Bauxite on igneous rocks from Amazonia and Southwestern of Australia: Implication for weathering process

摘要

Severe weathering processes acting on bedrock cause enrichment of Al, Fe and Ti and depletion of Si, forming ferruginous and bauxitic duricrusts. Although this process is more common in the inter-tropical belt, bauxitic duricrusts also occur in other climatic regions. To relate the environment influences weathering in distinct climatic region, we studied profiles developed on igneous rock from two sites: Pitinga, located in the northwest Brazilian Amazonia, and in Boddington, in southwestern Australia. Both sites are in long term stable tectonic Precambrian shields and during the last 100 Ma northwest Brazilian Amazonia experienced warm and seasonal low latitude conditions, whereas Australia migrated from a cool climate in high latitudes to the drier modern conditions in the lower latitudes. Under these distinct climate conditions both sites have similar regolith horizons upward consisting of saprolite, hard bauxitic zone and pisolitic duricrust. However, covering these horizons, Pitinga has a clayey topsoil, whereas Boddington has a loose covering of lag. These important differences in the upper bauxite zone in both sites concern a second aluminization which reflects the superposition of subsequent intense leaching process but as consequence of the contrasting modern climatic conditions: tropical warm and humid at Pitinga and a Mediterraean climate at Boddington. This modern climate in both sites has caused more extensive differences in the weathering product than the time and condition necessary for the initial bauxite development.