THE TERRANE SETTING AND METALLOGENESIS OF THE ROSH PINAH MASSIVE ZN-PB SULPHIDE DEPOSIT IN THE GARIEP TERRANE OF SOUTHERN NAMIBIA

The Rosh Pinah massive Zn-Pb-Cu sulphide deposit in the Neoproterozoic Gariep Terrane of southern Namibia is related to intracontinental rifting and associated bimodal volcanics. The main volcanic complex comprises an acid, shallow-water/subaerial suite of magmas, pyroclastics and volcanosediments with a recently discovered cogenetic granite pluton. This provides a convenient heat engine for the driving of base-metal-bearing hydrothermal fluids.

The setting for the Rosh Pinah deposit is a failed rift-graben with a dominantly clastic fill sourced from the coeval volcanics. Periods of drowning and/or sediment paucity resulted in shale intercalations, one of which hosts the massive sulphide ore. Carbonate beds mark regressive phases and become dominant in the upper part of the succession as the graben became stuffed.

Further rifting occured to the present-day-west of the failed graben, developing into spreading and evolution of the Adamaster Sea with deposition of tholeiites and pelagic sediments. Passive-margin sedimentation occurred on the edge of the Kalahari Craton. Later subduction below the Rio de la Plata craton resulted in ocean re-closure.

Collision of the Kalahari and Rio de la Plata cratons occurred at about 540 Ma. The graben-bounding faults of the failed rift were reversed, pushing the graben-fill sedimentary wedge upwards. Part of the Adamaster sea floor was thrust over the passive margin. A proforeland basin with mollasse sedimentation developed at the leading edge of the Kalahari craton and marks the culmination of the Wilson cycle.

Metallogenesis of the Rosh Pinah Zn-Pb sulphide deposit is a function of hydrothermal fluid circulation driven by the Spitskop Volcanic heat engine. Base-metal-bearing brines and associated fluids were conduited up the graben-bounding fault zones and precipitated within the volcanic-derived, rift-fill sediments. The result is a hybrid VMS-SEDEX Zn-Pb deposit with replacement phases including silicification and carbonatisation.