Mg^ ＋ Fe^2 ＋） ratios of the biotites from the Baoshan granodiorite porphyry are 0.18 ～0.3 and 0.42～0.58,respectively,which are well consistent with those of magmatic biotites.The coefficients of oxidation from biotites are 0.16～0.26.The values of MF are 0.38～0.5 and the contents of MgO are 8.17%～ 11.72%.These characteristics suggest that the Baoshan granodiorite porphyry belongs to Ⅰ-type granite with a mantle-crust mixed source.The solidification pressures （P） of the biotites which are positively related to the content of TAl are 97～174 MPa,corresponding to solidification depth （H） of 3.67～6.57 km.The oxygen fugacity （log f（O2）,of the magma is-14.5～-12.8.These crystallization conditions indicate that the Baoshan granodiorite porphyry was formed from relatively high oxidized magma,which was favorable for copper mineralization.