Sedimentary evolution of the Longchuanjiang basin and favorable ore-bearing layers for sandstone-type uranium deposits, Tuantian, Yunnan
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摘要: 在构造-沉积-成矿背景、铀源地质体配置研究的基础上，选择滇西腾冲地块龙川江盆地南部团田地区老矿床外围部署验证工程，获得了重要找矿发现。本文以第一手钻探资料为素材，通过分析上新世龙川江盆地演化过程，建立含矿层系芒棒组的沉积体系和圈定有利赋矿微相组合，重建赋矿古冲积扇体的岩相古地理格局，为该地区和同类盆地砂岩型铀矿勘查提供科学依据。芒棒组沉积期，龙川江盆地演化经历了断陷成盆期、火山沉积期及盆地回返期3个阶段，其中断陷成盆期沉积了砂岩型铀矿主力发育层段-芒棒组下段，此时未形成具规模的湖盆，具备汇水盆地的古地理模式。盆地东、南部分布多个潜在赋矿的古冲积扇体，本文的研究重点团田地矿区位于旱坝寨扇体。团田矿区芒棒组下段识别出3种沉积相和6种亚相，根据其沉积演化序列显示，在冲积扇扇中/扇端沼泽的纵向序列转换面附近，形成“沼泽相炭质泥岩层-煤层/扇中砾质辫状河道”的有利控矿微相组合。分析团田矿区主矿层、矿层顶板层沉积期底古地理格局可发现，在扇中-扇端沉积转换期的湖泛（相对水位上升）控制下，受晚期扇端（沼泽）覆盖的早期扇中沉积相区是最有利的赋矿相区和部位。基于上述认识，提出砂岩型铀矿勘查应加强沉积微相和岩相古地理研究，建立赋矿有利相带的空间分布模式；结合铀源和成矿作用类型的分析，可更准确地定位潜在的铀矿体。Abstract: 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.
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