ORE DEPOSITS AND THE EARLY EARTH

Some of the most critical and striking geological information concerning the conditions at the surface of the Archean Earth comes from examination of important ore deposits. Two types of sedimentary deposits, banded iron formations (BIF) and placer-type gold/uraninite/pyrite, are classic indicators of surface oxidation state, and especially insofar as they point to a vanishingly low level of atmospheric oxygen. Although these deposits initially constituted serious difficulties for those trying to provide uniformitarian explanations based upon current surface conditions, those problems largely disappeared once it was acknowledged that surface conditions during virtually all of the Archean were profoundly different. One of the most fascinating aspects of Eric Cheney’s courses in economic geology, one of which I was fortunate enough to take as a graduate student, was his careful presentation of then recent results concerning the important ore deposits that tell this story.

For the past 50 years the prevailing view has been one in which a profound change occurred near the end of the Archean, a result of oxygenic photosynthesizers generating free oxygen necessary to oxidize organic and inorganic compounds at the Earth’s surface, leading to accumulation of free oxygen in the atmosphere. A few dissenting voices persist in the view that free oxygen was present much earlier in the Archean, based on disputed geological evidence from both Archean and some rare modern occurrences in sedimentary settings. At the same time, views of the precise chemical composition of Earth’s atmosphere have largely converged on an atmosphere dominated by nitrogen and carbon dioxide. The difficulties of such a composition for the Archean atmosphere and surface are considerable and have been recently critiqued, with the proposal that Archean surface conditions were generally far more reducing, with a large pool of regularly recycled condensed organic compounds and important trace levels of atmospheric methane and ammonia. This model, which hearkens back to proposals made in the first half of the twentieth century, solves many problems inherent in the “conventional wisdom” model, and is consistent with what is known about Archean ore deposits formed by surficial processes.