Combining multiple correspondence analysis with factorial kriging analysis for geochemical mapping of the gold–silver deposit at Marrancos (Portugal)

Abstract This study concerns the application of multiple correspondence analysis and factorial kriging analysis to soil data, and aims to identify spatial patterns and superficial soil anomalies of the Au and Ag deposit at Marrancos, Vila Verde. The mineral deposit can be described as a quartz auriferous shear-zone, consisting of a quartz breccia of tectonic origin hosted by metamorphic rocks (hornfels). Gold is associated with arsenopyrite and pyrite, and Ag with galena and galenobismuthite. A total of 286 soil samples were analysed for Fe, Cu, Zn, Pb, Co, Ni, Mn, Ag and Bi by atomic absorption spectrometry, As, Se, Te and Sb by atomic absorption spectrometry–hydride generation system and Au by inductively coupled plasma–atomic emission spectroscopy after extraction of the metal by an organic solvent (methyl-isobutylketone). The methodology used included (a) multiple correspondence analysis applied to soil data to obtain some factors that summarize geochemical information, (b) a structural analysis (variography) in order to account for spatial variability of these factors, and (c) factorial kriging analysis used to split these factors into their spatial components. This methodology allowed an efficient multi-element characterization of the spatial patterns as well as the identification and interpretation of significant anomalies, not always associated to Au-bearing geological structures.