Rare-earth element geochemistry of supergene manganese deposits from Groote Eylandt, Northern Territory, Australia

Abstract

This study examines the rare-earth element (REE) geochemistry of manganese ores and associated rocks in the Mn-oxide deposit at Groote Eylandt in the Northern Territory of Australia. In vertical stratigraphic sections that have undergone lateritization, most REEs show a decrease with depth (except for Yb). Ce and Eu are the most mobile elements, exhibiting increasingly negative anomalies relative to the depletion of the others. Mineral normalization demonstrates that pyrolusite predominantly concentrates La, Ce, Eu, Nd, and Sm, and that goethite is strongly enriched in Gd. All other minerals are of minor importance in determining the REE composition of the bulk sample. The main laterite components show a distinct fractionation in the intermediate and heavy REEs. Kaolinite is depleted in the heavy REEs, whereas iron hydroxides are enriched in these elements. It is not yet possible to compare the REE composition of the ores directly with other deposits of similar origin and alteration history, because of a general lack of analytical data, but marine Mn-oxides from the Gulf of Bothnia (discoidal and flat concretions) display comparable REE patterns and concentrations. Also, similarities to diagenetic manganese oxide nodules are evident, which may indicate the existence of similar redox conditions during the formation of both deposits.