Abstract

The processes which affected the development of soft iron ores in the Marquette and Gogebic ranges were examined and evaluated by petrographic and chemical techniques. By tracing oxidation through mineral transitions from original rock to ore the conclusion was reached that soft iron ores of these two ranges were the result of mildly alkaline hydrothermal solutions acting upon silicated iron formations for a long period of time.Oxidation, as exhibited by major and minor minerals of the iron formation, was the initial step in the development of soft ores. Alterations and oxidation of other rocks affected by the ore forming solutions confirm the processes recognized in the iron formation. Some of the reactions that took place during oxidation were evaluated according to the most recent thermodynamic data.The steps in soft ore origin on these two ranges are commonly multiple and overlapping; however in their most simple form they are: 1) silication of the original iron formation, 2) oxidation of silicate formation, and 3) leaching of silica from the oxidized silicate rock. The last two steps were brought about by hydrothermal solutions.

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