Barite precipitation was associated with low temperature hydrothermal venting in the 3.2 Ga Mapepe Formation, Barberton Greenstone Belt

Sulfate minerals are rare in the geological record prior to 2.2 Ga. This is consistent with prevalent anoxic conditions in the Earth’s surface environment prior to the great oxidation event, and correspondingly low levels of sulfate in the Archean ocean. In contrast to this general scarcity, extensive barite (BaSO4) deposits are found in sedimentary rocks of Paleoarchean age from 3.4 to 3.2 Ga. The origin of these barite minerals and barite-rich rocks has remained controversial to date. The sulfate in the barite was derived from volcanic aerosols as shown by multiple S isotope signatures distinctive for atmospheric photolysis [1]. This is thought to have mixed with hydrothermally-sourced barium to create primary seafloor precipitates [2]. The origin of the barite is important since it is a potential archive of variations in seawater chemistry. Here we explore the chemistry and mineralogy of barite deposits from the 3.2 Ga Mapepe Formation of the Barberton Greenstone Belt. Barite occurs here in close association with chert in the remnants of hydrothermal spring systems, and in a range of crystalline forms from bladed to granular [2]. Many beds of barite are reworked and contain detrital chromite, pyrite and Fe, Ti oxide phases. We used electron microprobe and Laser ablation ICP-MS analysis to determine the major and trace element composition of major lithologies in the spring systems. We used a range of trace element and stable isotope data to model the relative contributions of seawater and hydrothermal fluid required for barite deposition. Our results show that, depending upon the chosen mineral and lithological association, it is possible to obtain estimates of seawater composition for some elements and radiogenic or stable isotope systems. [1] Roerdink, D.L., Mason, P.R.D., Farquhar, J., Reimer, T. 2012. Multiple sulfur isotopes in Paleoarchean barites identify an important role for microbial sulfate reduction in the early marine environment. Earth and Planetary Science Letters, 331-332, 177-186. [2] Lowe D.R, Drabon, N., Byerly, G.R., 2019. Crustal fracturing, unconformities, and barite deposition, 3.26–3.23 Ga, Barberton Greenstone Belt, South Africa. Precambrian Research, 327, 34-46.