Supergene gold characterization by geochemistry, grain morphology and Au-Ag-Cu-Te classification

Abstract

Primary characteristics and transformations during laterization processes were investigated in lateritic profiles integrating geochemical analysis and gold grain morphology provided by ICP-ES, electron microprobe and electron microscope in nine sites in three distinct gold provinces in Midwest Brazil. The acquired chemical data indicate a supergene concentration of Al, Fe, Au, As, Co, Cr, Cu and V allowing to detect higher leaching rates of the lateritic profiles in Area I where lateritic duricrust is well developed, relative the lower leaching related to truncated lateritic profiles in Area II. The totally free Au grains of veins, colluvium and alluvium deposits in area II and III show sub-rounded to rounded, irregular, elongated, delicate and polygonal-shaped grains with corrosion cavities, indicating primary vein source, feeble dissolution related to subtler lateritization effects and little mechanical reworking in the colluvium and alluvium deposits. These results allowed to identify subtler weathering effects and small distance transport for the Au grains collected in colluvium and alluvium samples. The quantification of Au-Ag-Cu-Te chemical composition of the free Au grains offered important preliminary discrimination, proving to be a fast, easy and useful approach. It allows classifying the studied deposits in five groups, strongly related to their primary sources (gold-rich porphyry deposits and epithermal deposits).