Abstract

The Taunsa uranium mineralisation is hosted by the upper Miocene–Pliocene Litra Formation, part of the molasse sediments (Siwalik Group) deposited in the Himalayan foreland basin of the eastern Sulaiman Range, Dera Ghazi Khan, Pakistan. The Litra Formation is the only rock unit hosting economic-grade uranium occurrences in the stratigraphic record of Pakistan. Surface radioactivity is mainly associated with cemented layers of the host sandstone, known as hard-bands, whereas weakly cemented sandstone is non-radioactive. The outcropping sandstone is dominantly whitish grey owing to bleaching, which is probably related to hydrocarbon migration from the underlying marine black shales. The main sandstone-type uranium mineralisation is phreatic owing to its parallelism with the water-table, whereas the concordantly oriented tabular (roll-type) ore is subordinate. The host sandstone layers have high dips ranging from 70 to 85°E indicating a deformed geological setting that makes the Taunsa uranium ore unique. Uranium is trapped mostly by scant organic matter, probably related to hydrocarbons, and also by Fe–Ti-oxide phases, phyllosilicates (chlorite and biotite), graphitic schist and black shale clasts in the host sandstone. Uranium ore minerals are mainly coffinite, pitchblende and minor brannerite. Uranium in the ore phases is accompanied by the high contents of Ca, Fe, Ti, Si and Al. Radiometric disequilibrium in the phreatic ore is strongly positive, whereas that in the tabular (roll-front) is moderately positive, which in turn suggests the former is younger than the latter. The coexistence of the two ore-types, characterised by different intensities of radiometric disequilibrium within the same uranium deposit, suggests that the phreatic uranium ore has possibly resulted from remobilisation of the tectonically uplifted tabular (roll-type) ore and its reprecipitation at the horizontally oriented redox interface following the water-table in the host sandstone aquifers.

1. KEY POINTS

2. The Taunsa uranium resource is dominantly phreatic that has resulted from oxidation and remobilisation of the earlier tabular (roll-type) mineralisation.

3. The phreatic uranium mineralisation is very young, which is indicated by low radioactivity and high positive disequilibrium in comparison with that within the tabular (roll-type) ore.

4. Uranium phases are mainly pitchblende, coffinite and brannerite, associated with organic matter of hydrocarbon origin, with phyllosilicates and Fe–Ti-oxide phases.

5. Outcrop of the host sandstone in the Taunsa area is pervasively bleached and is marked by flat topography.

摘要

Taunsa 铀矿化位于上中新世-上新世 Litra 组，该组是沉积在巴基斯坦德拉加齐汗东部苏莱曼山脉的喜马拉雅前陆盆地的磨拉石沉积物（Siwalik 组）的一部分。Litra 组是巴基斯坦地层记录中唯一拥有经济级铀矿的岩石单元。表面放射性主要与主体砂岩的胶结层有关，称为硬带，而弱胶结砂岩是非放射性的。由于白化作用，出露砂岩主要呈灰白色，这可能与下伏海相黑色页岩的碳氢化合物迁移有关。由于与地下水位平行，主要的砂岩型铀矿化是潜水的，而一致定向的板状（辊型）矿石则是次要的。主砂岩层具有 70 至 85°E 的高倾角，表明变形的地质环境使陶恩萨铀矿独一无二。铀主要被少量有机物捕获，可能与碳氢化合物有关，也被铁钛氧化物相、页硅酸盐（绿泥石和黑云母）、石墨片岩和寄主砂岩中的黑色页岩碎屑捕获。铀矿矿物主要为铜镍矿、沥青闪石和次要的镁铝榴石。矿相中的铀伴随着高含量的钙、铁、钛、硅和铝。潜水矿中的放射性不平衡是强正的，而表格（滚动前沿）中的不平衡是中度正的，这反过来表明前者比后者年轻。两种矿石类型并存，

1. 关键点

2. 陶恩萨铀资源主要是水下的，这是由于早期的板状（卷状）矿化作用的氧化和再活化所致。

3. 与板状（卷状）矿相比，潜水铀矿化年龄很小，放射性低，正不平衡高。

4. 铀相主要是沥青闪石、铜锌矿和镁铝榴石，与烃类有机物伴生，具有页硅酸盐和铁钛氧化物相。

5. Taunsa 地区主砂岩的露头普遍白化，并以平坦的地形为标志。