Tourmaline as Metallogenic Indicator: Examples from Paleo-Proterozoic Pb-Zn and Cu-Au Deposits of Rajasthan

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.