鄂尔多斯盆地西北部华池-环河组绿色砂岩成因探讨

Genesis of Green Sandstone From Huachi-Huanhe Formation in Northwestern Ordos Basin

鄂尔多斯盆地西北部华池-环河组赋矿层中存在大套的绿色砂岩,这套绿色砂岩致绿因素至今尚不清楚。文章在野外观察的基础上,利用测色仪对不同颜色的砂岩进行对比研究,基于CIE1976 LAB色度空间,通过a值的差异将华池-环河组砂岩分为灰色砂岩与绿色砂岩,利用高倍显微镜、X射线衍射、扫描电镜等,分析赋矿砂岩的矿物组成,最后结合主微量元素、全岩S和有机碳含量,综合分析了绿色砂岩致绿成因。研究表明,绿色砂岩的主要致绿矿物是绿泥石和绿/蒙混层,其中绿泥石的成因主要是砂岩中的黑云母、角闪石等含铁矿物中的铁离子与镁离子不断被流体萃取出来,致使砂岩中的蒙脱石经由绿/蒙混层向绿泥石转换。

Large sets green sandstones occur in the Huachi-Huanhe Formation in northwestern Ordos Basin,but the key factors result in the green are not clear. In this paper, based on field observation, colorimeter was used to identify sandstone with different colors. Based on CIE1976 LAB color space, sandstones in the Huachi-Huanhe Formation were objectively divided into gray sandstone and green sandstone by the value of “a”, so as to further compare and study the green factors of green sandstone. Then, the characteristics of rocks and minerals were analyzed by the high-power microscope, X-ray diffraction, scanning electron microscope.Finally, combined with the analysis of major and trace elements, whole-rock S and organic carbon, the genesis of green sandstone was comprehensively analyzed. This study showed that chlorite and chlorite/montmorillonite mixture are the main green minerals for the alternation of green sandstone. The origin of chlorite is due to the continuous extraction of iron and magnesium ions from Fe-bearing minerals, such as biotite, amphibole etc. in sandstone by fluid, which leads to the transition from montmorillonite clay to chlorite through chlorite/montmorillonite.