Sedimentary evolution of the Longchuanjiang basin and favorable ore-bearing layers for sandstone-type uranium deposits, Tuantian, Yunnan

In this paper, based on the first-hand drilling data, the evolution process of the Upper Pleistocene Longchuanjiang basin in western Yunan has been analyzed, the sedimentary system of the uranium -bearing Mangbang Formation in the basin has been established, and the microfacies combinations which are favorable for sandstone-type uranium deposits have been determined. The evolution of Longchuanjiang basin has gone through three stages, which are the basin-forming fault-depression period, the volcanic-sedimentary period and the recurved basin period, respectively. The main uranium-bearing layers are the lower number of the Mangbang Formation, which are alluvial fans deposited during the basin-forming fault-depression period. In the east and south of the Longchuanjiang basin, there are several palaeo-alluvium fan bodies which are potential for uranium deposits. Three sedimentary facies and six sedimentary microfacies have been identified in the lower number of the Mangbang Formation. The favorable ore-bearing micro-facies are paludal carbonaceous mudstone layers, coal seams, and gravelly braided channel of fans, which were formed near the vertical conversional surfaces of alluvial fans and fan-end swamps. As a matter of fact, the Tuantian deposit is located at the Hanbazai fan. By analyzing the paleogeographic pattern and the roof layers of main ore seams in Tuantian mining area, it is found that the middle parts of earlier fans covered by following fanheads (swamp) are the most favorable ore-bearing sites. It is confirmed that the study of sedimentary facies and paleogeography is very helpful to locating potential uranium orebodies precisely in the region.