VARIATIONS IN AMPHIBOLE CHEMISTRY IN THE IRONWOOD IRON-FORMATION, GOGEBIC IRON RANGE, WISCONSIN, USA

The 1.87 Ga Ironwood Iron-Formation (IIF), located in the western Gogebic iron range in northern Wisconsin, is one of the largest undeveloped taconite iron resources in the USA. Contact metamorphism of the IIF by the 1.1 Ga Mellen Intrusive Complex (MIC) of the Midcontinent Rift System results in the development of amphiboles, the presence of which may have environmental implications for any future development of this resource. Samples of the IIF from four drill cores located ~2 - 4.5 km from the MIC were examined to document the relationship between amphibole morphology and composition, and chemical variability with respect to distance from the MIC.

The predominant amphibole species present in the IIF are members of the cummingtonite - grunerite and actinolite - ferro-actinolite series. Most samples contain amphiboles from both series, which commonly develop in a massive or prismatic habit, but occasionally assume a fibrous habit in the Pence, Norrie, and Plymouth members of the IIF. The presence of fibrous amphibole is an important factor in potential mine planning and may impact the viability of this resource.

Electron microprobe analyses document that Mg/(Mg+Fe²⁺) values are highly variable in massive and prismatic amphiboles (0.06 – 0.87), but restricted to more Fe-rich values in fibrous amphiboles (0.14 – 0.35) regardless of metamorphic grade. Additionally, in drill holes where amphiboles are present at varying distances from the MIC, an increase in the range of Mg/(Mg+Fe²⁺) values with distance is observed. Amphiboles present in higher grade (pyroxene-bearing) assemblages consistently exhibit a narrower range of Mg/(Mg+Fe²⁺) values (0.07 – 0.60) than those of medium-grade assemblages (0.15 – 0.87) regardless of morphology.

These data document the variability of Fe and Mn content and the increasing Mg enrichment of amphiboles with distance from the MIC. Increasing Fe content of fibrous grunerites is positively correlated with metamorphic grade, and is associated with the incorporation of Mg into neighboring minerals like pyroxenes or carbonates. Fibrous amphiboles have been identified locally in each of the members comprising the ore zones that could potentially be mined for iron.