Description du livre
This book provides a comprehensive overview of the major supergene mineral deposits formed in intensely weathered lateritic terrains. It discusses both contemporary and pre-existing supergene deposits, describing their geological, mineralogical and geochemical characteristics. Supergene processes of enrichment are those that occur under ambient near-surface conditions, compared to hypogene processes mostly at depth under higher temperatures and pressures. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. Descending meteoric waters oxidize the primary (hypogene) minerals and redistribute the chemical elements. Residual supergene enrichment occurs as a physical process when the predominant rock-forming minerals oxidize and dissolve, concentrating ore elements hosted in resistant stable minerals; absolute chemical enrichment occurs when the ore elements themselves are leached and migrate in groundwater and precipitate due changes in the pH, oxidation potential and chemical composition of water. These processes can enrich commercially important elements to produce orebodies formed entirely by supergene processes. These include Al (bauxite), Fe ore, Ni-Co laterites, kaolinite, REE (clay deposits), Nb and REE (on carbonatites), base metals (secondary sulfides and oxidate minerals including gossans), gold and surficial U (in calcretes).