A review of some problems of African carbonatites

Publisher Website
Google Scholar
Abstract
Summary: Carbonatites ar known at many places in eastern and central Africa, from near Mount Elgon in Uganda to Spitzkop and Palabora in nort-eastern Transvaal. They reange in age from pre-Karroo, post-Waterbag in the south to Miocene—Pliocene in the north. Owing to this great age difference denudation has revealed carbonatites at various levels of erosion, the deepest corresponding perhaps with the carbonatites exposed at Alnö in Sweden and Fen in Norway. The composition and structure of the carbonatites, the associated alkaline igneous rocks, and the altered country rocks known as fenites are all described. The carbonatites are mainly pure calcite but some are ankeritic and dolomitic and, locally, sideritic and manganiferous. They carry characteristics accessory minerals, particularly pyrochlore. The associated igneous rocks are ijolites with, less frequently, nephelinesyenite and at some centres pryoxenite. Fenitization results in the formation of aegirine-fėlspar rocks, nearly pure felspar rocks and felspathic breccia. Evidence as to chemical changes involved in fenitization is not always consistent, but addition of K or Na, or both, and loss of SiO 2 are satisfactorily demonstrated. Current theories are reviewed. The carbonatites are believed to owe their origin to concentration of carbon dioxide of or carbonatitic fluid of magmatic origin, derived perhaps from pyroxenite highly charged with volatiles, among which carbon dioxide played the most important part, associated with phosphoric acid, fluorine, water etc., and in which the elements niobium and cerium, among others, were also concentrated.