Sulfide-coated grains are enigmatic sedimentary particles that occur in sediments ranging in age from Archean to Jurassic. Although their oolitic and pisoidal texture suggests energetic and agitated conditions, their mineralogy seems to require a reducing and stagnant environment. Because of this conflict, they have long been considered diagenetic replacements of precursor iron oxide, chamosite, or carbonate ooids.
Coated iron sulfide (CIS) grains from the Ordovician Winnipeg Formation of Saskatchewan, Canada, formed as primary diagenetic precipitates under shallow burial conditions, involving intermittent episodes of tractional reworking, grain abrasion, reburial, and precipitation of further concentric laminae. These iron sulfide grains are found in shallow marine sandstones with indications of episodic wave reworking, and indicate sufficient organic-matter burial to promote reducing pore waters and iron sulfide formation. Horizons with strong enrichment of iron sulfide grains (up to 40%) indicate negative net sedimentation to facilitate geochemical "reworking" of iron. CIS grains formed during intervals of maximum sediment starvation, when organic matter was enriched in surface sediments, and rare storm waves intermittently reworked the surface sediment.
Alternating laminae of pyrite and marcasite within these grains point to substantial fluctuations in pore-water pH, driven by intermittent oxidation of previously formed iron sulfides. Filamentous and coccoid structures within cortical laminae suggest microbial involvement in lamina formation through providing a growth medium for sulfate-reducing bacteria and potentially a template for mineral growth.
Coated iron sulfide grains of the Winnipeg Formation preserve a combined record of condensation, seafloor reworking, fluctuating chemical conditions in the surface sediment, and high surface productivity. This information-rich record is made even more valuable by the fact that the host sediment consists largely of heavily bioturbated monotonous sandstones that could not provide these insights on their own merit.