An investigation of Fe-Ti, V in the north-east Karbi Hills, Shillong Plateau, north-east India: Implication for mineralization

The Kulamati gabbro-anorthosite complex in the Karbi Hills of the Shillong Plateau, north-east India is a newly discovered stratified Fe-Ti-V-bearing magmatic body that has been classified as a titano-magnetite ore. The host gabbro-anorthosite is found intruded into the Mesoproterozoic Shillong Group. The stratified Fe-Ti-V oxide body consists mostly of magnetite-rich and magnetite-poor layers. The magnetite-rich bands are composed of titanomagnetite, haematite, ilmenite, and coulsonite with minor Al-spinel, while the mineral constituents of silicate bands contain plagioclase, clinopyroxene, olivine, apatite, and chlorite. Titaniferous magnetites display a wide variety of subsolvus features, including Al-spinel-magnetite-ulvospinel exsolutions and crystallographically oriented ilmenite exsolutions, but magnetites are exclusively vanadiferous. Comprehensive chemical analyses show that the ore is rich in TiO2 (10.98-12.78 wt%) and V2O5 (1.32-1.47 wt%). Ti is attributed by the presence of rutile, titanite, and ilmenite, while V is attributed by coulsonite as a solid solution component in the titanomagnetite. It records moderate to high Al2O3 (4.51-5.46 wt%), with enrichment of FeO and Fe2O3 (26.22-28.32 wt% and 39.34-42.86 wt%, respectively), which can be attributed to the presence of magnetite-spinel-ulvospinel. Higher Fe2O3 and lower FeO are indicative of oxidizing conditions of the ore-forming environment. The rare earth element (REE) patterns have (La/Yb)(N) and (Ce/Yb)(N) ratios comparable to an evolved basaltic melt, with light REE being enriched compared to heavy REE and a positive Eu anomaly. The electron probe micro analyser data of magnetite is conformable with the bulk chemistry, and shows consistently high FeO (70.87-88.06 wt%), Cr2O3 (1.02-2.44 wt%), TiO2 (0.16-6.20 wt%), and V2O3 (1.47-2.26 wt%), with trace amounts of Al, Si, Mg, Mn, and PGE. As per targeted parameters, the ore occurrence distinctly belongs to a stratified Fe-Ti-V class, where alternate thick (>1.0 m) and thin (<20 cm) strata resulted from the late magmatic crystallization of the original tholeiitic composition.