Bauxites

Bauxites are residually enriched rocks, mostly composed of Al-hydroxides (gibbsite, boehmite and diaspore), kaolinite, Fe-oxy-hydroxides and Ti-oxides, which form from intense lateritic weathering of aluminosilicate protoliths. Lateritic weathering and bauxite formation are generally favored by largely humid, tropical and subtropical climates. Bauxites are industrially important for both Al metal supply and non-metallurgical use and they represent the only raw material used in the production of alumina on a commercial scale. Gallium is a common by-product of Al extraction from bauxites, and they may also be considered as a possible future resource for REE. Bauxites are classified into: lateritic bauxites and karst bauxites. Lateritic bauxites occur within a lateritic profile directly overlying the original alumosilicate-rich lithotype (e.g., clay-rich sandstones, gneisses, granites, dolerites and gabbros). Karst bauxites are bauxite deposits that directly overlie a carbonate bedrock and do not exhibit a typical lateritic profile. With few exceptions (clayey/marly carbonate rocks) these deposits commonly lack an “unweathered” protolith at their base. Lateritic bauxites are generally located in regions currently characterized by tropical to sub-tropical climates and, more extensive examples started to form in Late Cretaceous times, experiencing peaks of intense lateritic weathering in the Paleocene-Eocene and the Middle Miocene. The majority of karst bauxites are located in the northern hemisphere, in most cases outside regions currently characterized by tropical to sub-tropical climates. This is because they represent fossil bauxites, formed in past geological periods (e.g., Paleozoic or Mesozoic) when more favorable climatic conditions prevailed.