Redistribution and fractionation of U, Th and rare-earth elements during weathering of subalkaline granites in SW Cape Province, South Africa

Abstract

Major and trace element behaviour during initial and advanced weathering stages in a weathering profile and saprolith developed on two subalkaline granites respectively indicated that Ca, Na and Sr are preferentially removed. Fe²⁺, K and Y are also lost, followed by P, Mg, Rb and Si. Zr and Nb remained constant relative to Ti. Mn, Fe³⁺, Al, Ga and Th initially increase eventually decreasing during advanced stages of weathering.

U, Th and the REE behaviour could be related to the primary and secondary mineralogy of the granites. The HREE are removed in solution, while the concentrations of LREE are significantly increased relative to HREE during early weathering stages. Ce is initially slightly depleted relative to other LREE, but positive Ce animalies develop during advanced stages of weathering in the profile. Previous hydrothermal alteration of the granite is largely responsible for the behaviour of REE, U and Th during weathering. U and Th values increase with increasing oxidation. U correlates with Fe³⁺ adn Th with Mn and Fe³⁺.