Uraniferous Organic Matter in the Sandstone-Type Uranium Ore from Domiasiat, Meghalaya, India

Uranium in the sandstone-hosted uranium deposit at Domiasiat, Meghalaya is mainly associated with organic matter (OM), which is also enriched in several trace elements, viz. Zn, Pb, Ni, V, Co, Ba, Sn, Mo, Cr, Ag, P etc. This paper is an attempt to characterize and discuss the origin of the OM and the associated uranium.

The OM occurs in several forms - lumps, layers, veins, dispersions, fracture fillings, coatings and impregnations. Based on structural features and optical properties, seven types of OM are identified. The structural and compositional features indicate that the OM is type II kerogen with macerals mainly belonging to liptinite group. Some of the primary structures have acted as loci of deposition of uraninite and other inorganic minerals like pyrite, marcasite, sphaleri'te and apatite, thereby giving rise to certain interesting textural features. Development of asecondary granular structure and bIades of secondary OM are features related to maturation. Uranium occurs in the form of uraninite of three generations, minor coffinite and a distinct organo-uranyl complex.

Both the South Khasi granite batholith and the tuffaceous matter related to Sylhet Trap episode appear to be the source for various inorganic elements enriched in the OM. From structures and association it appears that uranium fixation in the OM has taken pIace both due to anaerobic reduction by bacterial H₂S and complexation to form organo-uranyl complexes. The latter, on maturation, was reduced to form fine precipitates of uraninite dispersed in the OM. It is suggested that the maturity of OM, extending up to semi-anthracitic stage, has taken place due to radiolysis and radiogenic heat.