Archaean iron formation (>3.1 b.y.), contact-concordant at the base of the Stillwater Igneous Complex, Montana, was contact-metamorphosed to temperatures >800°C at a pressure ≥2 kbar. Temperature estimates are based on the presence of low-Ca, Fe-rich pyroxene with two-stage exsolution textures suggesting inversion from Fe-pigeonite; the thermal stability of identical pyroxene compositions is known from experimental data. The assemblage fayalite–magnetite–quartz is rarely found; rather, the fO2-dependent assemblage ortho-pyroxene–magnetite–quartz is common throughout the iron formation. Comparisons with the younger (~2 b.y.) contact-metamorphosed Gunflint Iron Formation, Minnesota, show striking similarities in composition and conditions of metamorphism: both iron formations were metamorphosed at fO2 conditions near fayalite–magnetite–quartz stability. In contrast, the available data for regionally-metamorphosed iron formations show that they reach comparable temperatures at higher fO2. The distinctive low fO2 of contact-metamorphosed iron formations reflects restriction of fO2 under the influence of the contacting igneous body. Exchange along fractures in the Stillwater Iron Formation included limited introduction of Cu, S, and Al, probably from the Stillwater Complex.

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