ELEMENT CONCENTRATION AND FORMATION OF ORE-DEPOSITS BY WEATHERING

Most important supergene ore or mineral deposits are those of Al, Fe, Mn, Cu, Ni, Co, an residual minerals: Au, PGE, REE, Nb, W, Sn, Cr, Ti- minerals, phosphates, diamond. During earth history. directed trends o f weathering are observed which depend on the following parameters: te ctogenesis and morphogenesis, and evolution of the atmosphere and the biosphere. Several - at least three - cycles of weathering can be dist inguished during Early Precambrian, Late Precambrian-Paleozoic, and Me so-Cenozoic times. The weathering cycle of the Mesozoic-Cenozoic times is described here. Each weathering cycle starts under special world-w ide conditions which are marked by monotonous and flat relief, by gree nhouse effect, and by an initial lateritic sequence followed by increa sing differentiation of the weathering sequences. The initial lateriti c weathering provokes the main process of rock decomposition developin g profiles with a thickness of several tens of metres. The vertical an d lateral variations of the profiles and, therewith, the quality patte rn of ore deposits mainly depend on: 1. morphology and drainage during the initial weathering; low drainage activity which causes very thick saprolites with a lower smectitic and an upper kaolinitic zone. Optim al drainage restricts saprolite evolution favouring the formation of t hick Box-horizons; 2. lateritic cementation by chemical displacement a nd neoformation of textures during a progressive stage of lateritisati on; 3. Mid-Tertiary to Quaternary changes of climate and morphology pr oduce a laterite degradation, followed by erosion and/or resedimentati on accompanied by mainly lateral, mechanical and/or chemical displacem ent of element or mineral concentrations. The evolution of laterites o n parent rocks with preconcentration of certain elements leads to thei r destruction and reorganisation of geochemical provinces in new super gene element associations by relative or absolute enrichment creating special supergene geochemical provinces.