Analyzing the Spatial Occurrence of High-Alumina Clays (Brazil) Using Electrical Resistivity Tomography (ERT)

摘要

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 system. 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 discuss 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 Fe2O3 from Gleysol and correspondingly increasing Al2O3 and SiO2. 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 sensitive wetland system in which high-alumina clays may occur.