Evolution of iron formation to ore during Ediacaran to early Paleozoic tectonic stability

Abstract

The enrichment of iron formation (IF) to form economically important hematitic iron ore has been linked to a range of mechanisms from interactions with hydrothermal or metamorphic fluids during active tectonics to chemical leaching driven by meteoric water in the very near surface. Direct geochronologic constraints on hematite that can link ore-forming processes to geologic events have only recently become possible. Here, we present spatially resolved hematite U-Pb and paleomagnetic data for Paleoproterozoic IF, and associated hematite ore bodies from Northern Michigan, USA, to assess the timing of hematite crystallization in these units. Our paired paleomagnetic and radiometric dating approach indicates that hematite crystallization in different iron oxide facies spanned the Proterozoic into the early Paleozoic. Notably, hematite ore enrichment occurred during a period of regional tectonic quiescence in the latest Neoproterozoic and early Paleozoic. At this time, Laurentia was at tropical paleolatitudes and for much of this interval Earth was in a particularly warm non-glacial interval. These results indicate that the hematite ore at the study locality developed through supergene process via the interaction of the host IF with warm meteoric water transported downward to the subsurface rather than hypogene hydrothermal fluids linked to tectonism in the Paleoproterozoic.