The 3.53 Ga St James Volcanogenic Massive Sulfide Deposit, Kaapvaal Craton: Links to Submarine Rhyolites, Not to Komatiites

Abstract

The Nondweni greenstone belt is located in the southeastern region of the Kaapvaal craton in South Africa and contains significant volumes of 3.53 to 3.45 Ga mafic-ultramafic lava flows (komatiitic and tholeiitic basalts). Minor felsic volcanic rock units, such as massive rhyolites and deformed quartz-feldspar-mica schists (felsic schists), also occur and are demonstrably linked to small base metal sulfide orebodies, interpreted here as volcanogenic massive sulfide (VMS)-type mineralization (e.g., sphalerite, chalcopyrite, pyrrhotite, pyrite, galena, acanthite). Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon analysis yields a rhyolite eruption age of 3531.91 ± 0.46 Ma for the felsic volcanic rock unit that hosts VMS-type mineralization at the St James deposit, which renders this Zn-Cu-Pb-Ag mineralization among the oldest preserved of its kind, offering insights into ore-forming processes that took place on and below the Paleoarchean sea floor. Rare earth element geochemical modeling suggests that the felsic volcanic rocks formed by moderate degrees of partial melting of hydrothermally altered basalts similar in composition to those from the Nondweni greenstone belt. Regarding ore formation, we envisage a scenario where basaltic ocean floor, in close proximity to a back-arc spreading ridge, had been intensively altered and subjected to elevated temperatures, which facilitated localized melting at low pressures (<2 kbar), resulting in the production of rhyolitic magmas accompanied by hydrothermal sulfide deposition. This model is supported by evidence from multiple sulfur isotope data (δ34S and Δ33S), which demonstrates that the basaltic rocks contain unfractionated magmatic sulfur and the rhyolitic rocks contain sulfur sourced from altered oceanic basalts. In contrast, the rhyolite-associated VMS-type mineralization records even more complex sulfur interactions, including contributions from surficial mass independent fractionated sulfur isotopic components; that is, these base metal sulfide ores exhibit a negative sulfur mass-independent fractionation signature of –0.53‰ Δ33S.