Geology, mineral chemistry and sulfur isotope geochemistry of the Shikebutai iron deposit in West Tianshan Mountains, Xinjiang: Constraints on genesis of the deposit

Abstract: 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. The δ34S 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.