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，该组隶属于西瓦利克群（Siwalik Group）的磨拉石沉积（molasse sediments），沉积于巴基斯坦德拉加齐汗地区苏莱曼山脉东部的喜马拉雅前陆盆地内。利特拉组是巴基斯坦地层记录中唯一产出经济品位铀矿化的岩石单元。地表放射性异常主要赋存于容矿砂岩的胶结层（即硬层，hard-bands）中，而弱胶结砂岩则无放射性。露头砂岩因褪色作用整体呈浅灰白色，该褪色过程可能与下伏海相黑色页岩的烃类运移有关。该砂岩型铀矿化主要为潜水型（phreatic），因其与潜水面平行，顺层产出的板状（卷状型，roll-type）矿石仅占次要地位。容矿砂岩层的倾角极高，为70°至85°（东倾），表明其处于变形地质环境中，这也使得陶恩萨铀矿独具特色。铀主要被容矿砂岩中少量有机质（可能与石油相关）捕获，同时也被铁钛氧化物相、页硅酸盐（phyllosilicates，包括绿泥石chlorite与黑云母biotite）、石墨片岩（graphitic schist）及黑色页岩碎屑捕获。铀矿石矿物主要为铀石（coffinite）、沥青铀矿（pitchblende），可能还含有钛铀矿（brannerite）。矿石相中的铀伴生有高含量的钙（Ca）、铁（Fe）、钛（Ti）、硅（Si）及铝（Al）。潜水型矿石的放射性不平衡呈强正值，而板状（卷状前锋型，roll-front）矿石则呈中等正值，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石类型，这表明潜水型铀矿化可能是经构造抬升的板状（卷状型）矿石发生活化迁移后，在容矿砂岩含水层中沿潜水面方向的水平氧化还原界面（redox interface）再次沉淀形成的。