On some geochemical features of the vanadiferous magnetite deposits of Kumhardubi and Betjharan, Mayurbhanj District, Orissa, India

Abstract

The magnetite-ilmenite ore bodies of Mayurbhanj, India, occurring as 10–320-m long massive lenses and pods within gabbro and anorthositic gabbro of the Lower Proterozoic Gorumahisani Banded Iron Formation show erratic fluctuations in vanadium content (0.3–1.6%) even at near-constant magnetite concentrations. Thin and polished section studies of ore and associated host-rocks with back-up XRD, electron probe and chemical analyses by AAS show that the ore mineral crystallised after plagioclase and pyroxene at 800-600°C to trellis-type magnetite-ilmenite intergrowths. The ore masses were later infiltrated along grain boundaries at higher f O 2 by haematite followed by alumina-rich chlorite. Supergene alteration of the magnetite to maghemite is patchily developed. Electron probe analyses indicate that the bulk of vanadium is in the magnetite-maghemite phase. No separate vanadium mineral has been identified. From the lower V and Cr contents in gangue magnetite as compared to the ore-forming magnetite, it is expected that the ore masses formed from immiscible liquid droplets, coalescing together. Absence of cumulate fabric including zones of rhythmic silicate-oxide layering is corroborative of this hypothesis, although diagnostic orbicular structures have not been identified so far. Erratic V concentration, within the ore bodies appears to be a product of: (a) variable exchange reactions between the silicate host and the late magmatic Fe-rich fluids, the smaller masses getting more depleted in V than the cores of larger segregations; and (b) subtle variability of V 3+ V 4+ in site preference within the Fe 3O 4Fe 2TiO 4 mix-crystals, as the threshold of solidus was being crossed under fluctuating f O 2 .