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

Abstract

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–ulvöspinel 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.