Rare Earth Mineralization in Regolith Zones of the Neoarchean Phoscorite from the Carbonatite-syenite Complex, Dombarahalli Area, Eastern Dharwar Craton, Koppal District, Karnataka

Abstract

Abstract Two plugs of phoscorite (1.4 km x 1.6 km) covering 3 km2 are reported in association with carbonatite and syenite complex for the first time from the Koppal alkaline complex (2528±9 Ma) Eastern Dharwar craton. Evidences of mingling and late magmatic alteration within the magmatic units of the complex are ubiquitous, as observed between Budihalu and Dombarahalli southwest of Koppal Town. Phoscorite variants are variably enriched in REE. Phoscorite variants analyzed REO (T) (+Y2O3) content ranging from 1002 to 6357ppm with P2O5 ranging from 2.46 to 13.40wt%. Highest concentration of REO(T) (9402ppm) and phosphates (33.81wt% P2O5) have been reported from apatite phoscorite west of Dombarahalli. Regolith zones over phoscorite containing REO(T) up to 6029 ppm with maximum P2O5 of 1.20 wt% have been noticed. Estimated resource as per UNFC-333 in regolith zones developed over phoscorite is 3300t with 0.2% REO(T) cutoff (over 0.34sq. km area) with an average grade of 0.38% REO(T). The resources were estimated on the basis of grid-wise regolith sampling with a spacing of 200 m X 400m. LREE is the dominant constituent in regolith zones over phoscorite, represented by allanite, britholite and parisite. Allanite also occurs as separate phases besides as rim around apatite and inclusions. However, this association is decoupled in the weathering regime and REEs are enriched in weathered silicate clay fractions over carbonatite zone. Secondary REE with partially altered silicates like diopside, pargasite, hornblende, actinolite, clinochlore with sub-ordinate vermiculite, saponite in fines indicate moderate weathering conditions.