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

Abstract

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 metalbearing 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.

Palladiferous placer gold from the Paraíba do Sul River in northern Rio de Janeiro (Brazil) and its geological implications

“…The São Bento gold has Ag contents of 20–50% (Márquez et al 2006). At São Sebastião, gold and Au-bearing silver (77–85% Ag) are spatially associated with bismuth (Brando Soares et al 2022). The aforementioned deposits are located within, or close to, the Quadrilátero Ferrífero of Minas Gerais – i.e.…”