A multi-methodological approach for mineral exploration and predictive metallurgy: the case of the Pilar gold deposit at the Quadrilátero Ferrífero, Brazil

Gold exploration is generally based on methods such as soil, stream and drill core geochemistry and geophysical techniques, but X-ray diffraction (XRD) and the Rietveld method-based mineral quantification are seldomly used. Here, a multi-methodological approach coupling traditional drill core macroscopic description and multi-element analysis, scanning electron microscopy/energy-dispersive X-ray fluorescence spectrometry (SEM/EDS), and electron probe microanalysis (EPMA) with XRD/Rietveld method and statistical techniques is presented as a useful tool for deposit characterization, mineral exploration and predictive metallurgy purposes. The Pilar gold deposit, situated in the Quadrilátero Ferrífero province, Brazil, was used as a case study for the proposed multi-methodological approach. Carbonate-facies banded iron formations (BIFs) and schist sequences showing an intense quartz and sulfide hydrothermal alteration represent the host rocks of Pilar mineralized intervals, which may be identified by three signatures: Au-As-W-S-quartz-chlorite-pyrrhotite-arsenopyrite-calcite for schists, and Au-As-quartz-chlorite-arsenopyrite-pyrrhotite and Au-Ag-S-Te-quartz-chlorite-pyrrhotite for BIFs. Within the mineralized BIFs, the expected average Au grades and gold textures vary according to the signature. In As-poor BIFs, gold is only observed within the structure of sulfide and telluride phases. On the other hand, native gold is present in As-rich BIF samples, which usually contain relatively higher Au grades, compared to the As-poor BIFs. Native gold is also identified in mineralized schists, whose average Au grades are similar to those of As-poor BIFs. Therefore, the proposed multi-methodological approach is useful in providing the following information: geochemical-mineralogical signatures and the expected average Au grades and mode of occurrence of gold for each one of these signatures. At Pilar, the geochemical-mineralogical behavior of mineralized zones is directly linked to Au grades and textures. In terms of a practical workflow, this study illustrates that, after the multi-methodological approach is applied and the representativeness of the ore-petrography is tested, the general described behavior of gold within each group of ores can be used for mineral exploration and metallurgical purposes without the need of repeated SEM/EDS and EPMA analyses. Instead, the average Au grades and the gold textures may be predicted solely by the signature definition based on the drill core geochemical-mineralogical analyses.