The Cuyuna iron range - Geology and Mineralogy
Peter McSwiggen and Jane Cleland
The Cuyuna iron range in east-central Minnesota is unique in the Lake Superior region because of its large manganiferous iron-ore resources. It consists of three areas--the Emily District, the North range, and the South range. Although much smaller than the Mesabi iron range to the north, the Cuyuna was a major producer of iron ore for 80 years. More than 106 million tons of ore were mined and shipped from the Cuyuna range between its discovery, in 1904, and 1984, when mining ceased. Although the iron ore contained more than 10 percent manganese, manganese was never economically feasible to recover because it was too difficult to separate from the iron. New technology, which involves in situ leaching of the manganese while leaving behind the iron, has been investigated as a possible mining method. The recent drilling and investigation of cores has also provided new data that geologists at MGS are using as a basis for reinterpreting the formation of the Cuyuna North range. This reinterpretation sets the Cuyuna range apart from the traditional Lake Superior-type iron-formations.
The accompanying maps show the geology of the east-central Minnesota region (Fig. 1) and in greater detail the Cuyuna range ( Fig. 2, inset from Fig. 1).
The North range is comprised of three stratigraphic units of Precambrian age-- the Trommald, Rabbit Lake and Mahnomen formations. The protolith to the iron ore of the Cuyuna has traditionally been considered a Lake Superior-type iron-formation similar to other Early Proterozoic iron formations in the region. The recent stratigraphic, mineralogical and geochemical studies indicate 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 active during deposition of the iron-rich strata. This modern interpretation has significant implications for developing exploration strategies for minerals other than iron and manganese in this part of Minnesota.
The following figures are photomicrographs of rocks from a zone approximately 30 m thick within the upper Trommald Formation. A common characteristic of these rocks is the occurrence of spheroidal structures consisting typically of rounded grains of one or more minerals coated by other minerals. These photomicrographs are backscattered images obtained from the University of Minnesota, Department of Geology and Geophysics JEOL 8900 electron microprobe; they show changes in the average atomic number that indicate compositional changes. False-color mineral maps are used to identify and show the minerals present. Further details about the mineralogy
- Figure 3. Example of internal structures within micronodules in the upper member of the Trommald Formation. Undeformed core grain surrounded by deformed layering. A, false-color image, b, back-scattered image.
- Figure 4. Micronodule consisting of a Ba-feldspar core and rhodonite rim. The rhodonite appears to be replacing the Ba-feldspar. Surrounding the rhodonite rim is a groundmass of hematite and rhodonite. A, false-color image; b, back-scattered image.
- Figure 5. Micronodule consisting of an aegirine core that is surrounded by rims of rhodonite, hematite, and rhodochrosite. A, false-color image, b, back-scattered image.
- Figure 6. Multiple carbonate micronodule with a rhodonite core surrounded by hematite, calcite, and rhodochrosite. A, false-color image; b, back-scattered image.