Supergene alteration since the upper cretaceous on alkaline igneous and metasomatic rocks of the Poços de Caldas ring complex, Minas Gerais, Brazil

This contribution presents field evidence, mineralogical observations and geochemical data of different modes of lateritic weathering which affected the alkaline magmatic rocks and their metasomatic alteration products since the Upper Cretaceous. Field relations demonstrate that ferralitic (bauxitic) weathering is confined to well-drained hill crest and upper slope settings. In the Poços de Caldas area bauxite normally rests directly on parent rock. Stagnant drainage and reducing conditions beneath tropical peat deposits, along valley floors and in lower slope situations favour sialitic (saprolitic) weathering. The additional importance of high porosity and permeability is demonstrated by co-occurring parent rocks with strongly contrasting response on weathering attack. In normal nepheline syenites and phonolites nepheline breaks down first and nearly instantly upon weathering. This results in high porosity, giving water and atmospheric gases easy access to the more resistant minerals. Thus in well-drained morphological positions bauxite is generated. If the same original magmatic parent rocks are “shielded” against pervasive weathering attack by pre-lateritic metasomatic alteration, no comparable early permeability arises. A fine-grained and dense metasomatic assemblage of kaolinite, illite, microcline and smectite, combined with the annealing of fractures and joints, is able to prevent bauxite generation, and saprolite is formed instead. This metasomatically induced saprolite is significantly richer in illite than “normal” saprolite. Bauxite formation results in a strong depletion of Mg, Ca, Na, K, Si, Ni, Co, Cu, Zn, Rb, Ba, Sr, Y and REE. Sialitic (saprolitic) weathering leads to a generally less pronounced depletion of essentially the same elements. Silicon and Ni are more efficiently retained than in bauxite. Mobile elements are Mn, Ca, Na, K, Mg, Co, Zn, Rb, Ba, Sr and REE. Cerium can be re-precipitated under oxidizing local conditions. Closely associated positive and negative Ce-anomalies are due to inhomogenously concentrated precipitates in small-scale spatial alternation with zones of nearly total depletion. A general correlation between the type of laterite (saprolite or bauxite) and Ce-retainment or depletion does not exist. Host phases for most of the immobile trace elements are goethite and related plasmas in bauxites and saprolites. Immobility is exhibited by V, Pb, Ga, Nb, Zr, Th and U. Relict zircon is too rare to explain a seemingly retarded HREE-loss in saprolites.