Deep weathering features of the Tapira Carbonatite, Alto Paranaíba Igneous Province, Brazil

[摘要] Carbonatite-Alkaline intrusive complexes represent an important source of rare and critical commodities including REEs, P, Nb, U, Th, Cu, Fe, Ti, Ba, F, Zr and others. Carbonatites are particularly susceptible to weathering in humid climates and under these conditions high tonnage, enriched lateritic deposits may develop, concentrating ‘immobile’ trace metals by up to 100 times from the source rock. The physical and chemical mechanisms which facilitate such extreme enrichment under weathering conditions are not well understood.The Tapira Carbonatite Complex is one of a large suite of alkaline-silicate and carbonatite complexes within the well-endowed Alto Paranaíba Igneous Province (south eastern Brazil) containing economic deposits of REEs, titanium, phosphate, in addition to accounting for approximately 85% of the world’s niobium supply. The Tapira Complex is host to an extensive weathered profile (100m), in which phosphates are enriched in the lower saprolite and Ti oxides are enriched in the upper saprolite. The weathering processes responsible for the fractionation of these elements are unknown. The mineralogical, textural, and chemical features of the fresh rock carbonatite-alkaline silicate, lower P enriched saprolite and upper Ti enriched saprolite were examined and a paragenetic sequence of weathering processes proposed.Textural and mineralogical features of the weathering profile support oxidising acidic meteoric fluids. In the lower saprolite these fluids dissolved large quantities of carbonate and altered silicate minerals to clay, without corroding apatite, leading to an accumulation of phosphates in this horizon through compaction. Ti was observed to be mobile in the weathering profile, inferred from the decrease in volume of Ti-bearing perovskite in the lower saprolite unit, relative to the fresh rock, and the high volume of precipitated anatase in the upper saprolite unit. The nature of the fluids which facilitated the transport of Ti in the weathering profile remains uncertain.

[发布机构] CSIRO

[效力级别] Geomorphology and Regolith and Landscape Evolution [学科分类] 地球科学(综合)