UC Davis

At about 2.65 Gya, the concentration of sulfide minerals increased in the Monteville Formation, Campbellrand Subgroup, Transvaal Supergroup, Griqualand West Basin, South Africa. Many textures of sulfides show evidence of pre-compaction lithification, suggesting that reduced sulfur and iron were available in the sedimentary environment. A large, Chixulub-scale meteorite impact during this sulfidic event does not change the concentration or morphology of pyrite, indicating that this event did not have long-term effects on the sulfur cycle.

Pyrite morphology can reveal how pyrite formed. The relative timing of formation of different textures of pyrite can be used to infer when during sedimentation and diagenesis a particular piece of pyrite formed. Such textural relationships have been used to reconstruct geobiological (Meyer et al, 2017) and geochemical (Izon et al, 2002) processes in the sulfur cycle in Archean-age rocks. The use of in situ analyses of sedimentary pyrite can be contrasted with whole-rock geochemical analyses which have been used to investigate long-term trends in trace element and redox conditions. However, late-stage diagenesis, metamorphism, and hydrothermal activity can affect pyrite geochemical signatures. An understanding of pyrite formation processes is essential for linking redox changes in the Neoarchean across continents (Cui et al, 2018; Partridge et al, 2008). This chapter illustrates a variety of pyrite textures observed in cores spanning the 2.65-2.63 Ga Jeerinah Formation, Western Australia, through a euxinic interval. Where possible, models for formation mechanisms are discussed. This work provides key constraints on the formation of various pyrite phases and helps refine prior interpretation of geochemical signatures through the euxinic interval.

The 2.63 Gya Jeerinah Formation contains pyrite nodules that show textural evidence of having precipitated on the Neoarchean seafloor. Shale and thin carbonate turbidites lap onto the nodules, demonstrating relief on the seafloor over the nodules. In addition, overhanging pyrite growths produced shelter porosity later filled with calcite. The petrographic textures at the tops of the nodules are sometimes laminated and columnar in contrast to euhedral crystal forms at the bases of nodules, demonstrating different growth conditions upward versus downward. This differential growth of nodules at the seafloor is analogous to polymetallic manganese nodules in the modern ocean. These very unusual iron sulfide seafloor nodules suggest that iron and sulfur cycling may have been unique in Earth history during this ca. 20 Ma interval, consistent with an overall exceptionally high concentration of pyrite in a subtidal siliciclastic/carbonate ramp of this age.