Mineralogia por DRX, petrografia e química mineral da jazida Córrego Brandão, Greenstone Belt Rio das Velhas, Quadrilátero Ferrífero

Abstract

Córrego Brandão (CBR) is a gold deposit located in the northeastern portion of the Quadrilátero Ferrífero (QF), in the municipality of Caeté, Minas Gerais. Its oxidized zone contains 1 Mt of ore with an average grade of 1.48 g/t Au, totaling 51,000 oz of gold. The deposit is hosted in metaigneous amphibolites of the Nova Lima Group, which forms the base of the Rio das Velhas Greenstone Belt. The synformal structure surrounding the granitoids and orthogneisses of the Caeté Dome (2.77 Ga) sets CBR apart from other Neoarchean gold deposits in the QF. This study used drill core samples and conducted XRD analyses, petrographic studies, mineral chemistry, and multielement geochemistry to characterize the host rocks. These rocks, of mafic origin, include metamorphic and metavolcaniclastic sequences with distinctive chemical, macroscopic, and microscopic features. The mineralization is hosted in metamorphic rocks derived from mafic igneous protoliths, within the lower amphibolite facies. This contrasts with most QF deposits, which are hosted in greenschist facies rocks. XRD analyses revealed a significant presence of carbonate group minerals, such as dolomite, in the mineralized rocks. The mineralized horizon is hydrothermally altered and contains sulfides, arsenides, tellurides, carbonates, and amphiboles. It occurs at the contact between a package of metaultramafic rocks and another of metamafic rocks, and is crosscut by a steeply dipping shear zone approximately 20 to 40 meters thick, while lithological contacts follow the CBR synform. The host rock is fine to medium-grained, with obliterated and inequigranular texture, and contains carbonate veinlets that crosscut foliation. It is referred to as tremolite/actinolite chlorite–carbonate schist. The mineralized zone contains disseminated sulfides, including sphalerite, arsenopyrite, pyrrhotite, pyrite, and chalcopyrite. In the proximal halo, magnetite is the dominant oxide phase, forming fine disseminated crystals. Pyrrhotite is rare and results from magnetite replacement. Fine, tabular ilmenite crystals occur subordinately. Gangue indicator minerals include amphiboles (hornblende, tremolite, and actinolite), epidote, chlorite, biotite, and garnet. Gold particles are associated with and/or included in magnetite and pyrite crystals. They appear as fine, elongated grains, often linked to microbands of chlorite.