新疆萨尔托海糜棱岩化石英菱镁岩元素地球化学特征及其对金成矿作用的贡献

新疆萨尔托海石英菱镁岩产在达拉布特蛇绿混杂岩带中,是蛇纹岩在剪切带深部发生热液交代作用的产物。蛇纹岩先转变为滑石片岩,再进一步转变为石英菱镁岩。石英菱镁岩局部发生剪切变形,形成糜棱岩化石英菱镁岩。与蛇纹岩相比,石英菱镁岩的MgO和SiO_2含量降低,Al_2O_3和CaO含量升高;而糜棱岩化石英菱镁岩的MgO含量较石英菱镁岩降低,Al_2O_3和SiO_2含量较石英菱镁岩升高。微量元素地球化学对比研究表明石英菱镁岩继承了蛇纹岩的微量元素特征;而糜棱岩化石英菱镁岩的微量元素含量较蛇纹岩发生了显著变化,稀土元素、高场强元素和自然金的含量明显升高,指示这些元素在剪切变形过程中发生迁移富集。剪切变形伴随强烈的流体/岩石反应,并导致剪切带流体组成和物理化学性质发生改变,从而影响矿物结晶或分解,并控制微量元素的带入迁出。蛇纹岩转变为石英菱镁岩的过程释放Au,可为糜棱岩化石英菱镁岩中的金矿化提供成矿元素。

Element geochemical characteristics of mylonitized listwaenite and its contribution to gold mineralization in Sartohay, Xinjiang

Listwaenites, located within the Darbut ophiolitic mélange belt in the Sartohay region, Xinjiang, are the product of metasomatism from serpentinite in deep levels of shear zones. Serpentinite was transformed into talc schist followed by listwaenite. Listwaenite lenses within the shear zone were deformed as mylonitized listwaenite along rims. Listwaenite is characterized by lower contents of MgO and SiO₂, but higher contents of Al₂O₃ and CaO compared with serpentinite. However, contents of SiO₂ and Al₂O₃ in mylonitized listwaenite are higher than those in listwaenite, whereas MgO is lower than that in listwaenite. Listwaenite has similar characteristics on trace elements as serpentinite, which could be inherited from serpentinite, whereas mylonitized listwaenite is characterized by distinctly different behavior of trace elements from serpentinite. Rare earth elements, high field strength elements and native gold apparently increase in mylonitized listwaenite, implying that these trace elements were concentrated during shearing deformation. Shearing deformation will cause high fluid/rock ratio and facilitate fluid-rock reaction, corresponding to variations on the composition of fluids and physic-chemical conditions in shear zones, accordingly control the decomposition or crystallization of minerals and the mobility of trace elements. Native gold released from serpentinite could be the source for gold mineralization in mylonitized listwaenite.