Origins of pyrites in the ∼2.5 Ga Mt. McRae Shale, the Hamersley District, Western Australia

Abstract

The Mt. McRae Shale is the footwall rock unit of the Brockman Iron Formation (∼2.5 Ga in age) in the Hamersley Basin, Australia. It is characterized by a high concentration of organic carbon (∼2–8 wt%), an abundance of disseminated pyrite (∼1 to ∼10 wt% S in the bulk rocks), and abundant pyrite nodules (∼1–10 cm radius). We have examined microscale (∼200 μm to 1 cm) variations in sulfur isotopic compositions of pyrite and S and C contents in six rock samples from a ∼27 m drill core section of the base of the Mt. McRae Shale at the Whaleback Mine. The microanalyses of sulfur isotope compositions were performed in situ on single or aggregates of pyrite crystals (eighty-four analyses) using a Nd-YAG laser microprobe. The carbon contents of thirty powdered samples, drilled from different parts of the six rock samples, are similar (∼2 to ∼8 wt% C) with the exception of one sample (0.5 wt%). However, the occurrence, morphology, abundance, and δ 34S value of pyrite reveal distinct differences between the two samples from the upper part and the four samples from the lower part of the drill core section. Pyrite crystals in the upper part occur mostly as disseminated fine grains (∼5 μm). The pyrite S contents are uniform within each sample, but the δ 34S values vary from −6.3 to +7.1‰. These data suggest that pyrite crystals in the upper section were formed by bacterial reduction of seawater sulfate. Pyrite crystals of the lower section occur in the form of veinlets, nodules, or laminae of coarse grains (∼200 μm): the pyrite contents are highly variable within each specimen (0 to >10 wt%), and the δ 34S values vary from +2.2 to +11.8‰. The formation process of pyrites in the lower section appears to have been complicated: pyrite laminae were first formed by bacterial sulfate reduction during early diagenesis, and then some of the early pyrites were dissolved and reprecipitated to form pyrite nodules by later diagenetic or hydrothermal solutions in a closed-system. The sulfur isotope data obtained in this study can be best explained by a model postulating that the seawater about 2.5 Ga ago in the Hamersley Basin had the δ 34S value of +10 to +15‰ and that the kinetic isotope effects accompanying bacterial sulfate reduction were 8–13‰ and 16–21‰, respectively, during the deposition of the lower and upper sections of the McRae Shale. The variable δ 34S values of microscale area and the magnitudes of the kinetic isotope effects suggest that: (1) the sulfate concentration of the 2.5 Ga seawater was already more than one-third of the present seawater value, and (2) the activity of sulfate-reducing bacteria in the 2.5 Ga ocean was generally higher than that in the modern ocean. Suggestion 1 further implies that, by 2.5 Ga, (3) the atmosphere became oxic, and (4) the chemical and isotopic characteristics of sulfur in the earth’s near surface reservoirs (oceans and sediments) were controlled by the Phanerozoic-style biogeochemical cycles, rather than by the mantle-buffer (i.e., magmatic) mechanism. Suggestion 2 may imply that (5) the Archean oceans were generally warmer compared to the modern oceans, and (6) they produced a higher abundance of organic matter that were easily metabolized by sulfate-reducing bacteria.