Analyzing the Spatial Occurrence of High-Alumina Clays (Brazil) Using Electrical Resistivity Tomography (ERT) C. V. S. COELHO, 1 C. A. MOREIRA, 2 V. ROSOLEN, 1 G. T. BUENO, 3 J. SALLES, 3 L. M. FURLAN, 1 and J. S. GOVONE 4 Abstract—Mottled and high-alumina clay horizons (Gleysols) above dismantled iron crusts (Ferralsols) are widespread in the soils that developed on the flat sedimentary plateau located in western Minas Gerais State (Brazil). Field exploration shows that the deposits of high-alumina clays are located at a lower topographic position, mottled horizons suggesting a lateral transformation sys- tem. Two-dimensional and pseudo-three-dimensional electrical resistivity tomography (ERT) techniques have been tested to investigate the distribution of high-alumina clay layers in a thick lateritic mantle, and to assess the potential of the technique to delimitate ore reserves. The figures of resistivity, based on spatial variations of electrical properties of the weathering layers, showed spatial changes in the subsurface structure of weathering mantle, expanding the distribution of iron crust and the high-alumina clay layers, which are strongly influenced by aquifer. Combining 2D and pseudo-3D geophysical images with soil morphology and geochemistry, we delimitate the high-alumina clay layer and dis- cuss its genesis. The ore is located exclusively on the edge of the plateau and is closely linked to the development of hydromorphic soils, exactly where the vertical water flow is restrained by the iron crust. This distinct water regime defines the geochemical transfers in soil mantle, depleting Fe 2 O 3 from Gleysol and correspondingly increasing Al 2 O 3 and SiO 2 . This study aimed to evaluate the potential of ERT as a prospecting tool for supergene ore, and as a technique with reduced environmental impact in the mineral research, when compared to the pre-existing exploration methods (trenches, drill holes and extraction) that are applied on this sen- sitive wetland system in which high-alumina clays may occur. Keywords: Wetlands, gleysols, ferralsols, hydromorphic soils, sedimentary plateau. 1. Introduction In the western part of Minas Gerais state, in Brazil, the flat plateau remnant of the Sul-Americana Surface (King 1956) is covered by lateritic ferrugi- nous materials (i.e., ferricrete and Ferralsols) that have been dismantling due to the initial stage of river incision. The current geomorphic processes constitute a system of surficial degradation and determine the pattern of soil distribution on the plateau, which is linked to hillslope hydrology (Pennock et al. 2014). Ferralsols are located upslope, under oxidic pedo- logical conditions, and Gleysols are located downslope, in the valley and shallow topographic depression, under oxidation–reduction conditions. The soils present in this region are environmentally and economically relevant. The Gleysol profiles display an iron-depleted layer enriched with high-alumina clays, consisting of kaolinite (Al 2 O 3 2SiO 2 2H 2 O) and gibbsite (Al(OH) 3 ), that have been exploited by the refractory industry (Coelho and Rosolen 2016). Supergenic non- metallic lateritic products, i.e., kaolin and bauxite, occur almost throughout the entire Brazilian territory (Melfi 1997; Montes et al. 2002), contributing sig- nificantly to its economy. In 2017, non-metallic mineral trading generated a total of US$ 25.8 billion (DNPM 2016). The minimum A1 2 O 3 content required to consider clay as high-alumina is set arbitrarily at 40% A1 2 O 3 in raw clay, just above the theoretical alumina content of 39.5% A1 2 O 3 in kaolinite (Keller 1963). Despite their economic relevance, high-alumina clays occur in very sensitive wetland environments. Wetland ecosystems are recognized as zones of per- fect synergistic relationship between water and soil, 1 Departamento de Petrologia e Metalogenia (DPM), Universidade Estadual Paulista (UNESP), Av. 24A, 1515, Bela Vista, CP: 178, Rio Claro, SP CEP: 13506-900, Brazil. E-mail: carla.vsc1@gmail.com 2 Departamento de Geologia Aplicada (DGA), Universidade Estadual Paulista (UNESP), Av. 24A, 1515, Bela Vista, Rio Claro, SP CEP: 13506-900, Brazil. 3 Instituto de Estudos So´cio-Ambiental (IESA), Universidade Federal Goia´s (UFG), Goiaˆnia, GO CEP: 74001-970, Brazil. 4 Centro de Estudos Ambientais (CEA), Universidade Estadual Paulista (UNESP), Av. 24A, 1515, Bela Vista, Rio Claro, SP CEP: 13506-900, Brazil. Pure Appl. Geophys. Ó 2020 Springer Nature Switzerland AG https://doi.org/10.1007/s00024-020-02444-w Pure and Applied Geophysics