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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 fO2) 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. [ABSTRACT FROM AUTHOR]

High-temperature conditions for ore deposition and anomalous abundances of base metals in orogenic gold deposits are frequently attributed to magmatic-hydrothermal fluids instead of metamorphic orogenic fluids. Zona Basal is a shear zone-related gold and base metals-rich mineral occurrence recently discovered in the northwestern portion of the Quadrilátero Ferrífero mining district in southeast Brazil. Mineralogical and geochemical characterization has revealed a two-stage mineralizing system. An Au-W-As stage is marked by a late- to post-tectonic arsenopyrite-pyrrhotite-pyrite assemblage (crystallized from ca. 491 to 404 °C), associated with native gold and scheelite, which was formed during the transition from the peak of greenschist facies metamorphism to retrograde metamorphism. A following, clearly post-tectonic stage comprises a pyrrhotite-pyrite-sphalerite-chalcopyrite-galena-ullmannite ± meneghinite ± fahlore ± miargyrite ± pyrargyrite assemblage crystallized from ca. 400 to < 170 °C, corresponding to the Ag and base metals stage. Fahlores from the Zona Basal mineral occurrence have been found to be sufficiently enriched in Ag, due to the Ag–Cu cation-exchange reaction 13PbS (galena) + 10AgSbS2 (in galena) = 3Ag3SbS3 (pyrargyrite) + Ag(Cu)−1 (exchange in fahlore) + CuPb13Sb7S24 (meneghinite), to jump across the (Cu, Ag)10(Fe,Zn)2Sb4S13 miscibility gap. Two mechanisms may explain the abrupt chemical change between the two mineralization stages: (1) participation of magmatic-hydrothermal fluids of relatively low temperature (acting mostly as a chemical source for fluids) associated with a syn- to late tectonic granite intrusion; or (2) local re-concentration of base metals by unmixing of an early metamorphic orogenic fluid due to fluid immiscibility after the transition from ductile to brittle conditions, generating coexisting fluids with different salinities (i.e., extremely low and moderate salinity). The findings in this paper illustrate the importance of considering retrograde or cooling reactions and mineral re-equilibrium while addressing complex ore-formation. [ABSTRACT FROM AUTHOR]

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