Prehnite from the Kalahari manganese field, South Africa, and its possible implications

Prehnite is described herein from the N'Chwaning II mine, Kalahari manganese field for the first time. The habit and composition are atypical for the species. The mineral occurs as elongated, prismatic orthorhombic yellow-orange crystals arranged in sprays and/or aggregates. It is associated with hydroxyapophyllite, datolite, ferroan inesite, calcite and pectolite. SEM, microprobe and XRF analyses reveal typical chemical compositions in terms of SiO₂, Al₂O₃ and CaO values, but the mineral has very low FeO and Na₂O contents, and MnO (±0.4 wt%) is an order of magnitude higher than published values for prehnite. Stable (H and O) isotope data for structurally-bound H₂O are consistent with a meteoric origin for the precipitating fluid. The existence of prehnite in the Kalahari manganese field provides evidence in support of low-temperature (150-250°C) formation of the mineral and possible metamorphism in the north-westernmost portion of the field, but its importance in terms of processes involving metal upgrading in the Kalahari manganese ores is uncertain.