Unique younger phreatic uranium mineralisation hosted by Neogene sediments from Taunsa area, Dera Ghazi Khan, eastern Sulaiman Range, Pakistan: New exploratory targets in deformed geological setting

The Taunsa uranium mineralisation is hosted by the upper Miocene–Pliocene Litra Formation belonging to molasse sediments of the 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 unradioactive. The outcropping sandstone is dominantly whitish grey due to bleaching which is probably related to hydrocarbon migration from the underlying marine black shales. This sandstone-type uranium mineralisation is mainly phreatic due to its parallelism with 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 which makes the Taunsa uranium ore unique. Uranium is trapped mostly by scant organic matter, probably related to petroleum, 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 probably 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 a younger age of the former than that of the latter. The coexistence of the two ore-types, characterised by different intensities of radiometric disequilibrium occurring in the same uranium deposit, suggests that phreatic uranium ore has possibly resulted from the 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.

陶恩萨铀矿化（Taunsa uranium mineralisation）赋存于晚中新世-上新世利特拉组（Litra Formation）中，该地层属于巴基斯坦德拉加齐汗地区苏莱曼山脉东部喜马拉雅前陆盆地（Himalayan foreland basin）内沉积的西瓦利克群（Siwalik Group）磨拉石沉积（molasse sediments）。利特拉组是巴基斯坦地层记录中唯一产出经济品位铀矿点（economic grade uranium occurrences）的岩石单元。地表放射性异常主要与赋矿砂岩的胶结层（即硬层（hard-bands））相关，而弱胶结砂岩则无放射性。露头砂岩因褪色作用（bleaching）整体呈浅灰白色，该褪色作用可能与下伏海相黑色页岩（marine black shales）的烃类运移（hydrocarbon migration）有关。该砂岩型铀矿化（sandstone-type uranium mineralisation）主要为潜流型（phreatic），因其与地下水位平行；而产状协调一致的板状（卷状）矿体（tabular (roll-type) ore）则占次要地位。赋矿砂岩层倾角高达70°至85°，倾向东，指示了变形的地质环境，这也使得陶恩萨铀矿独具特色。铀主要被赋矿砂岩中的少量有机质（可能与石油相关）、铁钛氧化物相（Fe-Ti oxide phases）、层状硅酸盐矿物（叶绿泥石（chlorite）和黑云母（biotite））、石墨片岩以及黑色页岩碎屑（black shale clasts）所捕获富集。铀矿石矿物主要为铀石（coffinite）、沥青铀矿（pitchblende），可能还包括钛铀矿（brannerite）。矿石相中的铀常伴生高含量的钙、铁、钛、硅及铝元素。潜流型矿体的放射性不平衡（radiometric disequilibrium）呈强正值，而板状（卷锋型）矿体的放射性不平衡则呈中度正值，这表明前者的形成时代晚于后者。同一铀矿床中产出两种放射性不平衡强度各异的矿体类型，这一现象表明潜流型铀矿可能是经构造抬升的板状（卷状）矿体发生活化迁移（remobilisation）后，在赋矿砂岩含水层（host sandstone aquifers）中沿地下水位形成的水平氧化还原界面（redox interface）处再沉淀（reprecipitation）而成。