新疆西天山式可布台铁矿地质、矿物化学和S同位素特征及其对矿床成因的约束

摘要

新疆西天山式可布台铁矿发育于伊犁裂谷内，赋存于上石炭统中酸性火山碎屑岩、浅变质片岩、千枚岩中，矿体呈层状、似层状以及透镜状顺层产出。金属矿物以赤铁矿、镜铁矿为主，含少量黄铁矿、菱铁矿；脉石矿物主要为碧玉、重晶石、石英以及少量方解石。矿石构造以条带状、纹层状和块状为主，矿物结构多为隐晶质结构、半自形结构以及充填结构。矿床分为4个成矿阶段，即黄铁矿−赤铁矿−铁碧玉−重晶石阶段、菱铁矿−软锰矿阶段、石英−镜铁矿阶段、氧化物阶段。矿体顺层产出和发育纹层状矿石构造指示矿床为沉积成因。电子探针分析显示：（1）块状赤铁矿Al2O3、Na2O、MgO、SiO2含量相对分散，推测这可能与块状矿石快速沉淀结晶有关，暗示了剧烈的流体喷流活动，而纹层状和条带状赤铁矿Al2O3、Na2O、MgO、SiO2含量相对集中则反映平静的沉积环境以及微弱的喷流活动，两者的比较可能暗示了成矿过程中流体喷溢速率以及沉积环境都不断改变；（2）黄铁矿中含有较高的Co、Ni比，显示其形成与火山作用关系密切；（3）菱铁矿的FeOT与MnO+MgO含量呈负相关关系，并形成两个聚集区，与镜下其具有不同特征相吻合，可能暗示了成矿后期菱铁矿随热液析出时候发生了分异作用。黄铁矿（δ34S=-6.1‰~6.5‰）和重晶石（δ34S=12.9‰）硫同位素组成显示曾发生过硫酸盐和硫化物之间的硫同位素分馏作用，成矿热液的硫可能来源于岩浆硫。综合分析认为，式可布台铁矿可能为海相火山喷流沉积型铁矿床。%The Shikebutai iron deposit is hosted in Upper Carboniferous intermediate-acidic volcaniclastic rocks, low-grade metamorphic schist and phyllite in the Yili rifting of West Tianshan Mountains. Stratiform, stratoid and phacoidal orebodies are distributed along the strata. Ore minerals are predominantly hematite and specularite, with small amounts of pyrite and siderite, whereas gangue minerals are dominated by jasper, barite and quartz, with a small quantity of calcite. The ores display banded, lamellar and massive structures. Ore minerals usually show cryptocrystalline, filling and subhedral textures. The ore-forming process of the Shikebutai iron deposit can be divided into four stages, i.e., pyrite-hematite-jasper-barite stage, siderite-pyrolusite stage, quartz-specularite stage and oxide minerals stage. Geological characteristics of lamellar structure and the distribution along the strata indicate that the formation of the Shikebutai iron deposit was related to sedimentation. Electron microprobe analyses show that hematite from massive ore has variable Al2O3, Na2O, Mgo and SiO2 values, which suggests that massive hematite ores are products of rapid sedimentation and crystallization, and also implies the rapid extravasation of iron-rich fluids. Nevertheless, hematite from lamellar ore and banded ore have concentrated Al2O3, Na2O, Mgo and SiO2 values, suggesting that they formed in calm sedimentary environment with slow extravasation of iron-rich fluids. In general, the extravasation rate of mineral-rich fluid and the sedimentary environment changed continuously during the metallogenic process. The high Co-Ni ratio of pyrite shows that it was derived from volcanism. A negative correlativity between FeOT and MnO+MgO is found in siderite which can be divided into two groups corresponding respectively to two different types of siderite under microscope. These phenomena imply that siderite experienced a process of differentiation at the late metallogenic stage. Theδ34S values of pyrite range from-6.1‰ to 6.5‰and the value of barite is 12.9‰, which indicates that the sulfur was derived from magma, and it was subjected to fractionation between sulfide and sulfate. In summary, the Shikebutai iron deposit belongs to submarine volcanic exhalative-sedimentary high-grade iron deposit.