Bauxitization scaffolded by quartzites 

Abstract

<p>Bauxites associated with quartzite karsts have not been previously reported.  In the Southern Espinhaço Range, Southeastern Brazil, bauxitic duricrusts reveal a unique geological-geomorphological setting. The spatial relationship between hematite-phyllites and quartzites and their different strengths play a key role in the development of thick bauxite profiles. The quartzites, resistant to physical and chemical denudation, scaffold the more easily eroded phyllites, protecting the later from physical erosion and allowing long-term – tens of millions of years – chemical weathering to transform phyllites into bauxites. We investigate the geochemical, mineralogical, geochronological and micromorphological signatures of bauxites in order to reconstruct their evolutionary history. Our data reveal periods of intense aluminum and iron dissolution-precipitation triggered by the strong bioturbation, revealing a crucial role for vegetation in bauxite evolution. (U–Th)/He geochronology of Al-goethite reveals bauxitization peaks in the Upper Miocene and Lower Pleistocene, during periods of higher rainfall. We identify a ferruginous duricrust that preceded bauxitization, suggesting perhaps an even more protracted history of weathering, now partially erased by the later-stage bauxitization. Our findings show that the elevated landscape positions and effective drainage into the exposed bedrock were crucial for ferruginous duricrusts to be chemically dismantled, leaving essentially a bauxitic duricrust residue. The chemically and mechanically resilient quartzites of the Southern Espinhaço Range were the main scaffolds that permitted bauxitization of the hematitic phyllites. A new model of bauxitization scaffolded by quartzites is revealed by this study.</p>