Patterns of alteration in chromites from ultramafic rocks of Kudada area, East Singhbhum district (Jharkhand)

Abstract

The ultramafic-mafic rock suite present in Kudada area is located close to the Singhbhum Shear Zone. Although the occurrences of these ultramafic-mafic bodies are reported in some earlier literatures but the detailed research work has not been undertaken so far. The main goal of this study is to find the detail mineralogy of the ultramafic rocks, identify the signatures that modify their primary composition and patterns of alteration in chromite. The major rock types found in Kudada area are talc-magnesite schist with accessory rutile, and serpentinite with accessory chromite. Details studies reveal intense compositional variability in accessory chromites of serpentinite. Core composition of chromites are characterized by high Cr# (0.74-0.86) and low Mg# (0.15-0.17). Compositional variability on the scale of a single chromite grain occurs in the form of zoning. To identify the pattern of compositional zoning, chromites are subdivided into four types depending on grain size, intensity of fracture and porosity. Smaller (< 20 μm) chromites (type-IV) are highly altered than comparatively larger chromites. Fracture and porosity are mainly developed in larger chromite (type-II) grains. Type-I chromites have medium grain size, less fractured and non-porous verity showing development of two rims. Outermost rim is magnetite surrounds the grey colour ferritchromit rim. Composition of rim-magnetite is similar in all types of chromite but it is compositionally different than the magnetite present in veins. Al, Cr, Mg depleted ferritchromit (R1) is formed surrounding the pore space of type-II chromites. From the texture and mineral chemistry, it is clear that metasomatism by CO2 bearing fluid causes significant increase in the rates of diffusion of the cations and this can facilitate the formation of regular and well-developed ferritchromit rims. Due to later hydrothermal event magnetite rims in all types of chromites and R1 ferritchromit in type-II chromites were formed around the pore spaces.