MORPHOLOGICAL CHARACTERIZATION AND MINERAL CHEMISTRY OF AMPHIBOLES IN THE IRONWOOD IRON-FORMATION, GOGEBIC IRON RANGE, WISCONSIN, USA

The ~1.87 Ga Ironwood Iron-Formation, located in the western Gogebic iron range in northern Wisconsin, constitutes one of the largest undeveloped iron resources of the United States. This study documents the morphological character and mineral chemistry of amphibole minerals in the Ironwood Iron-Formation in relation to contact metamorphism due to the emplacement of the Mellen Intrusive Complex (MIC), and has implications for future development of this resource.

Samples along the E-W strike of the Ironwood Iron-Formation were collected from drill core at distances ranging from 2 to 5 km from the MIC, and examined using a field-emission scanning electron microscope. Metamorphic grade in this area decreases eastward from the sillimanite to the biotite zone. A high-grade mineral assemblage within the iron formation was found within 2 km of the MIC, whereas a medium-grade assemblage was found at distance of approximately 3 km from the MIC. Regardless of metamorphic grade, about 50 percent of all amphiboles observed occur in massive fine-grained crystalline aggregates with indistinct grain boundaries, whereas about 30 percent of all amphiboles observed occur as prismatic single crystals with an average aspect ratio of approximately 5:1. In the high-grade assemblage, the remaining amphiboles occur as equant crystals. In contrast, the remaining amphiboles in the medium-grade assemblage occur as fibrous amphiboles with average aspect ratios of approximately 32:1, typically displaying curvature and occurring in bundles. The most striking difference in morphological habit between medium- and high-grade assemblages is the absence of fibrous amphibole in the high-grade assemblage.

Electron microprobe analysis reveals that the amphiboles of the high-grade assemblage are more chemically homogenous than those of the medium-grade assemblage and contain greater amounts of Fe; they are classified as grunerite and ferroactinolite. In contrast, medium-grade amphiboles have greater amounts of Mg and show a range of compositions in the grunerite-cummingtonite series and actinolite-ferroactinolite series. Both series occur in massive and prismatic forms, but fibrous amphiboles are almost exclusively grunerite, restricted to a small compositional range, and formed under medium-grade metamorphic conditions.