The origin of early Archean barite deposits on the Kaapvaal and Pilbara cratons

Sulfate minerals are rare in the geological record prior to the oxygenation of the Earth’s atmosphere circa 2.4 billion years ago (Ga). An exception to this are a few isolated occurrences of early Archean (3.6-3.2 Ga) barite (BaSO4), hosted in volcano-sedimentary rocks in South Africa, India and Western Australia. The origin of these barite deposits is controversial, despite having been studied over decades. Here, we combine field observations and geochemical data from a multi-year investigation into barite occurrences on the Kaapvaal and Pilbara cratons to derive a holistic model for the formation of early Archean barite. Studied deposits include the 3.52 Ga Londozi deposit in Eswatini and the 3.49 Ga North Pole deposit in Western Australia that are hosted in volcanic rocks, and the 3.26-3.23 Ga Barite Valley deposit in South Africa and possibly time-equivalent but little-known Cooke Bluff deposit in Western Australia that are found in sedimentary successions. Our field observations indicate that barite is closely associated with chert on both the Kaapvaal and the Pilbara cratons, although the scale of barite mineralization is much larger in the Pilbara and cross-cutting barite veins are only observed at North Pole and Cooke Bluff. These findings suggest that the fluids from which the chert precipitated are the same as the fluids from which the barite formed, and geochemical data support an origin for these barium-rich fluids that is related to low-temperature hydrothermal circulation of seawater. Barite precipitation could have been triggered by silica removal from these fluids. The ubiquity of chert in the early rock record suggests that these settings may have been common in the early Archean and that barite formation was therefore limited by sulfate abundance, and could only occur in settings where hydrothermal circulation and local sulfate enrichment occurred together.