ORE DEPOSITS AND THE EVOLUTION OF THE ATMOSPHERE, THE OCEANS, AND THE BIOSPHERE--AN OVERVIEW

The oxidation state of the atmosphere-ocean system during the Precambrian has been a very interesting and somewhat contentious issue during the past 50 years. Arguments concerning the origin of ore deposits of the redox sensitive elements have played a major role in the controversy. Uraninite in ores of the Witwatersrand-Elliot Lake type have been variously assigned a hydrothermal origin, a placer origin, and a modified placer origin. The textures and the composition of the uraninite in these deposits favor a modified placer origin. If correct, this indicates that the O₂ content of the atmosphere prior to ca. 2.4 Ga was much lower than 0.2 atm. The appearance of hydrothermal uranium ores and the occurrence of enhanced uranium concentrations in highly carbonaceous shales ca. 2.0 Ga suggests that the oxidation state of the atmosphere-ocean system increased significantly between 2.4 and 2.0 Ga. A similar conclusion is supported by sedimentary ores of manganese and the composition of paleosols. The origin of the large oxide facies BIFs is still a matter of debate, but the composition of their near-shore facies is consistent with a major increase in the O₂ content of shallow oceans between 2.45 and 2.3 Ga.

The most compelling evidence in favor of this view comes from data for the history of the mass independent fractionation of sulfur in sedimentary sulfides and sulfates. At present the only cogent explanation of these data is that the level of atmospheric O₂ prior to ca. 2.4 Ga was £ ca. 10 ppm, and that P₀₂ has been ³ 10 ppm since ca. 2.3 Ga. The cause or causes of what has been called the Great Oxidation Event between ca. 2.4 and 2.0 Ga are still unclear. A minor increase in the average oxygen fugacity of volcanic gases may well have been the trigger for the Event, but evolutionary changes in the marine biota cannot be ruled out.