Origin, Distribution, Morphology, and Chemistry of Amphiboles in the Ironwood Iron-Formation, Gogebic Iron Range, Wisconsin, U.S.A.

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Green, Carlin J

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Abstract

The Ironwood Iron-Formation, located in the Gogebic Iron Range in Wisconsin, is one of the largest undeveloped taconite resources in the United States. Interest in the development of this resource is complicated by potential environmental and health effects related to the presence of amphibole minerals in the Ironwood Iron-Formation, a consequence of Mesoproterozoic contact metamorphism. The purpose of this study is to provide mineralogical information about these amphiboles to aid regulatory, medical, and mining entities in their evaluation of this potential resource. Optical microscopy, X-ray diffraction, scanning electron microscopy, and electron microprobe analysis techniques were utilized to study the origin, distribution, morphology, and chemistry of amphiboles in the Ironwood Iron-Formation. The development of amphiboles from Fe-carbonates and Fe-phyllosilicates at temperatures of approximately 300 -340º C has long been recognized as a result of regionally extensive contact metamorphism of the Ironwood Iron-Formation by the Mellen Intrusive Complex, however amphiboles related to the emplacement of diabase or gabbro dikes and sills in low-grade iron-formation were also recognized in this study area. Amphiboles in the Ironwood Iron-Formation most commonly developed in massive and prismatic habits, and locally assumed a fibrous habit. Fibrous amphiboles were locally recognized in the two potential ore zones of the Ironwood Iron-Formation, but were not observed in the portion considered to be waste rock. Massive and prismatic amphiboles show a wide range of Mg# values (0.06 to 0.87), whereas Mg# values of fibrous amphiboles are restricted from 0.14 to 0.35. Factors that influenced the compositional variability of amphiboles in the Ironwood Iron-Formation may have included temperature of formation, the presence of coexisting minerals, morphology, bulk chemistry of the iron-formation, and variations in prograde and retrograde metamorphism.

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Amphibole, Iron-formation, Metamorphism, Gogebic iron range, Ironwood Iron-Formation

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