Rajan Chopra1, Biswajit Panigrahi2, Ritesh Yadav3 and G.B. Joshi4

Atomic Minerals Directorate for Exploration and Research

Department of Atomic Energy

1 Jaipur-302033, 2Jamshedpur -831002, 3Shillong-793019, 4 New Delhi


Mahadek Basin of Meghalaya in northeast India is a prime target for sandstone-type uranium deposits. The basin comprises about 2400m thick platformal sediments deposited over Precambrian Gneissic Complex, Palaeo to Meso Proterozoic Shillong Group of rocks or Late Jurassic Sylhet trap in different parts of the basin. The lower part of sedimentary column is divisible into Jadukata (~235m) and Mahadek Formations (~200m) of Cretaceous age, which are overlain by Tertiary sequences. The Mahadek sediments are favourable for hosting sandstone type uranium mineralization due to their intra-cratonic set up, fertile granitic/ acidic provenance, Cretaceous fluvial braided channels containing immature permeable sandstone and presence of reductants like disseminated organic matter and pyrite. The western sector of the basin is dominated by fertile granitic provenance with uranium content varying from 7 to 110 ppm. The Lower Mahadeks (LM) comprise coarse to very coarse grained, grey to dull grey coloured, ill-sorted arkosic to feldspathic wacke of reduced nature deposited in braided channels. Tuffaceous layers containing 10 to 50 ppm of uranium are reported. Uranium mineralization is reported in lower Mahadek Formation (LM) all over the basin. The eastern sector of the basin, comprising conglomerate–grit-sandstone sequence of alluvial fan proximal environment, hosts uranium anomalies associated with carbonaceous lenses at several locales. Subsurface exploration indicated impersistency of mineralization. The western part of the basin in West Khasi Hills has tabular sandstone type uranium deposits such as Domiasiat- Phlangdiloin-Tyrnai, Wahkyn-Lostoin-Wahkut, Umthongkut and Gomaghat, containing nearly 20,000 tonne of uranium oxide. Ore grade enrichment of uranium is controlled by palaeo-channels containing arkosic sandstone of optimal thickness (40 to 60m). The palaeo channels deciphered from the proved deposits have south to southwesterly trend indicating continuity of mineralising features towards south and southwest below the cover sediments of Nongjri plateau.

Keywords: Uranium ore, Cretaceous, Tertiary, Mahadek Basin, Domiasiat, Wahkut, Meghalaya.