Abstract
Sao Sebastiao is a recently discovered gold deposit in the Pitangui greenstone belt, Minas Gerais, Brazil. Part of its hydrothermal system is characterized by chaotically distributed tourmaline that is concentrated in patchy domains, or pockets. The tourmaline-rich pockets obliterate the foliation of the host greenstones. Pyrrhotite and late chalcopyrite occur in the tourmaline-rich pockets. Bismuth, bismuth–gold intermetallic aggregates, and Bi-telluride minerals locally fill microfractures that are superimposed on preexisting, chalcopyrite-bearing microfractures in the tourmaline-rich pockets. Hedleyite (Bi7Te3) as well as the unnamed minerals Bi8Te3 and Bi2Te are noteworthy. Specifically, hedleyite and Bi8Te3 make up composite aggregates with joseite-B (Bi4Te2S) in which the Bi8Te3 is unambiguously distinguished from hedleyite, both optically and compositionally. The Au–Bi–Te mineral assemblages indicate precipitation below about 270 °C, under epizonal conditions, in a mesothermal (orogenic) setting. Comparison with the economically and historically important Passagem de Mariana gold mine shows similarities that are indicative of a possible widespread, but still poorly recognized, epizonal overprint in Minas Gerais.
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