RELATIONSHIP BETWEEN NON-SULPHIDES AND SULPHIDES AT THE SKORPION ZINC DEPOSIT, NAMIBIA

The Skorpion non-sulphide zinc deposit is situated within the southern part of the Pan-African Gariep Belt in Namibia. The host rocks consist of Late Proterozoic metasediments and metavolcanics of greenschist metamorphic grade.

Sulphide mineralisation is hosted by feldspar-bearing quartz-biotite-schists and quartzites. The Skorpion sulphides form irregular, sharp to diffuse, millimetre to centimetre-thick layers of euhedral to 'subrounded' pyrite, in a matrix of sphalerite, and minor galena and chalcopyrite along foliation planes. Pyrite and sphalerite also occur as isolated sheared anhedral aggregates. The ore textures suggest a post-depositional recrystallisation during metamorphism.

Sphalerite, galena and chalcopyrite have been partly dissolved and secondary Zn-, Cu- and Pb-minerals (e.g. sphalerite, chalcocite, atacamite) have been formed in secondary pore spaces and along thin veins, crosscutting the foliation. The occurrence of sulphides terminates abruptly at the contact with the non-sulphide zinc mineralisation. Cube-shaped cavities and pseudomorphs of iron (hydr-)oxide after pyrite have been found. Thus, postmetamorphic oxidising fluids must have migrated through the rocks.

The non-sulphide ores consist of sauconite, smithsonite and hemimorphite, hosted by feldspar-bearing quartz-mica-schists. Sauconite replaces mica and/or feldspar, whereas smithsonite replaces calcite in metacalcarenites. Euhedral hemimorphite and smithsonite have also been found in fractures and secondary pore space. These non-sulphide zinc minerals show no metamorphic textures and thus post-date metamorphism.

The main non-sulphide ore-forming processes at Skorpion appear to involve: (1) in-situ oxidation of a sediment-hosted sulphide body; this is supported by the occurrence of oxide minerals within the sulphide ores and (2) mobilisation of zinc from sulphides and reprecipitation at a distance; this is supported by the presence of late-stage zinc-silicates and -carbonates.