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Abstract

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.

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Auriferous sulphide mineralisation occurs in the Jagpura Formation of the Debari Group in the Sanjela Manpur- Dugocha belt The Baratalav-Dugocha mineralised zone characterized by occurrence of gossan and presence of ancient workings is confined to a shear zone Bedrock samples from a thick band of meta-exhahte within the dolomite horizon occurring northwest of Dugocha have analysed 1 to 4 ppm of gold The gold values in dolomite range between 0 10 ppm and 2 70 ppm in Baratalav and between 0 15 ppm and 8 52 ppm in Dugocha area The Manpur Lohagarh mineralised zone, characterised by linear ancient workings and gossans, also occurs in the proximity of a shear zone Richer concentration of gold mineralisation can be expected along shear zones that are believed to have offered easy passage for gold-sulphide mineralising fluids.

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Abstract

Petrology and mineral chemistry of ten types of tourmalines from Pb-Zn bearing strata-bound exhalative deposits and Cu-Au-Fe deposits, presumably of IOCG-type, from Rajasthan are presented. The Pb-Zn deposits studied include Agucha, Kayar and Dariba areas in central Rajasthan, each of which is widely separated in space and occurs in isolated but coeval basins within the Archaean basement. The Cu-Au deposits include Ghagri, Bada Talav, Dugocha and Bhukia areas in southeast Rajasthan. The syngenetic Pb-Zn type and the epigenetic Cu-Au-Fe type of deposits are formed during the initial phase of Paleoproterozoic Aravalli Basin evolution with dominant deposition of shelf facies pelite-carbonate sequence of Debari Group. The post-depositional tectono-metamorphic history of the host rocks of Pb- Zn type and Cu-Au type deposits differ significantly with the former deposits showing higher metamorphic impress (amphibolite to lower granulite facies) than the latter (green schist facies).

This study suggests that tourmalines on individual deposit scale exhibit both similarities and differences in optical and chemical features that are in accordance to their bulk host mineral assemblages. But on individual belt-scale, despite significant differences in the post-depositional metamorphic alterations, there is an overall chemical similarity of tourmalines that suggests prevalence of similar hydrothermal conditions during the formation of the tourmalines of individual deposits in each of the Pb-Zn and Cu-Au belts. In many of the discriminant diagrams using the critical elements, the tourmalines of these two belts maintain a distinct identity and assert their role as metal-type indicator in contemporaneous (Paleoproterozoic) deposits formed in different hydrothermal conditions.

This study also tests the reported role of tetrahedral alumina (Al^iv) and X-site vacancies (X^£) in the tourmaline in indicating the grade of metamorphism of the host rocks. Whereas this study confirms the assertions regarding the covariance of Al^iv with respect to the grade of metamorphism in calcareous and metapelitic sediments, the same assertions could not be made in respect of X^£ for the tourmalines from different ore deposits of Rajasthan. The latter is attributed to the diffusion of Na in the tourmaline structure from the abundantly available Na⁺-rich host rocks (albitites) in a postore formation event.

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The Salumber-Ghatol belt in Rajasthan, India, situated along southern margin of the Aravalli Craton, hosts a cluster of Cu-Au deposits in calcitic and dolomitic marbles that belong to Debari Group of the Paleo-mesoproterozoic Aravalli Supergroup. The Fe-Mn rich dolomitic marble of the Delwara Formation hosts Cu-Au-Fe-oxide mineralization at Ghagri and associated distal K-Fe-Mg rich altered rocks (cryptocrystalline microcline + magnesioriebeckite + magnetite + phlogopite) and proximal feldspathised carbonate rocks (medium grained albite + microcline + dolomite + magnetite). The calcitic marble of Mukandpura Formation hosts Dugocha Cu-Au deposit with development of distal graphite-tourmaline-bearing albitites and proximal albite-microcline-magnetite rocks. Calcite and dolomite carbonates of Bhukia region with development of albite-actinolite-bearing alteration assemblages host the largest of the Cu-Au deposits in this belt. The second generation folds and associated ductile-brittle shear zones of the multiply deformed events constitute conduits for the mineralizing fluids at all locations in this belt.

Carbon isotope data for the dolomitic marbles of Ghagri area suggest mixed isotopic signatures: (i) normal marine values close to zero per mil (mean= -0.269 ‰) and (ii) enriched positive values (mean= +4.04 ‰) akin to the 1.9 Ga global event of positive excursion. The calcitic marbles of Dugocha area have significantly depleted δ¹³C values that are interpreted to be the result of pervasive inter-mixing of isotopically distinct carbon derived from primary depositional carbonates, hydrothermally exchanged carbonates, with significant influence of the organic carbon. The petrographic distinction of calcitic and dolomitic marbles of Bhukia area is corroborated by δ¹³C values that show a bimodal distribution of near 0 per mil and depleted values of -3 per mil. Fluid inclusion studies suggest predominance of saline brines in the ore fluids. The δ¹³S values for all the three deposits are narrowly constrained between 10.4 and 15.1 ‰, which suggests a common metasedimentary source for sulfur.

Identical chondrite-normalized REE patterns and primitive mantle-normalized multi-element patterns suggest a common Na- and/or K-source for the formation of Ghagri and Dugocha-Bhukia feldspathic rocks. Based on mineralization styles and attendant alteration patterns, it is argued that the mineral deposits of Salumber-Ghatol belt in the Aravalli Supergroup (i) represent sedimentary carbonate-hosted Cu-Au deposits belonging to the wider class of Proterozoic Iron-Ore-copper-Gold type (IOCG-like) mineral deposits; (ii) the fluids responsible for the mineralization are generated from common sub-crustal source and (iii) during mineralization, the influx of Na-K-rich fluids is so intense that it created 'artificial' hyper-saline conditions in the normal depositional basin that facilitated crystallization of scapolite, brucite, spadaite, povondraite and phlogopite bearing mineral assemblage.

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