鄂尔多斯盆地东胜地区砂岩型铀矿床沉积微相与层序地层

SEDIMENTARY MICROFACIES AND SEQUENCE STRATIGRAPHY OF SANDSTONE-TYPE URANIUM DEPOSIT IN THE DONGSHENG AREA OF THE ORDOS BASIN

鄂尔多斯盆地东胜地区侏罗系直罗组砂岩型铀矿是在中国克拉通盆地中首次发现的大型铀矿。通过野外调研和多种室内分析手段,对该铀矿的含矿建造进行了岩石学、沉积学、层序地层学及其相互关系研究,认为直罗组沉积环境经历了辫状河(辫状河三角洲)向曲流河的转化,沉积相由辫状河向辫状河三角洲平原过渡的地区有利于铀矿的沉淀富集,层序地层学格架由1个长期基准面旋回、6个中期旋回和17个短期旋回层序构成,基准面升降旋回中形成的含矿建造具良好的地层结构。基准面上升早期形成的辫状河河道砂体岩性主要为浅灰、灰白色粗—中粒长石岩屑砂岩、岩屑长石砂岩,具连通性好、粒度粗、碎屑含量高、杂基含量低及孔渗条件好等特点,是重要的含矿层。

Sandstone-type uranium deposit in the Jurassic Zhiluo Formation in the Dongsheng area of the Ordos Basin is the first discovered giant uranium deposit in a cratonic basin in China. The petrology, sedimentology, sequence stratigraphy and their relationships with uranium deposits were investigated by synthesizing field and experimental data. Our data show that sedimentary facies of the Zhiluo Formation in the Dongsheng area underwent a transition from a braided stream system （braided deltas） to a meandering stream system. The transition from braided streams to braided delta plains was favorable to uranium deposition. The sequence of the Zhiluo Formation consists of one long-term base-level cycle, 6 middle-term base-level cycles and 17 short-term base-level cycles. Fluctuation of base-level cycles resulted in an ore-hosting stratal architecture. Lithology of braided channel sand bodies deposited at the beginning of baselevel rises is light-gray to gray, coarse to medium feldspathic litharenite and lithic arkose. Such sand bodies have good porosity, permeability, coarse granularity, high debris content, and low matrix content, which were most important for the formation of uranium ores.