Rare Elements—Markers of the Formation Setting of Manganese and Iron Ores in the Kalahari and Postmasburg Manganese Fields (South Africa): Communication 2. Postmasburg Iron and Manganese Field

Resumo

Deposits in the Postmasburg field, South Africa, confined to rocks of the Transvaal Supergroup, occupy a significant place in world resources of the manganese and iron ores. The ore deposits represent karst residual accumulations. Several elements (B, Cr, Ni, Zn, Ge, As, Se, Mo, Ag, Cd, Sb, Te, W, Pb, REE) in them make up an assemblage providing insight into the geochemistry of the processes of ore formation. Among these elements, Mo, As, Ag, and REE are most representative. Molybdenum is marked by the chemosorptional incorporative nature of accumulation in these ores, sometimes with the formation of epictic overgrowths of the ferrimolybdate-type minerals. Arsenic, which is leached from the substrate rocks and accumulated in karst ferromanganese and iron ores, reflects total impact of the dominating iron oxide minerals upon its mobility. The behavior of Ag is controlled by the processes of supergene alteration of Archean–Early Proterozoic carbonate rocks and ores of the banded iron formation (BIF). Comparison of the distribution of rare earth elements (REE) in the karst iron, ferromanganese, and manganese ores, as well as banded iron ores, demonstrates that these elements display similar values of the cerium (C/Ce*) and europium (Eu/Eu*) anomalies, but differ in terms of the HREE and LREE fractionation (typical values: Ce/Ce* = 0.7‒1.0; Eu/Eu* = 0.8‒1.1). Banded iron and associated manganese ores were accumulated in the marginal anoxic-dysoxic marine basin, which was constrained by the continental land. Anoxic and dysoxic conditions were created by an intense hydrothermal activity.