Original paper

Gold-tourmaline assemblage in a Witwatersrand-like gold deposit, Ouro Fino, Quadrilátero Ferrífero of Minas Gerais, Brazil: the composition of gold and metallogenic implications

Koglin, Nikola; Cabral, Alexandre Raphael; Brunetto, Wilson José; Vymazalová, Anna

Abstract

Ouro Fino, a Witwatersrand-like gold deposit in the Moeda Formation of the Minas Supergroup, Quadrilátero Ferrífero of Minas Gerais, Brazil, has visible gold lacking coeval sulfide minerals. The visible gold is characteristically intergrown with tourmaline. The gold-tourmaline aggregates truncate sub- to euhedral pyrite, which is epigenetic because it occurs as overgrowths on rounded pyrite that is regarded as detrital. Hence the gold is hydrothermal in origin. Compositionally, the gold is argentiferous, with mercury contents between about 2 and 6 wt.%. Palladium and platinum are insignificant (< 1 μg/g) in the gold. The mercury component in the gold and the gold-tourmaline assemblage suggest precipitation from boron-rich brines poor in sulfide sulfur. In the metallogenic context of the Quadrilátero Ferrífero, the gold-tourmaline assemblage of the Moeda Formation can be understood as a reducing equivalent of the mercury-bearing, sulfide-poor hydrothermal overprint on relatively more oxidised rocks of the overlying Itabira Group.

Keywords

goldtourmalinemercuryouro finomoeda formationquadrilatero ferrifero