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  /Applications/Mineral Characterization: Cuyuna Iron Range
  The Cuyuna iron range of east-central Minnesota has traditionally been considered a Lake Superior-type iron- formation. However it is unique in the Lake Superior region due to the high manganiferous content of its iron ores. Recent investigations have also identified the presence of such minerals as aegirine, barite, Ba-feldspar (hyalophane), and tourmaline. This association with the iron-formation implies that there were hydrothermal or exhalative processes underway during the deposition of the iron-rich sediments. This in turn has significant implications for mineral exploration in this part of Minnesota, suggesting the possibility of sediment-hosted, submarine exhalative Pb-Zn-Ag deposits.   Unraveling the paragenetic sequence of these rocks, and understanding the processes by which they were deposited, is difficult due to the complexity of the mineral associations. Below is a backscattered electron image and mineral map of a sample from an aegirine-rich unit in the the Trommald Formation — the principal iron-formation of the Cuyuna North range. The mineral map was constructed from individual element maps collected simultaneously with the backscattered electron image on a JEOL electron microprobe. The two images show the complexity of the mineral textures. The prominent features in the mapped area are two spheroidal structures, which have been described in the literature as oolites, but which have physical attributes similar to oncolites.
  The upper feature has a core of mostly rhodonite, which is surrounded by hematite. That in turn is surrounded in succession by the Ba-feldspar, hyalophane, and then by the Mn-carbonate, rhodochrosite. The coatings of hyalophane have an enigmatic paragenesis, but must represent periods of barium, aluminum, and silica depositions on pre-existing grains. The fact that the hyalophane coating surrounds a hematite coating and is in turn surrounded by rhodochrosite, indicates that these phases or their precursors were deposited from a very complex and changing aqueous solution.

Below are quantitative electron microprobe analyses of selected minerals shown in the previous images. The hyalophane is a feldspar in which Ba has substituted for the K. In the samples, the feldspars contain from trace amounts of BaO to over 13 weight percent. The aegirine is a Na-rich pyroxene. It occurs in many different textural settings in these rocks and often occurs compositionally near its ideal end-member, though there are some substitutions with MnO and CaO. Rhodonite is a Mn-silicate, which in these rocks contains up to a few percent FeO, MgO, and CaO. There is a wide range of carbonates in these rocks, most of which are low in Mg, and fall predominantly within the ternary CaCO3- MnCO3- FeCO3. In the upper member of the Trommald Formation, the carbonates are mostly rhodochrosite, kutnahorite and Mn-rich calcite.

  In the middle and lower members, the carbonates are more iron-rich and tend to be either manganiferous siderite or ferroan kutnahorite.

The abundance of manganese oxides on the Cuyuna North range has long proven to be an enigma. However, the occurrence of aegirine and hyalophane in the iron-formation, as well as associated barite and tourmaline within the stratigraphic section, suggests that the protoliths of these rocks have a significant exhalative hydrothermal component.

McSwiggen, P.L., Morey, G.B., and Cleland, J.M, 1994a, The origin of aegirine in iron-formation of the Cuyuna Range, east-central Minnesota: Canadian Mineralogist, v. 32, p589-598.
________ 1994b, Occurrence and genetic implications of hyalophane in manganese-rich iron-formation, Cuyuna iron range, Minnesota, USA: Mineralogical Magazine, v. 58, p. 387-399.
________ 1995, Iron-formation protolith and genesis, Cuyuna Range, Minnesota: Minnesota Geological Survey Report of Investigation 45, 54p.