David Machado, MSc

D. Machado¹, A. Vasconcelos¹, M. Soares², T. Souza³

¹Department of Geology, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil

²Department of Geology, Rio de Janeiro State University, Rio de Janeiro, Brazil

³Jaguar Mining Inc., Santa Bárbara, Minas Gerais, Brazil

The Pilar underground mine (Jaguar Mining Inc.) is located in the eastern portion of the Quadrilátero Ferrífero, within the Neoarchean Rio das Velhas greenstone belt of the São Francisco Craton, Brazil. The area was affected by multiple deformational and hydrothermal events in the Neoarchean, Rhyacian-Ororsirian and Ediacaran. The structural and stratigraphic complexity of the region requires detailed mapping to constrain the primary controls on gold mineralization. 

Underground mapping at a 1:100 scale, combined with macroscopic logging of 584 m of drill core, allowed identification of several lithotypes, including: metachert; banded iron formation (BIF) in oxide, carbonate, and silicate facies; quartz veins with sulfides and mafic minerals; quartz–carbonate veins; carbonaceous schist; chlorite-schist; chlorite–quartz-schist; and talc- schist. The dominant hydrothermal alteration styles include carbonation, chloritization, silicification, and sulfidation. Gold mineralization is hosted within BIF and mafic schists, being composed by replacement-style sulfide zones as well as quartz-sulfide veins. Ore minerals include chalcopyrite, pyrrhotite, pyrite, and arsenopyrite. Structural analysis reveals two main deformational phases: (1) an early Archean event responsible for penetrative foliations and isoclinal folds with SW–NE vergence and ENE–WSW axial planes dipping to the SW; and (2) a Paleoproterozoic event associated with localized shear zones, producing open to asymmetric folds with SW–NE vergence and NNE–SSW axial planes dipping to the SE–SW. 

fIn this framework, BIF units acted as preferential fluid conduits due to their rheological contrast, whereas carbonaceous schist behaved as an impermeable barrier, promoting fluid trapping, quartz vein development, and sulfide precipitation near lithological contacts. Two generations of quartz–sulfide veins were distinguished. The first generation is more deformed, sulfide-rich, and absent of carbonate minerals, developing boudinage and folding. The second generation shows intense carbonation, lower sulfide content, better preservation of primary textures, and lower strain, reflecting a later hydrothermal pulse. 

These combined geological, structural, and textural observations indicate that gold deposition at Pilar was controlled by the interplay between progressive deformation, lithological contrasts, and multiple episodes of hydrothermal fluid flow.