Rediscovery of the high-grade Viana orebody: a missing link in the Archean Cuiabá microbasin framework and the timing of a world-class BIF-hosted gold system

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The Cuiabá world-class gold deposit, hosted by the Archean Rio das Velhas green-stone belt (Minas Gerais, Brazil), is a major gold system with >7 Moz produced. Re-cent resource delineation drilling at Levels L19 and L20 intercepted a 30-m-thick mineralized zone (Viana) at the hangingwall of the Fonte Grande Sul and Serrotinho orebodies. The Viana orebody, represents a significant (re)discovery (>15 t Au) of a historically neglected shallow target. The Viana orebody is subdivided into two domains. Domain 1 features a 15-m-thick replacement-style ore zone (15–30 g/t Au) consisting of massive sulfide at the base and a gold-bearing metachert at the top, separated from barren metachert by a mylonitic sericite-chlorite schist. Domain 2 comprises a 2-m-thick zone of carbonate-sericite-quartz schists and laminated/smoky-quartz veins (2–10 g/t Au). Multi-element geochemistry and REE + Y systematics demonstrate that the carbon-rich Viana Metachert is geochemically distinct from the main Cuiabá BIF horizon, reflecting a stronger high-temperature hydrothermal input and organic matter presence during deposition. Structurally, the Viana orebody is interpreted as the result of flexural-slip folding along its stratigraphic level during the formation of the Cuiabá F1 upright fold (D1). Domain 2 laminated veins developed along refracted cleavages, channelizing fluids into Domain 1. A duplex system formed within Domain 1 due to flexural-slip, trapping syn -deformational fluids below the thrust plane. The current cigar-shaped oreshoot geometry results from refolding during the D2 penetrative event. Geochronological data expand the temporal window of gold mineralization in the Rio das Velhas belt. Hydrothermal monazite U-Pb dating from massive sulfides (51.7 g/t Au) yielded an age of 2775 ± 23 Ma, while Re-Os ages from arsenopyrites yielded 2723 ± 25 Ma. These results identify a previously unrecognized chemical sedimentary horizon and indicate that gold mineralization occurred through multiple pulses associated with the Rio das Velhas II and Mamona tectono-magmatic events. This study requires a reinterpretation of the local lithostratigraphic column and highlights significant new exploration opportunities within the Cuiabá system.