Geochemical evolution of a Devonian diaspore-crandallite-svanbergite-bearing weathering profile in the Middle Timan, Russia

A diaspore-crandallite-svanbergite-bearing occurrence in the Timan folded a rea of Russia is underlain by Riphean dolomite that bears dispersed francol ite and pyrite. The Devonian weathering profile reaches a thickness of 27 m . It contains francolite, crandallite, svanbergite, diaspore and berthierin e. Sericite, goethite and kaolinite are less common. There is no distinct b oundary between bauxite and phosphate zones. Crandallite and svanbergite of ten appear together with diaspore so that the P2O5 content in the bauxite z one reaches 10%. The Al2O3 content increases upwards through the section fr om similar to 7 to similar to 55%. The content of Sr varies from 0.02 to 5. 2%. Factor analysis (R-mode principal components) and other statistical met hods have been used to identify mineral and element associations and proces ses of mineral formation. Trace elements tend to associate with diaspore (T h, Pb, Y, Zr), svanbergite (Ba), phosphates (U), berthierine (Cr, V, Ni, Co ) and sericite (Rb); accessory minerals are hosts for Ti, Cr, V and Zr. Fra ncolite was inherited from the underlying rock during dissolution of dolomi te and accumulated in the lowest part of the weathering profile. Diaspore, goethite, crandallite and svanbergite were formed during dolomite leaching and weathering of phosphate and dispersed aluminosilicates. Mg, Ca and Sr d erive from the dolomite, and the source of Fe and S was oxidized pyrite. Be rthierine and kaolinite are diagenetic minerals. An epigenetic supply of al kali elements (K and Rb) was responsible for sericite formation. (C) 1999 E lsevier Science B.V. All rights reserved.