Li, B-rich Rhaetian metabauxite, Tuscany, Italy: reworking of older bauxites and igneous rocks

Abstract

Li, B-rich metabauxite layers occur in two outcrops of talus metabreccias intercalated between Norian and Liassic marbles in the Alpi Apuane, Tuscany, Italy. Field evidence, petrographical, geochemical, and paleogeographic, paleoclimatologic considerations suggest that bauxitic material was derived from a multistage reworking of Late Carboniferous–early Autunian primary deposits now completely removed. The identified lithotypes are: cookeite-rich metabauxite, cookeite-bearing metabreccia, chloritoid schist and chloritoid phyllite. The Mg content of calcite, and the Si content of muscovite and mineral assemblages indicate temperatures of 350°–380°C, pressures of <7 kbar and f O 2 -values of 10 −30 to 10 −24 for the Alpine metamorphism. In comparison with the average Mesozoic bauxite of Bronevoy et al. (1985)the metabauxite is: strongly enriched in Si, K, Ca, Mg, Li, B, Be, Ni and Ce; it shows comparable contents of Co, La, Eu, Tb, Ta and Th; it is depleted in Ga, Cr, V, Zn, Mn, Hf, U; and is strongly depleted in Sr, F, Cu. A metabauxite layer shows decreasing abundance of Fe and Ga from bottom to top with a pattern opposite to that of primary bauxite deposits (inverted profile). The strong enrichment of Li and B is not related to evaporitic conditions, since the MgO, Na 2O and Sr contents are too low, and indicator minerals such as dravitic tourmaline, albite and scapolite are missing. During the Permian, volcanic products similar to the coeval peralkaline–peraluminous rocks of Corsica–Sardinia were widespread in Tuscany. B, Li-rich minerals were produced by circulation of hot hydrothermal fluids within the Permian volcanic rocks during and soon after their emplacement. The erosion of such hydrothermal products supplied detrital material to the talus metabreccia.