Abstract

Newly developed techniques were used to characterize the age, origin, and temperature history of coarse hematite contained in high-grade iron ore. Three botryoidal specimens from two different mines in the Gogebic iron range near Ironwood, Michigan (USA), yielded (U-Th)/21Ne ages concordant with plateau 4He/3He ages. These data reveal that while two of the specimens grew at ca. 772 ± 41 Ma, the third formed 300 m.y. later (453 ± 14 Ma). 4He/3He polydomain thermochronometry indicates hematite formation at >80 °C for the older specimens and ∼60 °C for the younger. Fe mobilization and recrystallization in open cavities at 3–5 km subsurface has occurred episodically or possibly continuously over hundreds of millions of years despite a quiescent tectonic setting throughout this interval. While the analyzed specimens are distinct from typical Fe ore, their ages along with structural relationships constrain the onset of iron enrichment to between 1060 and 770 Ma. Cooling of ∼0.5 °C/m.y. ensued between ca. 800 and ca. 550 Ma, declining to ∼0.1 °C/m.y. for the last 550 m.y. These data suggest an erosion rate of ∼6 m/m.y. and ∼5 km of total erosion of the Superior Upland province through the Phanerozoic, and limit unroofing associated with Pleistocene glaciation to <1 km.

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