RI-45 Iron-Formation Protolith and Genesis, Cuyuna Range, Minnesota

Abstract

The Cuyuna iron range in east-central Minnesota is unique in the Lake Superior region because of its large manganiferous iron ore resource. The protolith to the ore has traditionally been considered a Lake Superior-type iron-formation similar to other Early Proterozoic iron-formations in the region. However, recent stratigraphic, mineralogical, and geochemical studies show that the Trommald Formation-the principal iron-formation of the Cuyuna North range-is not the product of a simple sedimentological regime. The presence of the minerals aegirine, barite, Ba-feldspar, and tourmaline within or associated with the iron-formation shows that hydrothermal and exhalative processes were very important during deposition of the iron-rich strata. This new interpretation has significant implications for mineral exploration in this part of Minnesota. It implies that parts of the Cuyuna range may be likely areas for exploration for sediment-hosted, submarine exhalative, PbZn- Ag deposits.