Significance of tourmaline-rich rocks in the North Range group of the Cuyuna iron range, east-central Minnesota

J.M. Cleland; G. B. Morey; P.L. McSwiggen

doi:10.2113/gsecongeo.91.7.1282

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Significance of tourmaline-rich rocks in the North Range group of the Cuyuna iron range, east-central Minnesota

J.M. Cleland·G. B. Morey·P.L. McSwiggen

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Abstract

Concentrations of tourmaline in Early Proterozoic metasedimentary rocks of the Cuyuna iron range, east-central Minnesota, provide a basis for redefinition of the evolutionary history of the area. Manganiferous iron ore forms beds within the Early Proterozoic Trommald Formation, between thick-bedded granular iron-formation having shallow-water alepositional attributes and thin-bedded, nongranular iron-formation having deeper water attributes. These manganese-rich units were previously assumed to be sedimentary in origin. However, a reevaluation of drill core and mine samples from the Cuyuna North range has identified strata-bound tourmaline and tourmalinite, which has led to a rethinking of genetic models for the geology of the North range. We interpret the tourmaline-rich rocks of the area to be a product of submarine-hydrothermal solutions flowing along and beneath the sediment-seawater interface. This model for the depositional environment of the tourmaline is supported by previously reported mineral assemblages within the Trommald Formation that comprise aegirine; barium feldspar; manganese silicates, carbonates, and oxides; and Sr-rich barite veins.In many places, tourmaline-rich metasedimentary rocks and tourmalinites are associated locally with strata-bound sulfide deposits. At those localities, the tourmaline-rich strata are thought to be lateral equivalents of exhalative sulfide zones or genetically related subsea-floor replacements. On the basis of the occurrence of the tourmaline-rich rocks and tourmalinites, and on the associated minerals, we suggest that there is a previously unrecognized potential for sediment-hosted sulfide deposits in the Cuyuna North range.

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J.M. Cleland, G. B. Morey, P.L. McSwiggen. 1996-11-01. Significance of tourmaline-rich rocks in the North Range group of the Cuyuna iron range, east-central Minnesota. https://doi.org/10.2113/gsecongeo.91.7.1282

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