SUPERGENE ALTERATION SINCE THE UPPER CRETACEOUS ON ALKALINE IGNEOUS AND METASOMATIC ROCKS OF THE POCOS 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 cres t and upper slope settings. In the Popes de Caldas area bauxite normal ly rests directly on parent rock. Stagnant drainage and reducing condi tions beneath tropical peat deposits, along valley floors and in lower slope situations favour sialitic (saprolitic) weathering. The additio nal importance of high porosity and permeability is demonstrated by co -occurring parent rocks with strongly contrasting response on weatheri ng attack. In normal nepheline syenites and phonolites nepheline break s down first and nearly instantly upon weathering. This results in hig h porosity, giving water and atmospheric gases easy access to the more resistant minerals. Thus in well-drained morphological positions baux ite is generated. if the same original magmatic parent rocks are ''shi elded'' against pervasive weathering attack by pre-lateritic metasomat ic 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 abl e to prevent bauxite generation, and saprolite is formed instead. This metasomatically induced saprolite is significantly richer in illite t han ''normal'' saprolite. Bauxite formation results in a strong deplet ion of Mg, Ca, Na, K, Si, Ni, Co, Cu, Zn, Rb, Ba, Sr, Y and REE. Siali tic (saprolitic) weathering leads to a generally less pronounced deple tion of essentially the same elements. Silicon and Ni are more efficie ntly 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 oxidiz ing local conditions. Closely associated positive and negative Ce-anom alies are due to inhomogenously concentrated precipitates in small-sca le spatial alternation with zones of nearly total depletion. A general correlation between the type of laterite (saprolite or bauxite) and C e-retainment or depletion does not exist. Host phases for most of the immobile trace elements are goethite and related plasmas in bauxites a nd 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 i n saprolites. (C) 1997 Elsevier Science Ltd.