Fluid evolution in and around the Rosh Pinah massive sulphide deposit in the external Pan-African Gariep Belt, Namibia

Abstract

A regional fluid inclusion and stable isotope (H, O, C) study of the external part of the Pan-African Gariep Belt, with emphasis on the Rosh Pinah Zn-Pb sulphide deposit, demonstrated that the bulk of those fluids present at the time of syn-tectonic peak metamorphism and subsequent uplift and exhumation are of aqueous composition with low salinity. Exceptions are few moderately saline inclusions that are limited to the tectonic contact with the pre-Gariep basement near the volcano-sedimentary Rosh Pinah Formation. These are explained by derivation from a nearby, locally developed lagoonal, possibly evaporitic, facies. The infiltrated host rocks largely controlled the composition of the syn-orogenic fluids and no evidence was found for a regional pervasive flux of orogenic brines through the external parts of the belt. Very rare pre-orogenic fluids discovered in Rosh Pinah mine samples display markedly different composition and density that are in line with a derivation from a volcanic source. These fluid inclusions are interpreted as remnants of the original ore fluid that infiltrated the host sediments and pyroclastic rocks at a temperature of around 400°C and a hydrostatic pressure of a few hundred bars. This infiltration led not only to intense brecciation and silicification of arenitic footwall and sulphide precipitation but also to the dolomitisation of original limestone host rock. The good preservation of the syn-rift Rosh Pinah ore bodies during the Gariep orogeny is explained by the lack of regional orogenic brine flow.