SEDIMENTOLOGY AND GEOCHEMISTRY OF THE GLACIOGENIC LATE PROTEROZOIC RAPITAN IRON-FORMATION IN CANADA

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 Archean 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 Archean 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 paleomagnetic 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.