Abstract

The ways in which evidence derived from the mineralogy of uraninite-bearing quartz pebble orthoconglomerates has hitherto been used in attempts to determine the oxygen content of the early Proterozoic atmosphere are critically reviewed in the light of recent studies of the conglomerates of the Quirke Ore Zone, Elliot Lake, Ontario, Canada (Robinson 1982; Robinson & Spooner 1982 Robinson & Spooner in press). The presence of detrital uraninite and pyrite in the ores is not, as once was thought, evidence for a 'reducing' atmosphere because persistence of these minerals as detrital phases has been documented in recent sediments of the Indus fluvial system. Theoretical models of the kinetics of uraninite dissolution are hard to interpret quantitatively in terms of atmospheric partial oxygen pressures because of the large number of assumptions that need to be made. Post-depositional uranium mobility, which has been conclusively documented at Elliot Lake and in Upper Witwatersrand ores, does not necessarily indicate the presence of free-oxygen-rich groundwaters, even if transport was as hexavalent species. In addition, post-depositional uranium mobility was a diagenetic, groundwater-mediated process. Hence, while the chemistry of groundwaters may be inferred from mineralogy, that of surface waters, and by extension that of the atmosphere, may not, because surface and groundwaters are not necessarily in mutual chemical equilibrium. The same restriction applies to the analogous use of the diagenetic mineralogy of any rock type. Sulphur isotope systematics of pyrite concentrates from the Elliot Lake and Upper Witwatersrand ores do not unequivocally demonstrate the involvement of sulphate reducing bacteria or, thus, the presence of aqueous sulphate. The presence of sulphidized limonitic pebbles in Upper Witwatersrand ores can only place an extremely low, lower limit on the oxygen content of the atmosphere at the time.

It is concluded that it is by no means as straightforward a matter as has hitherto been assumed to make deductions concerning the oxygen content of the early Proterozoic atmosphere from evidence provided by either the detrital or Secondary mineralogy of uraninite-bearing quartz pebble orthoconglomerates, and that, to date, the problems involved have not been sufficiently considered.

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