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-
McSwiggen, P.L., Morey, G.B. and Cleland, J.M., 1995, Iron-Formation Protolith and Genesis, Cuyuna Range, Minnesota, Minnesota: Minnesota Geological Survey Report of Investigations 45, 54 p.
McSwiggen, Peter L.; Morey, G.B.; Cleland, Jane M..
RI-45 Iron-Formation Protolith and Genesis, Cuyuna Range, Minnesota.
Minneota Geological Survey.
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