Mineralogy of Chromiferous Quartzites from South India

The mineralogy of chromiferous quartzites from Archaean greenstone belts of southern Peninsular India has been studied. The majority of the quartzites are metamorphosed clastic sediments derived from a sialic source with enclaves of ultramafic chromite-bearing rocks. For the quartzites from the Ghatti Hosahalli belt, however, a volcanic-exhalative origin is most likely. The chromium in the sediments was fixed in the detrital chromite and/or the clay minerals.

The typical mineral assemblages developed in a wide range of metamorphic conditions from greenschist to amphibolite facies comprise fuchsite, quartz, kyanite and accessory rutile, tourmaline and Cr-spinel. Microprobe analyses show high chromium contents for fuchsite (2-4wt. % Cr₂O₃), kyanite (up to 6wt. % Cr₂O₃), rutile (1.0-2-6wt. % Cr₂O₃) and tourmaline (2.5-7.6 wt. % Cr₂O₃). Cr-spinels are essentially solid solutions between hercynite and chromite (Mg/(Mg+Fe²⁺)=0.060.15; Cr/(Cr+Al)=0.42-0.75). Comparison with igneous chromites indicates that compositional reequilibration during metamorphism was characterised by replacement of Mg by Fe²⁺ and, to a lesser extent, of Cr³⁺ by Al. The micas of chromiferous quartzites from the Chitradurga area have high Cr and Ba contents (1.4-3.9wt. % Cr₂O₃; 0.5-8.5 wt. % BaO) and represent solid solutions extending from muscovite to Cr-oellacherite, A baryte-bearing quartzite from the Ghatti Hosahalli belt contains the rare assemblage quartz, baryte, celsian (Cs₈₅Or₁₁Ab₄), fuchsite (15 wt. % Cr₂O₃; 8.5 wt. % BaO), uvarovite (Uv₇₁Gro₂₈) and tremolite.

The regular distribution of chromium between mica and associated Cr-spinel, rutile and tourmaline indicates equilibration during metamorphism.