Gold refinement by the fractionation of Bi-enriched partial melts at the Quadrilátero Ferrífero, Brazil: implications on the formation of hypozonal deposits

Mariana Brando-Soares, Felipe Emerson André Alves, Atlas Vasconcelos Corrêa Neto, Luiz Bertolino Bertolino, Ivan Mendes Caixeta de Pamplona Araujo, Phillip Gopon, Mariana Sathler Mozart

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Several gold deposits display complex Bi-bearing mineralogy. Bismuth-bearing phases are generally sensitive to changes in physicochemical conditions such that the interpretation of Bi–Au associations is useful to understand these mineralizing systems. However, gaps in the physicochemical constraints given by the Bi enrichment in these systems prevent robust metallogenic models in some complex deposits. São Sebastião is a hypozonal gold deposit in the Quadrilátero Ferrífero historic mining district in Brazil, and unlike other Archean gold deposits in this area, it displays a disseminated high-temperature and high-grade sulfide overprint with Bi enrichment. The Bi-bearing mineralogy at São Sebastião is heterogeneously distributed in the BIF-hosted ore bodies. Bismuth-sulfosalts, Bi-chalcogenides, and native bismuth have crystallized in a variable set of intercalated patches, yielding different ore facies given by the fractionation of hydrothermally assisted melts. Base metal–bearing Bi-sulfosalts, electrum, and acanthite result from high-temperature (~ 600 °C) immiscible melts rich in chloride ligands, while late crystallization of high-fineness native gold associated with native bismuth at 271 °C is evidence for Au scavenging by protracted low-temperature Bi-enriched metallic melts. This mechanism has favored gold remobilization from early sulfide assemblages and the development of localized upgraded ore stringers. Rapid uplift from a reduced (low fO 2) hypozonal environment to oxidized brittle conditions is suggested by crosscutting bismuthinite-bearing veins’ fluid inclusion patterns and the absence of maldonite (Au–Bi alloy). Many hypozonal deposits like São Sebastião show similar mineralogy and have developed at the boundaries of Archean terrains, registering multiple tectonic overprints where partial melts may have had an essential role in the final ore formation.

Original languageEnglish
Pages (from-to)781-800
Number of pages20
JournalMineralium deposita
Issue number5
Early online date23 Feb 2022
Publication statusPublished - Jun 2022

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Publisher Copyright: © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.

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