Mineralogy


The spheroidal structures have textures similar to oolites and oncolites, but with mineral compositions that are quite variable. The mineral assemblages in both cores and rims may contain any of the following phases: rhodonite, aegirine, rhodochrosite, hematite, Ba-feldspar, and calcite.

These spheroidal structures clearly did not form in a simple sedimentary environment, but rather in a rapidly changing, dynamic chemical system in which the cation ratios (hence composition) changed significantly over time. It is likely that the rounded grains formed on or just beneath the sea water-sediment interface where the grains could react with a complex brine solution. This solution was part of a submarine hydrothermal system that vented into small basins and resulted in several phases of mineralization, each with a unique chemical fingerprint. The mineralogical data and resulting environmental interpretation suggest the possibility of regional Pb-Zn-Ag mineralization.

mineral zonation

Although the Cuyuna area has undergone extensive mineral exploration in the past, that exploration has been mostly limited to the manganiferous iron ore. Very little is known about the geology stratigraphically beneath the iron-formation itself.

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