Alcoa World Alumina and Chemicals (AWAC) is an unincorporated global joint venture between Alcoa Corporation and Alumina Limited. Alcoa Corporation owns 60% and Alumina Limited owns 40% of these entities. Alcoa provides the operating management for AWAC. AWAC entities' assets include the 100% interests of the Juruti bauxite deposit and mine in Brazil.
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Summary:
The Juruti area is located in the lower part of the Amazonian basin, south of the Amazon River, between the Guyana and Brazilian Shields. Parent deposits which form the base ofthe bauxite sequence belong to the Alter-do-Chao Formation comprising continental sedimentary deposits that accumulated in a fluvial-lacustrine environment and consist ofsandstones, siltstones, mudstones, and quartz breccias. Bauxites are known to have formed during intense lateritic alteration of the Cretaceous (145 to 66 million years ago, Ma)siliciclastic parent deposits which is estimated to have occurred during the Eocene (56 to 34 Ma). Cretaceous deposits were later covered by tropical soils as a product of rootactivity resulting in a kaolinitic and alumina-goethite deposited during flooding events in the Miocene (23 to 5 Ma).
The evolution of bauxite in this region is generally accepted to have occurred through a combination of intense weathering and geochemical alteration, leaching by meteoricwaters, and accumulation of alumina and iron-rich horizons, in addition to periodic erosion and redeposition of upper horizons. These horizons form gently undulating plateausranging from 100 m to 170 m above the level of the Amazon, surrounded by drainage erosion channels. While these plateaus cover extensive areas both north and south of theAmazon, not all are bauxitic, despite being within the same geomorphological and climatic area on top of the same sedimentary formation.
Cretaceous deposits were later covered by tropical soils as a product of rootactivity resulting in a kaolinitic and alumina-goethite deposited during flooding events in the Miocene (23 to 5 Ma). The evolution of bauxite in this region is generally accepted to have occurred through a combination of intense weathering and geochemical alteration, leaching by meteoricwaters, and accumulation of alumina and iron-rich horizons, in addition to periodic erosion and redeposition of upper horizons. These horizons form gently undulating plateausranging from 100 m to 170 m above the level of the Amazon, surrounded by drainage erosion channels. While these plateaus cover extensive areas both north and south of the Amazon, not all are bauxitic, despite being within the same geomorphological and climatic area on top of the same sedimentary formation.
Deposit Types
Bauxite deposits, sedimentary deposits with economic concentrations of aluminum oxide, represent the world’s major source of aluminum and consist primarily as the mineralsgibbsite (Al(OH3)), boehmite, and diaspore, and commonly found alongside iron oxide minerals including goethite and hematite, kaolinite clay minerals, and minor concentrationsof titanium oxide minerals such as anatase and ilmenite.
Bauxite formation is widely known to occur through two main depositional mechanisms:
• Lateritic bauxite: formed through intense chemical weathering and accumulation of residual and transported material on top of aluminosilicate-rich parent rocks.The Juruti deposit is classified as a lateritic bauxite deposit.
• Karstic bauxite: formed on top of carbonate / paeleokarstic surfaces and karst depressions by the accumulation of aluminosilicate-rich clays at the time of chemicalweathering and dissolution of carbonate rocks.
Mineralization
Bauxite mineralization principally occurs as microcrystalline gibbsite (Al(OH)3), along with accessory minerals of hematite (Fe2O3), goethite (FeO(OH)), kaolinite (Al2Si2O5(OH)4),and anatase (TiO2). . The mineralization of each of the stratigraphic horizons observed across the Juruti deposits are classified based on a combination of visual inspection during drilling and sampling, and the results of chemical analysis.
The stratigraphy is comprised of, with increasing depth, a mottled clay horizon at the base topped with a massive bauxite layer, overlain by a ferricrete crust with hematite andgibbsite nodules, and an overlying yellow clay at surface. In comparison to their lateral extent over tens of kilometers, the overall thickness of the bauxite deposits is relativelythin with the depth of drilling typically in order of 20 m.