Placing further geochemical constraints on the genesis of the Gamsberg Zinc Deposit, Namaqualand Metamorphic Province, South Africa using sulphur isotope data

Abstract

The 1.6-2.0Ga Gamsberg deposit is a >200 Mt zinc reserve located in the Aggeneys-Gamsberg mining district in the Northern Cape Province of South Africa, which is host to a cluster of world-class base metal deposits. These four proximal SEDEX-type deposits were formed in an extensional basin, in which Gamsberg occupies the eastern side of the district. The Gamsberg deposit is characterized by an anomalous enrichment in manganese, which greatly complicates ore re?finement. Previous work has shown a high abundance and variability of Mn in sphalerite minerals throughout the stratigraphy at the West orebody of Gamsberg. This study goes beyond previous work by determining for the fi?rst time sulfur isotope ratio variability in individual sphalerite and iron sul?fide minerals with stratigraphic position through the deposit. LA-ICP-MS analysis of the Fe-sulfi?de- and sphalerite minerals in the samples from the West orebody revealed initially low d34S values around 0‰ that increased abruptly to high d34S values of up to +40‰ relative to Vienna Canyon Diablo Troilite (VCDT). This was interpreted as the source of reduced sul- fur for initial mineralization being decoupled from the source of reduced sulfur after initial mineralization, thermochemcial reduction of seawater derived sulfate. The strongly positive d34S values were interpreted as the result of Gamsberg developing later than the other Aggeneys deposits in the closed depositional basin. Manganese variability is interpreted as a fi?nite sediment enrichment as a distal expression of the other Aggeneys SEDEX deposits which was remobilized and progressively depleted during the formation of the Gamsberg deposit. Metamorphism and associated metasomatism during the Namaquan orogeny (1.06-1.03 Ga) were interpreted as a mechanism for introducing secondary sulfi?de minerals to ore units with an oxidized primary mineral assemblage and mobilizing manganese for incorporation in metamorphic gangue minerals.