Abstract

The Rapitan iron-formation in the Northwest Territories and Yukon of Canada was formed between 755 and 730 Ma. This iron-formation is approximately coeval with several other extensive iron-formations worldwide, all of which are associated with glaciogenic sequences. The mineralogy and major element geochemistry of the Rapitan iron-formation is very simple (consisting mainly of hematite and chert) and is distinctly different from that of most major banded iron-formation types of Archcan and Early Proterozoic age. The rare earth element (REE) chemistry of the Rapitan iron-formation appears to be much less distinctly influenced by hydrothermal input into an ocean system than that of these earlier Archcan or Early Proterozoic iron-formation sequences. This suggests a hydrothermal input that was highly diluted by ocean waters at Rapitan time.The sedimentologic setting of the Rapitan iron-formation, among a thick sequence of glaciogenic materials, the occurrence of dropstones and faceted pebbles in the iron-formation itself, and the very large aerial extent of these sequences strongly suggest an origin as part of glaciomarine conditions. The iron-formation was deposited during a major transgressive event with a rapid rate of sea-level rise during an interglacial period. The deposition of the iron-formation took place under clear water conditions, with the addition of siliciclastic materials as subaqueous gravity flows.The occurrence, during Late Proterozoic time, of several, mineralogically and chemically very similar iron-formations worldwide that are in close association with major glaciogenic sequences, and palcomagnetic data for that same period (Kirschvink, 1992), which suggest widespread continental glaciers within a few degrees of the equator, lead to the concept of a "snowball-type Earth": an earth that would have resembled a highly reflective "snowball" with floating pack ice over most of the ocean surface (Kirschvink, 1992). In such an environment there would have been a buildup of dissolved iron during glacial periods and a deposition of iron during interglacial periods.

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