鄂尔多斯盆地白土井地区环河组灰绿色砂岩成因分析OA

Huanhe Formation of Lower Cretaceous contains thick gray-green sandstones in Baitujing area of Ordos Basin.Studying their genesis is significant for identifying redox zoning and clarifying exploration directions for sandstone-type uranium deposits.This study used macroscopic core observations and scanning electron microscopy analyses to determine the causes of the gray-green coloration in the sandstones of the study area.The results show that the primary chromogenic mineral giving the sandstone its gray-green color is the chlorite,followed by chlorite-smectite mixed layers.Chlorite occurs in three forms:grain coatings,pore linings,and pore fillings,representing different formation stages in the sequence of grain coatings → pore linings → pore fillings.The green color in the sandstone is uniformly and continuously distributed without features of later alteration such as"red core with green edges,"and the edges of purplish-red mud clasts also show no signs of green alteration.The three forms of chlorite closely envelop detrital grains and develop around intergranular pores,with no evidence of iron oxide replacement.Based on sedimentary environment analysis,it was believed that this set of gray-green sandstones was formed in a weakly reducing underwater environment at the delta front,representing a primary sedimentary origin.Therefore,the gray-green sandstones in the study area collectively represent a original reducing zone,and the contact boundary on their western side with red or yellow sandstones can serve as the oxidation front.This recognition has practical significance for guiding uranium exploration in the region.