Ce anomalies and trace element distribution in Sardinian lithiophorite-rich Mn concretions

Abstract

Supergene concentrations of lithiophorite found in alluvial conglomerates and residual sandy clay deposits of Messinian age in northern Sardinia, were investigated for minero-chemical composition and fractionation processes. The Mn concentration occurs as cement in conglomerate, coatings on pebbles and as concretions in clays. In the mineralisation, some transition metals including Ni, Cu, and Zn along with Ba, U, Pb, and REEs, are enriched relative to the composition of the upper continental crust. Zn and Ni co-vary with Mn, suggesting that they are hosted in lithiophorite. Also the REEs, with the exception of Ce, co-vary with Mn. Ce, due to its redox chemistry, fractionates relative to the other REEs and precipitates as cerianite during Mn4+ reduction. Mn mineralisation has a different Ce signature as expressed by the Ce anomaly, which varies between 0.22 and 11.75. A two-stage model was proposed for explaining the Ce anomaly fluctuations. Precipitation of the samples with positive Ce anomalies occurred in the first stage from organic matter-free fluids which favored the oxidation of cerium on the Mn phase surface; in the second stage, instead, Ce-depleted solutions, resulting from the first stage, were responsible for the precipitation of samples with negative Ce anomalies.