Rare Elements—Markers of the Formation Setting of Manganese and Iron Ore Deposits in the Kalahari and Postmasburg Fields (South Africa): Communication 1. Kalahari Manganese Field

Resumo

Manganese ores of the Lower Proterozoic Hotazel Formation (Transvaal Supergroup) associated with the banded Fe-silicites are marked by high concentrations of several rare elements (B, Ge, W, Mo, Cr, Ni, Zn, Cd, Pb, Ag, Bi, As, Sb, Te, Se). High boron contents in the oxide‒carbonate ores (Mn-lutites) are attributed to the chemosorptional concentration of this element on Mn-carbonates. Owing to hydrothermal transformations, a wide range of the ore-forming (mainly Fe and Mn) and rare elements (REE included) was removed from the underlying hyaloclastic basaltic andesites of the Ongeluk Formation. Manganese ores and Fe-silicites are characterized by the typical values of the cerium (Ce/Ce* 0.28–1.72) and europium (Eu/Eu* 0.57–16.31) anomalies that can suggest that primary sediments were deposited in a marginal shallow-marine basin with a prominent oxic surficial water layer and subanoxic conditions near the floor. At different stages of lithogenesis, metalliferous (Mn, Fe) sediments of the shallow-water basin were enriched in Eu (positive Eu/Eu*) and subjected to metasomatism (with the redistribution of Mn and the formation of manganese carbonates) and the consequent regional metamorphism (up to the stage of green sericite schists).