Phreatic uranium mineralisation hosted by Neogene sediments from the Taunsa area, Dera Ghazi Khan, Eastern Sulaiman Range, Pakistan: unique exploration targets in a deformed geological setting

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.

KEY POINTS

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

2. 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.

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

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

Keywords:

• uranium
• phreatic ore
• deformed geological setting
• Litra Formation
• Siwaliks
• Taunsa
• Sulaiman Range

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Acknowledgements

We acknowledge the critical review of this manuscript by Prof. Dr Guoxiang Chi, Regina University, Canada, and Prof. Dr Christophe Bonnetti, ex-professor of economic geology (uranium geology) in the ECUT, Nanchang, China, and IAEA uranium geology consultant. Dr Bonnetti suggested many changes through several rounds of review. We pay special gratitude to Prof. Dr Michel Cuney, Research Director Emeritus of the French National Centre for Scientific Research, Paris for a very insightful discussion through Researchgate forum on many problems of the Taunsa uranium ore. We thank all the authorities of the China National Nuclear Corporation and PAEC for their overall support and supervision during this research. The Chemistry Division of AEMC is thanked for conducting geochemical and radiometric analyses. The National Centre of Excellence in Geology (NCEG), University of Peshawar (Pakistan), kindly provided laboratory facility for fluorescence microscopy. Prof. M. Hanif, University of Peshawar (Pakistan), Prof. Dr Iftikhar Ahmad Abbasi, Sultan Qaboos University, Muscat (Oman) and Dr Birkhez Aslam Shami, consultant geologist in the Hydrocarbons Development Institute of Pakistan, Islamabad, kindly spared time to discuss the possible origin of the OM associated with the Taunsa uranium ore. Dr Muhammad Sajjad, Dr Muhammad Hanif (Jr) and Dr Sulaiman Khan, University of Peshawar, and their students helped in using sedimentology laboratories under their control in NCEG, and for their overall guidance in carrying out some important analyses on OM. Dr Liaquat Ali, Director NCEG, is thanked for granting permission to use laboratories in NCEG, University of Peshawar. Dr Fahad Ali Alizai, Department of Geology, Bacha Khan University Charsadda (Pakistan), Mr Muhammad Sadiq and Mr Muhammad Irfan, NCEG, University of Peshawar, helped in the fluorescence microscopy of OM. Special thanks are due to geophysicist colleagues at AEMC namely, Mr Usman Shokat, Mr Shah Nawaz Khan and Mr Zaheer Hussain for informative discussion on gamma-ray spectrometry. The authors are highly indebted to Ms Xuefen Zhang, Ms Zhao Jiao and Dr Li Kaixuan of the Laboratory of Nuclear Resources and Environment, ECUT, for their help in SEM and EPMA studies.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was funded by China National Nuclear Corporation, Pakistan Atomic Energy Commission and ECUT, China.