Abstract:
The chromite mineralization in the southern fringe of the Singhbhum Craton around Ghutrigaon, Dhenkanal district, Odisha, Eastern India shows a unique nature of occurrence. The structural features, mineral assemblage, and spinel composition are different from the two well-known Sukinda and Boula-Nuasahi chromite deposits occurring within the mafic/ultramafic complex in Odisha. The chromite-bearing quartzite from Ghutrigaon shows typical sedimentary features, such as colour banding, cross-bedding, broomsticks, and slump structures and is closely associated with large volumes of volcanic tuff. The chromite-bearing quartzite contains major quartz and chromite along with minor accessories such as fuchsite, chromian muscovite, chromphyllite, chromceladonite, chromechalcedony, rutile, and zircon. Detailed mineralogical, textural, and spinel compositional analysis support to its detrital origin having been derived from a volcanic source. The Cr-spinel composition shows very low Mg-number (0.01-0.04) and high Cr-number (0.69-0.79) in contrast to chrome ores from different localities of India. The EPMA results when plotted in various established discrimination diagrams show a distinctly different domain, indicating that these are affected by a pro-grade metamorphic event. Our study shows that the Ghutrigaon chromite ore owes a mixed origin where chromite detritus from a volcanic provenance (island arc to intra-plate terrain) got deposited along with terrestrially derived silica-rich clastics. Paucity in magnesium content in the spinel may be the result of replacement of Mg by Fe2+ due to compositional re-equilibration during metamorphism. Grain coarsening took place at the onset of metamorphism. The volcaniclastics added to the arenaceous sediment gave rise to different Al-silicate minerals. The chrome-content in most of these minerals is attributed to redistribution of Cr through solid-state diffusion during metamorphism. Rutile and Zircon detritus are considered as continental input. The presence of Zn, Mn, and Ni in the chrome spinel is most likely contributed by the precursor rock. The formation of large idioblastic chromite grains, preferred grain orientation, band development, elevated values of some elements in the spinel, Zn in particular attest to their formation under low-grade regional metamorphism (greenschist facies).