Mineralogy and geochemistry of a bauxite-bearing lateritic profile supporting the identification of its parent rocks in the domain of the huge Carajas iron deposits, Brazil

摘要

A bauxite-bearing lateritic profile, developed along a borehole within the supergenic Fe-deposits of the Caraj ' as Mineral Province in Brazil, is investigated after its textural aspects, mineralogy, and geochemistry. Petrographic studies, XRD mineralogical analysis, SEM-EDS, and whole-rock chemistry were used to study selected samples. The 156 m depth borehole is composed of basaltic andesite at the substratum and an overlying lateritic profile well-structured in distinct horizons, from base to top composed of: a saprolite, with rock relicts within a clayey matrix of mainly vermiculite and kaolinite; a clayey horizon dominated by kaolinite; a bauxitic horizon with gibbsite as the main phase; a ferruginous crust mainly composed of hematite and goethite. XRD mineralogical quantifications indirectly confirmed the chemistry of Si, Al, Fe, and Ti, as the most abundant elements in the lateritic horizons. Statistical factor analysis of the whole chemistry and mass balance calculations showed groups of elements arranged according to their enrichment or depletion character in the distinct horizons. Si, Ca, Na, and K are leached out to the top of the profile following the destabilization of their carrier mineral phases feldspar, hornblende, and clinochlore, whereas Al, Fe, and Ti are enriched to the upper horizons, respectively in the structure of gibbsite, hematite/goethite, and anatase. The basaltic andesite at the substratum has its parental affinity confirmed after textural changes, REE normalized patterns, and Zr/Ti ratios throughout the profile, nevertheless, parent rock variations are expected. The studied lateritic profile is comparable to most of the bauxite-bearing laterites in the Amazon region, except for its unparalleled 22 m thick Fe-rich bauxitic horizon and absence of a truncating clay cover on the top of the profile. The lateritic sequence was probably formed during the Paleocene-Eocene bauxitization event that reached the Amazon region and survived as a complete unaltered fossil lateritic profile through the time.