内蒙古西胡里吐盆地下白垩统大磨拐河组沉积相及砂岩型铀矿成矿作用

Sedimentary facies and sandstone-type uranium mineralization of the Damoguaihe Formation of Lower Cretaceous in Xihulitu Basin, Inner Mongolia

西胡里吐盆地是位于内蒙古东部的一个早白垩世断陷盆地，盆地盖层由下白垩统大磨拐河组构成。受同沉积断裂作用的影响，盆地的古地理格局总体为南陡北缓，南部主要发育冲积扇相、扇三角洲相，盆地北部以河流相、三角洲相为主，仅上部层位发育冲积扇相，而盆地中部的广大区域则以巨厚的湖泊相沉积占优势。大磨拐河组可进一步划分为3个岩性段。一段形成于盆地断陷发育的初始阶段，以冲积扇、河流、三角洲发育为特征；二段形成于湖泊迅速扩张至最大时期，沉积了一套巨厚的湖相泥岩，构成盆地盖层的主体，晚期有河流、三角洲等粗碎屑沉积；三段以冲积扇十分发育为特征，代表盆地进入萎缩阶段。3个岩性段总体表现为一段、三段岩性粗、煤层少且薄，二段岩性细、泥岩厚、煤层发育。从一段到三段，基本构成了一次完整的盆地沉积充填史。主要含矿层位大磨拐河组的沉积特点不仅控制着盆地内铀矿化的空间展布，而且也控制着本区的铀矿化类型。“粗—细—粗”的地层结构特点决定了本区的铀成矿作用主要发生于湖泊最大扩张的上部地层，即二段上部及三段的粗碎屑沉积物中，主要矿化层位为该套地层内的冲积扇、辫状河与三角洲相沉积，含矿岩石主要为渗透性好、富含有机质的砂砾岩、砂岩；同时由于上部含矿层位具有“砂多泥少”的特点，决定了本区铀矿化类型主要为具垂直分带的潜水氧化带型，仅局部存在潜水转层间氧化带型。

The Xihulitu Basin, located in the eastern Inner Mongolia, is a downfaulted basin and the Damoguaihe Formation of the Lower Cretaceous constitutes its depositional cover. Influenced by syndepositional fault in the southern margin, the palaeogeographic framework of the basin is characterized by being steep in the southern part and gentle in the northern part. Alluvial fan and fan delta facies are mainly developed in the southern part of the basin while the braided fluvial and delta facies are developed in its northern part, with alluvial fans only developed in its upper strata. However, the very thick lacustrine sediments are dominant in the central part of the basin. Studies on sedimentology show that the Damoguaihe Formation can be further divided into three members. The first member was formed in the beginning of downfaulting period during which alluvial fan, fluvial and delta facies were mainly developed. The second member, which is the main component of the depositional cover, was formed in the extensively enlarging period of the lacustrine basin with very thick lacustrine mudstone deposited, while the fluvial and delta facies were dominant at the end of this period. The third member was characterized by the development of large-scale alluvial fan facies, reflecting that the basin entered the shrinking stage of its evolution. Totally, the lithologic members 1 and 3 are coarsegrained with a few and thin coal beds, while the member 2 is fine-grained with thick mudstone and abundant coal bed. A complete sedimentary filling history is basically composed of the 3 lithologic members. The above characteristics of the Damoguaihe Formation reveal that the sedimentary facies plays an important role in controlling both the distribution and the type of mineralization of the epigenetic sandstone-type uranium deposits in the region. The stratigraphic structure of “coarse - fine - coarse” from bottom to top of the cover determines that the uranium mineralization mainly occurs in the upper part of depositional cover, that is, in the coarse detrital sediments in the upper part of the second member and in the third member. Main ore-bearing beds are in the alluvial fan, braided fluvial and delta facies of the second and third members, and the U-bearing host rocks are mainly the glutinites and sandstones of granule-supported massive and crossbedded lithofacies, which are abundant with organic matter and good in infiltration. At the same time, due to lack of the mudstone layer, which usually acts as groundwater confining bed, the uranium mineralization in the region is typically a kind of phreatic oxidized mineralization with vertical zoning, while phreatic-interlayered oxidized mineralization is only developed locally.