新疆北山小常山铁矿矿物电子探针分析及矿床成因探讨

小常山铁矿位于新疆北山裂谷带西段,矿体赋存于辉长岩、辉长岩和大理岩接触带中,部分赋存于花岗闪长岩和大理岩接触带。前人研究辉长岩年龄为276±1.2Ma,与坡北铜镍硫化物岩浆矿床形成年龄一致,同属早二叠世。小常山铁矿中可见有明显的岩浆贯入现象。矿体主要呈透镜状、块状、脉状、薄层状。金属矿物主要为磁铁矿,含极少量的褐铁矿和黄铁矿;近矿围岩蚀变较弱,主要有石榴子石化、绿帘石化、大理岩化。电子探针研究表明,磁铁矿FeOT含量范围较大,主要分布在85%范围以上,Al_2O_3含量相对较高,TiO_2-Al_2O_3-(MgO+MnO)图解、TiO_2-Al_2O_3-MgO图解均显示热液接触交代成因特点,表明小常山铁矿的形成和岩浆热液的交代作用有关。石榴子石属于钙铝榴石-钙铁榴石系列,与典型矽卡岩矿床的石榴子石端员组分组成有一定差别。结合小常山铁矿体地质特征以及电子探针分析测试等研究,认为小常山铁矿是多成因的复合型矿床,具有岩浆成因和热液成因特征,但后者是主要成因。

Electron microprobe analysis of minerals and genesis of the Xiaochangshan iron deposit in Beishan area, Xinjiang

Located in the west of the Beishan metallogenic belt in Xinjiang, the Xiaochangshan iron deposit is hosted in gabbro and the contact zone of the gabbro and marble. Parts of the orebodies are hosted in the contact zone of the granodiorite and the marble. Previous researchers hold that the age of gabbro in the mining area is (276±1.2) Ma, similar to the age of gabbro in the Pobei copper-nickel-sulfide orebodies, so they both belong to Early Permian. The magma injection is an obvious phenomenon in the Xiaochangshan iron deposit. The orebodies occur as lenses, lumps, veins and thin layers. Metal minerals are mainly magnetite, with very little limonite and pyrite. The country rock alteration in the Xiaochangshan iron deposit is weak, comprising mainly garnetization, epidotization and marbleization. The electron microprobe analyses of magnetite show that the content of FeO_T in different iron ores has a large distribution range, and the content of Al₂O₃ is relatively higher. The ternary plot of TiO₂-Al₂O₃-(MgO+MnO) and TiO₂-Al₂O₃-MgO suggest that the formation of the Xiaochangshan iron deposit was associated with hydrothermal metasomatism of magmatic-hydrothermal fluid. The electron microprobe analyses of garnet show that the garnets from the Xiaochangshan iron deposit belong to andradite-grossular solid solution series, somewhat different fromthings of the typical skarn deposits. Combining the geological characteristics of the Xiaochangshan iron deposit with the study of the electron microprobe analyses, the authors believe that the Xiaochangshan iron deposit belongs to the polygenetic and compound deposit with magnetic and hydrothermal genesis, with the latter factor probably playing the major role.