Geochemistry of trace elements in Paleozoic bauxite profiles in northern Russia

Abstract The data on trace-element distribution in eight Paleozoic bauxite deposits of the North of the East-European Platform are presented. Statistical methods are used in analyzing the data. The distribution and behavior of trace elements in bauxites are controlled by the following factors: 1. (1) Composition of parent rocks. Bauxites after acidic rocks are relatively enriched by felsiphile elements. On the contrary, iron-rich bauxites after ultrabasic rocks contain high amount of femaphiles. Bauxites after alkali rock can contain very high amount of Th, Nb, Zr and some other elements. 2. (2) Physico-chemical conditions of bauxite formation (pH, E h , drainage intensity etc.) have a significant effect on the mineral composition of bauxites and content of rock-forming elements. 3. (3) Mineral forms of elements in parent rocks. The same element can be concentrated in relatively insoluble accessory minerals and also dispersed in the main rock-forming minerals. The behaviour of elements during bauxitic weathering is determined by relative proportions of these different mineral phases. 4. (4) Chemical properties of elements, e.g. solubility, pH of hydroxide precipitation, possibility of complex formation etc. Sometimes very mobile element can become immobile in the presence of other elements (Sr in the presence of P). 5. (5) Diagenetic, epigenetic and recent transformations of bauxites. The redistribution of elements during such processes under reducing conditions or during hydrothermal alteration are considered. 6. (6) A carbonate footwall plays an important role, acting as a geochemical barrier to the removal of certain elements, resulting in the formation of hypergene carbonates or other minerals within a bauxite profile. The problem of temporal evolution of bauxite formation is also considered.