内蒙古东胜地区中侏罗统砂岩沉积物源的地球化学证据

基于显微薄片观察、稀土和微量元素分析探讨了鄂尔多斯盆地北部东胜地区中侏罗统直罗组砂岩的物源和铀矿化的地球化学特征。砂岩样品含有多种的变质岩岩屑应来自盆地北部的变质岩,那里的花岗片麻岩和斜长角闪岩分析显示:花岗片麻岩LREE/HREE为19.02,斜长角闪岩LREE/HREE为4.47。这两类母岩LREE/HREE比值分别接近于砂岩最高和最低LREE/HREE比值,其稀土配分曲线模式与砂岩LREE/HREE比值端元岩样可以对比,说明砂岩的母岩主要来自这两类变质岩。花岗片麻岩Th/U高,表明发生铀丢失,可作为本区铀成矿的重要铀源。有意义的是,铀含量高的样品,其LREE/HREE比值高,铀含量与亲硫的微量元素Pb、Zn和Mo具有相关关系,这些特征可作为铀矿化的重要标志。

GEOCHEMICAL EVIDENCE FOR SEDIMENTARY PROVENANCE OF MIDDLE JURASSIC SANDSTONES IN DONGSHENG AREA, INNER MONGOLIA

Sandstone-hosted roll-type uranium deposits have recently been found in the Middle Jurassic Zhiluo Formation in Dongsheng area, northern part of the Ordos Basin. In order to assess the sandstone provenance and geochemical characteristics of the uranium mineralization, the host sandstones were analyzed for rare earth elements(REE) and trace elements. The sandstone samples contained different kinds of metamorphic rock fragments. Meanwhile, the granitic gneiss and amphibolite sampled from outcrops north to the Ordos Basin were analyzed for REE. The results showed that ratios of LREE/HREE were 19.02 for the granitic gneiss and 4.47 for the amphibolite, respectively. The LREE/HREE ratios were close to the highest and the lowest LREE/HREE ratios of the sandstones with well-compared chondrite-normalized REE patterns, respectively. So these results consistently indicated that parent rocks of sandstones were mainly contributed from these two kinds of metamorphic rocks. There was a very high Th/U ratio for the granitic gneiss, which was a mainly potential uranium resource. Interestingly, the uranium contents of the sandstones were positively correlated to LREE/HREE ratios and contents of sulphophile element, such as Pb, Zn and Mo. These characteristics were important marks of uranium mineralization.