SEAFLOOR PYRITE NODULES IN THE 2.6 GA JEERINAH FORMATION

Disseminated pyrite in the Jeerinah formation has been used to infer surface processes in the lead-up to the Great Oxidation Event. However, petrographic textures and mineral relationships observed in macroscopic pyrite bodies reflected light microscopy show that there are multiple generations of pyrite growth.

Criteria were developed to identify nodules that potentially grew on the seafloor in contact with the overlying seawater, versus those that grew within the sediment. The criteria for seafloor nodules are:

1. Inclusions of shale minerals within pyrite nodules.
2. Void-filling carbonate in shelter porosity.
3. Shale and detrital carbonate laminae onlap onto protuberances on the top of the nodule.
4. Differential growth morphology on top and bottom of the nodule.

Seafloor nodules contrast with concretionary pyrite growths within the sediment, which tend to be spherical to oblate, have uniform textures on the top and bottom, and deform the top and bottom shale symmetrically. The laminations around concretions are often continuous around the pyrite body and show no evidence of onlap geometry. While these concretions formed prior to lithification, they are likely to record porewater rather than seawater chemistry. Both types of pyrite bodies were observed in polished thin sections from core AIDP-2.

One seafloor nodule is about 2 cm horizontally and about 0.5 cm maximum thickness, with an approximately flat bottom and multiple upward-oriented protuberances. Calcite fills overhangs under some protuberances, and shale inclusions increase upward and are abundant near the top of the nodule. Although we interpret the nodule as having grown on the seafloor, diagenetic and epigenetic processes produced euhedral crystal habits and crosscutting veins containing carbonate, pyrite, sphalerite, and chalcopyrite. The effect of these processes on the geochemistry of pyrite needs to be evaluated in order to determine the suitability of seafloor nodules for inferring the chemistry of 2.6 Ga seawater.

However, the precipitation of pyrite directly on the seafloor, which requires sulfide and iron to be present near the sediment-water interface, points to potentially unique ocean chemistry at this time of enhanced sulfide deposition.