The hydrothermal footprint of the Crixás deposit: New vectors for orogenic gold exploration in central Brazil

Abstract

The main gold deposits of the Archaean-Paleoproterozoic Terrain of Goiás, in central Brazil, occur in the Crixás Greenstone Belt, one of the five volcanosedimentary sequences of this terrain. The Crixás orogenic gold deposit consists of several orebodies structurally controlled by low to moderate angle thrust faults, hosted in different stratigraphic units that exhibit contrasting mineralization styles and hydrothermal alteration assemblages.This study characterizes the hydrothermal footprints of the Palmeiras Structure and Structure IV, which control the orebodies hosted in the hydrothermally-altered mafic metavolcanic rocks and carbonaceous schists, respectively. We assessed the magnitude of fluid-rock interactions by integrating mineralogical, geochemical, and petrophysical data with the reflectance spectroscopy results for drill core samples for both structures.The Palmeiras Structure hydrothermal footprint is characterized by a distal halo composed of Fe-Mg chlorite + biotite ± carbonates (calcite and Fe dolomite), followed by the intermediate halo defined by epidote + carbonates (dolomite, Fe dolomite, and ankerite) ± Fe-Mg chlorite, and proximal halo composed of muscovite + Fe-enriched chlorite ± paragonite, magnetite, tourmaline, Fe dolomite, and ankerite. Additionally, the chemical pattern of the hydrothermal alteration along the Palmeiras Structure has significantly increased concentrations of alkali (K2O, Na2O, Rb, Cs, Li, and Ba), as well as Fe2O3, Al2O3, TiO2, LOI, U, W, Th, and Ta towards the gold mineralization. Also, the high magnetite concentration resulted in high magnetic susceptibility and density, with values above 1432.00×10–3 SI. In Structure IV, the proximal alteration assemblage comprises muscovite + Fe dolomite, Fe-Mg chlorite + arsenopyrite. The lithogeochemical signature is characterized by enrichment in SiO2, CaO, Na2O, K2O, P, Ba, and pathfinder elements As, W, Sb, Te, that increase towards the gold mineralization. A new potential target hosted in the hydrothermalized metabasalt was identified, whose signature is defined by the occurrence of muscovite, chlorite and enrichment in K2O, Ba, Rb, Li Cs and pathfinder elements such as As, W, Sb, Te.This study demonstrates that the integrated application of reflectance spectroscopy, petrophysics and chemical analyses provide a powerful tool for characterizing the hydrothermal footprint and determining the vectors toward gold mineralization in regional-scale mineral exploration.