临兴地区太原组下段煤系泥页岩中战略性金属元素镓富集特征及成因分析

鄂尔多斯盆地东缘临兴地区太原组下段煤系泥页岩中镓富集,镓含量平均为24.7 μg/g。泥页岩中镓含量在平面上表现为研究区北部、西南部靠近紫金山岩体,镓含量降低。研究区太原组下段煤系泥页岩中镓主要以类质同象取代铝的形式赋存于高岭石中,碎屑中的碱性长石是镓的另一种重要赋存矿物。通过元素地球化学分析,认为阴山造山带花岗岩系是富镓泥页岩的主要物源。加里东期华北克拉通长期缓慢稳定隆升,为源岩风化形成富镓物源提供了有利的构造环境。风化和沉积条件是泥页岩中镓富集的控制性因素,包括温暖湿润、间歇性干热的气候,淡水为主、缺氧还原的沉积环境。成岩期,煤系有机质热演化释放出大量有机酸,溶蚀碱性长石碎屑并将其中的镓释放,致使镓元素进一步富集。由于煤系中煤层—泥页岩—砂岩组合体系的开放性较差,镓在成岩期的富集不能持续进行。研究区泥页岩中镓的富集过程以陆源富集型为主。成岩改造作用在一定程度上促进镓元素进一步富集。岩体侵位导致了低熔点的镓在温度作用下发生迁移,导致靠近紫金山岩体的镓含量降低。

Enrichment Characteristics and Genetic Analysis of the Strategic Metal Element Gallium in Coal-measure Shale,Lower Taiyuan Formation,Linxing Area

Gallium is enriched in the coal-measure shale of the lower Taiyuan Formation in the Linxing area of the Ordos Basin, with an average content of 24.7 μg/g, but is lower in the northern and southwestern regions of the study area adjacent to the Zijinshan rock mass. The gallium in the coal-measure shale in the study area mainly occurs in kaolinite in the form of isomorphic replacement of aluminum. Alkaline feldspar in the clasts is another significant source of gallium. Elemental geochemical analysis indicates that the granite series of the Yinshan orogenic belt was the main source of gallium-rich shale. The Caledonian North China Craton uplifted slowly and steadily over a long period of time, providing a favorable environment for weathered source rocks to develop gallium-rich provenances. Weathering and depositional conditions are the main conditions for gallium enrichment in shale: a warm, humid climate with intermittent periods of dry, hot climate, together with a predominantly freshwater anoxic reducing sedimentary environment. During diagenesis, the thermal evolution of organic matter in the coal measures released a large amount of organic acid, which dissolved the alkaline feldspar debris and released the gallium contained in it, thus further increasing the gallium content. The tightness of the coal-shale-sandstone system in the coal measures did not allow diagenetic-type enrichment of gallium to continue. The enrichment of gallium in the mud shale in the study area is predominantly terrigenous. Diagenesis and re-formation promoted further gallium enrichment to some extent. The heat generated during emplacement of the rock mass stimulated migration of the low-melting-point gallium, resulting in lower gallium content near the Zijinshan rock mass.