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

Abstract

A diaspore–crandallite–svanbergite-bearing occurrence in the Timan folded area of Russia is underlain by Riphean dolomite that bears dispersed francolite and pyrite. The Devonian weathering profile reaches a thickness of 27 m. It contains francolite, crandallite, svanbergite, diaspore and berthierine. Sericite, goethite and kaolinite are less common. There is no distinct boundary between bauxite and phosphate zones. Crandallite and svanbergite often appear together with diaspore so that the P 2O 5 content in the bauxite zone reaches 10%. The Al 2O 3 content increases upwards through the section from ∼7 to ∼55%. The content of Sr varies from 0.02 to 5.2%. Factor analysis ( R-mode principal components) and other statistical methods have been used to identify mineral and element associations and processes of mineral formation. Trace elements tend to associate with diaspore (Th, 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. Francolite was inherited from the underlying rock during dissolution of dolomite 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 derive from the dolomite, and the source of Fe and S was oxidized pyrite. Berthierine and kaolinite are diagenetic minerals. An epigenetic supply of alkali elements (K and Rb) was responsible for sericite formation.