Platiniferous gold–tourmaline aggregates in the gold–palladium belt of Minas Gerais, Brazil: implications for regional boron metasomatism

Abstract

The platiniferous gold–palladium belt of Minas Gerais, Brazil, forms an approximately 240-km-long, roughly north–south-trending domain that includes numerous auriferous lodes and platiniferous alluvium. The belt transects two Precambrian terranes, the Quadrilatero Ferrifero in the southern part, and the southern Serra do Espinhaco in the northern part. Both terranes were overprinted by regional fluid flow that led to tourmalinisation, with or without hematitisation, and precious-metal mineralisation. Here, we report the occurrence of coarse-grained gold–tourmaline aggregates and integrate recently obtained ages and tourmaline boron-isotope values published elsewhere. One type of aggregate is unique because it has patches that are close to stoichiometric PdPt, in which gold content varies from 2.5 to 33.5 at.%. The gold–tourmaline aggregates seem to be the ultimate expression of the boron metasomatism.