URANIFEROUS ORGANIC-MATTER IN THE SANDSTONE-TYPE URANIUM ORE FROM DOMIASIAT, MEGHALAYA, INDIA

Uranium in the sandstone-hosted uranium deposit at Domiasiat, Meghalay a is mainly associated with organic matter (OM), which is also enriche d in several trace elements, viz. Zn, Pb, Ni, V, Co, Ba, Sn, Mo, Cr, A g, P etc. This paper is an attempt to characterize and discuss the ori gin of the OM and the associated uranium. The OM occurs in several for ms - lumps, layers, veins, dispersions, fracture fillings, coatings an d impregnations. Based on structural features and optical properties, seven types of OM are identified. The structural and compositional fea tures indicate that the OM is type II kerogen with macerals mainly bel onging to liptinite group. Some of the primary structures have acted a s loci of deposition of uraninite and other inorganic minerals like py rite, marcasite, sphalerite and apatite, thereby giving rise to certai n interesting textural features. Development of a secondary granular s tructure and blades 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 g ranite batholith and the tuffaceous matter related to Sylhet Trap epis ode appear to be the source for various inorganic elements enriched in the OM. From structures and association it appears that uranium fixat ion in the OM has taken place both due to anaerobic reduction by bacte rial H2S and complexation to form organo-uranyl complexes. The latter, on maturation, was reduced to form fine precipitates of uraninite dis persed in the OM. It is suggested that the maturity of OM, extending u p to semi-anthracitic stage, has taken place due to radiolysis and rad iogenic heat.