A Multistage Genetic Model for the Metamorphosed Mesoproterozoic Swartberg Base Metal Deposit, Aggeneys-Gamsberg Ore District, South Africa

Abstract

The polymetamorphosed Swartberg Cu-Pb-Zn-Ag deposit in the Namaqua Metamorphic Province of South Africa is a major metal producer in the region, yet its genesis remains poorly understood. The deposit comprises several stratiform to stratabound units, namely the Lower Orebody and Dark Quartzite, the overlying Barite Unit, and the Upper Orebody, all of which are folded by an F2 isoclinal syncline and refolded by an open F3 synform. A discordant Garnet Quartzite unit surrounds the Upper Orebody in the F2 hinge, where it overprints the Lower Orebody and Barite Unit. The Lower Orebody comprises sulfidic, pelitic lenses with fine-grained pyrite, sphalerite, galena, and lesser pyrrhotite, hosted by sulfide-poor but magnetite- and barite-bearing siliceous rock. The overlying Barite Unit is poorly mineralized and grades from massive magnetite-barite close to the F2 hinge to distal laminated baritic schist and quartzite. The Dark Quartzite is the stratigraphic equivalent of the Lower Orebody and Barite Unit but comprises siliceous quartzite and schist, with lenses of conglomerate and minor Fe-Mn-Zn phases. The Upper Orebody displays rapid zonations from massive magnetite-rich iron formation in the F2 hinge, rich in coarse galena, pyrrhotite, and chalcopyrite, to sulfide-poor, magnetite-bearing schist and quartzite. The Garnet Quartzite is dominated by quartz and almandine garnet and mineralized with pyrite and chalcopyrite. Geochemical discriminant plots show that the Lower Orebody has a significant detrital component, whereas the Upper Orebody and Barite Unit are strongly zoned, with the greatest chemogenic component close to the F2 hinge. This corresponds to a deposit-scale metal zonation from the Cu-rich F2 hinge to more Pb- and then Zn-dominated areas. Mineral assemblages and paleoredox proxies suggest generally oxic conditions, with a more reduced signature close to the hinge and in the sulfidic Lower Orebody lenses. The Lower Orebody is interpreted as a mixed chemogenic-pelitic unit, with sulfides deposited on or near the seafloor during stage 1 hydrothermal activity. The sulfidic lenses formed from fine mud and clay deposited in quiet seafloor depressions, in which warm, dense, reducing, Pb-Zn-Ba–rich stage 1 brines accumulated, while the siliceous portions formed from higher-energy clastic sediments on aerated seafloor highs. The Barite Unit forms a baritic cap to the Lower Orebody, while the Dark Quartzite is their shallower-water equivalent. Thereafter, clastic sediment with lesser hydrothermal input was deposited during stage 2a exhalations, forming the poorly mineralized portions of the Upper Orebody. During stage 2b hydrothermal activity, hot Cu-Fe–rich fluids invaded part of the Upper Orebody, creating the highly chemogenic protolith to the well-mineralized, magnetite-rich portion. Associated hydrothermal alteration in a discordant subseafloor feeder zone created the Garnet Quartzite protolith. The F2 hinge thus corresponds closely to the original vent zone. Swartberg therefore resembles a deformed and metamorphosed Selwyn-type sedimentary exhalative deposit, with both proximal- (Upper Orebody, Garnet Quartzite) and distal-style (Lower Orebody) mineralization. The close association of these styles suggests that differences in the mineralizing fluids and depositional environment, rather than proximity to a vent, determine the deposit style.

Pre‐Klondikean oxidation prepared the ground for Broken Hill‐type mineralization in South Africa

“…Independent support for supergene mobilization of base metals at a deposit scale can be derived from the base metal distribution at Black Mountain. There, both limbs of an Okiepian (1,210–1,180 Ma) F1 fold (Ryan et al., 1986) display a metal zonation from Cu‐rich (NNW) to Zn‐rich (SSE) (Cawood & Rozendaal, 2020). Stedman (1980) explained this by a hypothetical hydrothermal feeder zone that would have been fortuitously placed exactly in the position of the later fold hinge.…”

A classification of Broken Hill-type deposits: A critical review

“…11). Rozendaal et al (2017) and Cawood and Rozendaal (2020) proposed that each deposit (except Broken Hill and its downplunge extension, Broken Hill Deeps) formed in a sub-basin of a larger extensional basin. Using the nomenclature of Ryan et al (1986), the Bushmanland Formation at Aggeneys consists of the Aluminous Schist Formation (~80 m thick) at its base that is composed of quartzmuscovite-K-feldspar-sillimanite-biotite rock.…”

“…Metamorphosed exhalative and/or premetamorphic alteration units in the Bushmanland Group include the iron formations, barite-bearing rocks, quartz-garnet rocks, nodular sillimanite rocks, and gahnite-bearing rocks (e.g., Spry, 1987b;Stalder and Rozendaal, 2004;Rozendaal et al, 2017). Quartz-garnet rocks occur in the stratigraphic footwall of the Broken Hill and Swartberg deposits and likely served as the alteration zones to ore (e.g., Stedman, 1980;Cawood and Rozendaal, 2020). Zones of barite ore occur at Gamsberg but they occur in significantly smaller amounts in the Aggeneys deposits; it is absent in the recently discovered Broken Hills Deep deposit.…”

“…At Gamsberg, pyrite, pyrrhotite, marcasite, sphalerite and galena, and gangue minerals quartz, garnet, amphibole, alabandite, graphite, and pyroxene are the main constituents of the ore. Recently, Cawood and Rozendaal (2020) argued that the Swartberg deposit was a Selwyn-type metamorphosed SEDEX deposit rather than a BHT deposit. Rozendaal et al (2017) had previously proposed that Gamsberg was a metamorphosed SEDEX deposit.…”

Syn-metamorphic sulfidation of the Gamsberg zinc deposit, South Africa

“…gahnite) cannot be explained easily by this genetic model. Furthermore, various geochemical anomalies in the immediate vicinity of the ore district (e.g., Willner et al 1990) and several studies, which point to inconsistencies in the redox conditions of the deposits (e.g., Stalder and Rozendaal 2005b;Cawood and Rozendaal 2020), all suggest a more complicated metallogeny. An oxidation event affecting the whole ore district between the Okiepian (1210( -1180 and Klondikean (1040-1020 Ma) orogenic events (Clifford et al 2004) has been suggested based on Cu isotope data (Höhn et al 2020).…”