URANIUM EXPLORATION IN THE CRETACEOUS MAHADEK SEDIMENTS OF THE MEGHALAYA PLATEAU

D.B. Sen¹, A.S. Sachan², A.K. Padhi² and S.K. Mathur³
Atomic Minerals Directorate for Exploration and Research
Department of Atomic Energy
¹New Delhi – 110 066, ²Nagpur – 440 001 and ³Hyderabad-500 016

Abstract

The Upper Cretaceous Mahadek sediments of the Meghalaya Plateau are being explored for uranium since more than four decades.  During this period, the geoscientists of the Atomic Minerals Directorate have identified the terrain occupied by these sediments as uranium ore province and also delineated a number of deposits and promising occurrences.  Six such important areas have been identified so far. These are Pdengshakap-Tarangblang, Gomaghat, Mawkyrwat, Phlangdilion, Domiasiat and Wahkyn.  Among these, Domiasiat is the country’s largest sandstone-type uranium deposit and subsurface exploration results from the near by Wahkyn area are very promising.  Also, the recent finds at Laitduh (Cherrapunjee) and Plangsynnei (Mawsynram) may develop into important areas.

Uranium investigations in this part of the Meghalaya Plateau started by selection of areas for radiometric survey on the basis of favourability criteria like age of Formation, geological setting, provenance, migration and concentration of uranium, etc., and subsequent subsurface exploration by borehole drilling was carried out in the more promising blocks.  Genetic concepts with variable constraints were considered for various exploration models that were constantly modified on accumulation of data. Field tests on pilot-plant scale for uranium extraction by pug-cure leaching process were for the first time carried out on the bulk samples recovered by exploratory mining.

As a result of the systematic and sustained efforts and multi-disciplinary approach, not only uranium deposits were discovered but many important fundamental concepts regarding geology, sedimentology and uranium exploration in the Mahadek sediments of the Meghalaya Plateau were also developed.  Worth mentioning among them are recognition of fluvial component of the Mahadek Formation, which was otherwise widely reported as marine, and identification of its two stratigraphic subdivisions.  The concepts of Plateau and Ghat domain, and characterisation of parameters for ore-grade mineralisation are significant contributions in this direction.