Petrography and chemistry of chromite phases from the Mesoarchean chromitite bodies of the Boula-Nuasahi ultramafic complex, India: Indicators of magmatic evolution and hydrothermal alteration

Occurrence of “ferrian chromites” have earlier been reported from the Mesoarchean chromite deposits in the Boula-Nuasahi ultramafic complex (BNUC) of India. We have investigated the chromitite bodies in the southern part of the BNUC (i.e., Bangur area) with respect to the mode of occurrence, petrography, chemistry and structure of the chromite types. Although morphologically five varieties of chromite ore were found, chemically only three types of chromite can be distinguished based on EPMA analysis. These are: 1) Type I: magnesiochromite [high Cr₂O₃ (57–65 wt.%), low iron (FeO: 13–17 wt.%), X^Fe3+: < 0.1 apfu]; 2) Type II: ferrian chromite [moderate Cr₂O₃ (43–53 wt.%), high iron (FeO: 27–30 wt.%), X^Fe3+: 0.1 to 0.5 apfu]; and 3) Type III: ferrichromite [low Cr₂O₃ (19–29 wt.%), very high iron (FeO: 55–67 wt.%), X^Fe3+: 0.5 to 1.0 apfu]. Stoichiometrically calculated Fe₂O₃ content is very high in some grains (maximum 47 wt.%). Geochemical discrimination diagrams for the Type I pristine magnesiochromite suggest a dominantly boninitic parental magma. Trace element data obtained from LA-ICP-MS indicate that the Type II chromite has formed from a more evolved magma and is richer in trace elements such as V, Mn, Co, Cu, Pb, Ga, and Nb whereas the Type III ferrichromite shows unusually high Ti and erratic high concentrations of trace elements. Alterations in chromite is noticed in two different thermal regimes: 1) 100–200 °C related to serpentinization of dunite and peridotite rocks where chromite grains show an unaltered core, an intermediate ferrian chromite rim and an outer magnetite rim; 2) 500–600 °C where the entire chromite grain is converted to ferrichromite which can be linked to later intrusion of the Bangur gabbro. While HR-TEM study reveals that all three chromite-types have face-centered cubic structure, Raman spectroscopy indicates that there is a gradual transition of the structural state from normal spinel structure (Type I) through Type II to a fully inverse spinel structure in case of ferrichromite.