西昆仑造山带塔什库尔干地区寒武纪条带状铁矿（BIF）成因的地球化学研究

The Taxkorgan area, located in the Western Kunlun orogenic belt, is characterized by abundant magnetite mineralization. These magnetite ore bodies are generally stratiform and parallel the strata which predominantly composed of metamorphic schist and rhyolite. In addition, the magnetites alternate with layers of gangue minerals, which resulted typical structures of banded iron formation (BIF). However, previous studies proposed that the magnetite precipitated during 537-509 Ma as indicated by the zircon U-Pb dating, which makes the magnetite ore deposits in this area are distinct to the Precambrian BIF and belong to unique Phanerozoic age BIF. This proposal is going to choose Ta’axi, and Ziluoyi magnetite deposits as the candidates which remained the original typical banded structures, by using the trace element and Sr-Nd-Pb-Fe isotopic compositions of magnetite as indicators, combining the geochemistry features of the metamorphic rhyolites which alternate with layers of magnetite orebodies, in order to decipher the ore genesis of the magnetite in this area, and further to discuss the implications for the tectonic evolution and palaeoenvironment during the proto-Tethys period provided by this unique Cambrian BIF in this Taxkorgan area. Moreover, this proposal also aims to guide the exploration of magnetite resources in this area by establishing regional metallogenesis, which can promote the development of economy and keep the society stable in this Taxkorgan area as well.

西昆仑造山带塔什库尔干地区以富集磁铁矿为特征，磁铁矿矿体为明显的层状，与地层变质片岩、流纹岩夹层整合接触，且磁铁矿与脉石矿物互层形成典型的条带状构造，表现出明显的条带状铁矿（BIF）特征。然而，与BIF主要形成于前寒武纪不同，已有的锆石U-Pb年代学研究显示，该地区磁铁矿沉积的时间为537~509 Ma，属于独特的显生宙BIF。本课题拟选择该地区条带状构造保存最完好的塔阿西和孜落依两个铁矿床为研究对象，通过对磁铁矿单矿物的微量元素、Sr-Nd-Pb-Fe同位素组成进行分析，结合地层中与铁矿体整合接触的流纹岩夹层的岩石地球化学组成，目的在于对该地区独特的寒武纪BIF的矿床成因进行解析，并探讨该BIF对地球早期环境及原特提斯的构造演化的指示意义。此外，通过建立该地区磁铁矿富集的区域成矿理论，为磁铁矿资源的进一步勘查提供理论指导，有利于当地民计民生的发展和社会稳定，具有重要的经济和社会价值。

条带状铁矿（BIFs）的形成需要还原的海洋环境，因此其主要形成于地球早期海水较还原的太古代、元古代以及新元古代，并在埃迪卡拉纪之后消失。然而，最近在我国西昆仑造山带塔什库尔干地区发现了大量具有类似BIFs条带状构造的磁铁矿矿床，但是前人的研究却显示其形成的年龄可能属于寒武纪，与典型的BIFs存在明显的差异，这导致该地区铁矿的类型和成因尚不明确。本课题基于详细的野外观察和系统的取样工作，对赞坎、塔阿西以及孜落依三个代表性铁床体中与矿体整合接触的流纹岩进行了锆石U-Pb定年工作，并对赞坎铁矿中与磁铁矿共生的黄铁矿进行了原位硫同位素和微量元素分析，以期解决上述问题，并进一步认识BIFs的成因。结果显示，这三个矿床流纹岩的年龄分别为540.2 ± 2.4 Ma、540.2 ± 2.4 Ma至515.2 ± 2.7 Ma以及526.4 ± 2.5 Ma至508.8 ± 3.6 Ma，指示铁矿沉积的年龄为寒武纪，且可以被划分为4个沉积阶段：540 Ma、526 Ma、515 Ma以及508 Ma。此外，赞坎铁矿的黄铁矿具有富集重硫和低Co/Ni比值的特点，属于典型的海水中沉淀的产物，并且该时期海洋的硫酸盐浓度相对较低。结合锆石年龄以及黄铁矿的组成，本研究确定塔什库尔干地区的铁矿属于特殊的寒武纪条带状铁矿，寒武纪早期该区域可能存在局部还原的盆地，并且海水在该区域可能存在分层的现象，即硫化的海水呈楔形嵌在上层的氧化和下层的还原海水之间。其中，赞坎铁矿沉淀的位置浅，靠近该硫化水域，从而在磁铁矿沉淀的时候同时沉淀了大量的黄铁矿和硫酸盐；而塔阿西和孜落依沉淀的位置逐步加深，矿体中硫化物和硫酸盐的缺失指示其沉淀在富集还原铁的深水环境中。本课题的结果暗示，全球范围内可能存在更多的尚未被发现的寒武纪BIFs。