Carbonaceous Metaexhalite of Shungitic Affinity in Palaeoproterozoic Aravalli Supergroup, Dugocha Area, Rajasthan

The Palaeoproterozoic Debari Group of Aravalli Supergroup near Dugocha, Rajasthan exposes carbonalbite-tourmaline bearing exhalite rock termed carbonaceous metaexhalite. This unit occurs within dolomites of Jagpura Formation and hosts gold-bismuth bearing sulphide mineralisation either within it or along dolomite-metaexhalite contact zones. Polyphase deformation and mineral recrystallisation under green schist facies of metamorphism characterise these rocks.

Geochemically, the carbonaceous metaexhalites have a wide silica range (51 wt% to 63 wt%) and high total alkali (7.85 wt % to 13.20 wt%). EPMA studies of silicates indicate that the main feldspar in the rock is albite and the tourmalines are richer in titanium and chromium. The associated sulphides possess high salinity fluid inclusions (up to 19 wt% NaCl equivalent) and δ³⁴S isotopic values of 12-14 permil. The latter values match well with those of the modern hydrotherms along Discovery Deep median trench of Red Sea.

The most outstanding feature of the rock is abundant carbon and its distribution pattern vis-a-vis associated silicates. The carbon distributed mainly as tiny graphite flakes of low order crystallinity often exceeds 30% by volume and analyses upto 15% by weight. There is an intimate intergrowth of the carbonaceous matter and the composite silicates that gives unique identity to the rock as for the texture and constituent minerals. These features together with their similar age and geological set-up have been discussed in terms of these rocks being similar to the multi-utility carbonaceous rocks with high economic significance found in Shunga Region, Karelia, Russia called Shungites. These high sodacarbon rocks are interpreted here as metamorphic products of mixed volcanic-clastic-exhalite facies rocks (felsic lavas, tuffs, exhalite and carbonate rocks) or the metamorphosed product of the original soda-carbonate volcanic flows during initial stages of Aravalli ensialic rifts.