Origins of pyrites in the similar to 2.5 ga Mt. McRae Shale, the HamersleyDistrict, Western Australia

The Mt. McRae Shale is the footwall rock unit of the Brockman Iron Formatio n (similar to 2.5 Ga in age) in the Hamersley Basin, Australia. It is chara cterized by a high concentration of organic carbon (similar to 2-8 wt%), an abundance of disseminated pyrite (similar to 1 to similar to 10 wt% S in t he bulk rocks), and abundant pyrite nodules (similar to 1-10 cm radius). We have examined microscale (similar to 200 mu m to 1 cm) variations in sulfu r isotopic compositions of pyrite and S and C contents in six rock samples from a similar to 27 m drill core section of the base of the Mt. McRae Shal e at the Whaleback Mine. The microanalyses of sulfur isotope compositions w ere performed in situ on single or aggregates of pyrite crystals (eighty-fo ur analyses) using a Nd-YAG laser microprobe. The carbon contents of thirty powdered samples, drilled from different part s of the six rock samples, are similar (similar to 2 to similar to 8 wt% C) with the exception of one sample (0.5 wt%). However, the occurrence, morph ology, abundance, and delta(34)S value of pyrite reveal distinct difference s 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 oc cur mostly as disseminated fine grains (similar to 5 mu m). The pyrite S co ntents are uniform within each sample, but the delta(34)S values vary from -6.3 to +7.1 parts per thousand. 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 (similar to 200 mu m): the pyrite contents ar e highly variable within each specimen (0 to >10 wt%), and the delta(34)S v alues vary from +2.2 to + 11.8 parts per thousand. The formation process of pyrites in the lower section appears to have been complicated: pyrite lami nae were first formed by bacterial sulfate reduction during early diagenesi s, and then some of the early pyrites were dissolved and reprecipitated to form pyrite nodules by later diagenetic or hydrothermal solutions in a clos ed-system. The sulfur isotope data obtained in this study can be best expla ined by a model postulating that the seawater about 2.5 Ga ago in the Hamer sley Basin had the delta(34)S value of +10 to =15 parts per thousand and th at the kinetic isotope effects accompanying bacterial sulfate reduction wer e 8-13 parts per thousand and 16-21 parts per thousand, respectively, durin g the deposition of the lower and upper sections of the McRae Shale. The va riable delta(34)S values of microscale area and the magnitudes of the kinet ic isotope effects suggest that: (1) the sulfate concentration of the 2.5 G a seawater was already more than one-third of the present seawater value, a nd (2) the activity of sulfate-reducing bacteria in the 2.5 Ga ocean was ge nerally 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 i sotopic characteristics of sulfur in the earth's near surface reservoirs (o ceans and sediments) were controlled by the Phanerozoic-style biogeochemica l cycles, rather than by the mantle-buffer (i.e., magmatic) mechanism. Sugg estion 2 may imply that (5) the Archean oceans were generally warmer compar ed to the modern oceans, and (6) they produced a higher abundance of organi c matter that were easily metabolized by sulfate-reducing bacteria. Copyrig ht (C) 1998 Elsevier Science Ltd.