Magnetic signature of t he world-class, Archean Cuiabá gold deposit in the Quadrilátero Ferrífero

Abstract

The world-class Cuiabá orogenic gold deposit is hosted in the Archean Rio das Velhas greenstone belt, located in the Quadrilátero Ferrífero, southeast Brazil. Gold mineralization is stratabound style in sulfidized banded iron formation (BIF) and shear-related quartz-carbonate veins hosted in mafic volcanic rocks. Gold is mainly associated with pyrite, arsenic-rich pyrite, arsenopyrite and pyrrhotite in BIF, and as the mine deepens there is evidence that the pyrite: pyrrhotite ratio decreases. Given that pyrrhotite can be magnetic, this property is herein applied to characterize the magnetic signature of mineralization and its host rocks by measuring the physical properties of samples from a 2,241.15 m borehole. In addition, polished thin sections were selected to define the mineralogy and hydrothermal alteration zones of the main lithologies. The results show that the mineralized zone is associated with high values of magnetic susceptibility and that pyrrhotite is, in fact, the more relevant sulfide mineral in these intervals and associated with hydrothermal magnetite. In order to map the extent of gold mineralization at depth, aeromagnetic data were also analyzed. The 2D analysis show that there is an important anomaly related to the selected borehole, and other anomalies of lower intensity in the west of the region. The inversion of the magnetic vector performed to build the 3D magnetic susceptibility model revealed that high magnetic susceptibility values occur at a depth lower than the anomalies identified by petrophysics and and that may be related to other magnetic lithologies. Intermediate anomalies were recovered at the same depths of levels 3 and 4 of the Cuiabá mine, where the quartz-carbonate-vein-hosted Viana orebody occurs. The recovered model suggests potential extensions of mineralized bodies and provides important indicators for near-mine gold exploration.