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Ore-Forming Processes Related to Lateritic Weathering Available to Purchase
Abstract Supergene lateritic deposits have played an important role in the global mineral resource economy for over 50 years, with lateritic Al, Fe, Ni, and Au deposits having a significant input to global metal production and reserves. Significant scientific research started in the 1970s, focusing initially on bauxites and, subsequently, on Ni laterites. In the 1980s, attention turned to Au, Fe, and Mn deposits with most research efforts focused on supergene Au deposits, because this also had implications for exploration of primary mineralization. Most recently, research has focused on the formation of lateritic P-Nb deposits. This paper reviews the knowledge acquired, mostly during the past two decades, on the processes of metal enrichment during lateritization. The first section deals with lateritic bauxite deposits. Three type of bauxitic profiles are distinguished: (1) orthobauxites, the product of a single weathering phase; (2) metabauxites, which are very aluminous and result from the transformation of orthobauxites due to a change to less humid conditions; and (3) cryptobauxites, which are bauxites concealed by a clay-rich cover, typical of highly humid Amazonian conditions. The bedrock and geomorphology also influence the formation of bauxite deposits. The second section presents a classification of Ni lateritic deposits, describing three principal types: (1) oxide deposits dominated by Fe oxyhydroxides, with Ni mainly hosted in goethite; (2) hydrous Mg silicate deposits, dominated by the Mg-Ni silicates “garnierite”in the saprolite; and (3) clay silicate deposits, dominated by Ni-rich smectites. Clay silicate deposits represent 5 to 10 percent of global resources, with the remainder divided about equally between hydrous Mg silicates and oxides. Controls on the formation of Ni lateritic deposits include bedrock lithology, tectonic setting, age of weathering, paleoclimatic history, and the geomorphology. Lateritic iron deposits can be divided in two subtypes: (1) residual lateritic iron ores, developed normally on banded iron formation but submitted to a lateritic weathering that increased the goethite content; and (2) the channel iron deposits. The latter formed by the accumulation of fluvial sediments, during the Tertiary, in paleochannels incised into a ferruginized surface of Precambrian bedrock. Lateritic gold deposits can be classified according to the distribution of Au in the profile, which is generally dependent on the paleoclimatic history of the regolith. In savanna environments, Au has mainly accumulated residually, although it is partly leached in the ferruginous duricrust. Most deposits appear to have been initially of this type but have been modified following climatic changes. In humid tropical rain forest regions, the duricrust becomes degraded and Au may be enriched in the upper ferruginous horizons of the saprolite. In semiarid environments, postlateritic remobilization has occurred through the dissolution of Au by acid and saline ground waters, resulting in Au enrichment deeper in the profile. Lateritic phosphate and niobium deposits form from apatite-rich carbonate rocks. There are two types, depending upon the bedrock, namely, sedimentary phosphate, developed from mainly marine carbonates; and igneous phosphates, derived essentially from carbonatites. The dissolution of carbonates under humid tropical climates causes a large volume reduction and residual accumulation of less soluble elements, forming very specific weathering profiles. Given the long time frames under which the deposits have formed and evolved and their wide distribution, the mechanisms of formation of lateritic deposits must be considered in a global perspective. This paper addresses the main features of the deposits and the lithological, geomorphological, and paleoclimatological processes that have led to their formation.