Abstract

A U-Pb zircon age (1880 ± 5 Ma) has been obtained from a mineralized ultramafic body in the Setting Lake area of the Thompson nickel belt. This crucial new age for ultramafic magmatism in the Thompson belt, in conjunction with the precise age determinations established for the Molson dike swarm (Cross Lake dike, 1883.7+1.7–1.5 +Ma; Cuthbert Lake dike, 1883 ± 2 Ma) and the Fox River sill (1882.9+1.5–1.4 +Ma) by Heaman et al. (1986) provides the first direct evidence for the contemporaneous nature of mafic-ultramafic magmatism along the northwest margin of the Superior craton. This magmatic event also appears to be synchronous with mafic magmatism in other parts of the Circum-Superior belt and the Trans-Hudson orogen.

Re-Os isotope systematics of mineralized samples from seven deposits that span the length of the belt produce an age similar to the U-Pb zircon age but with a much larger error (1885 ± 49 Ma with an initial γOs of 4.6). The Os isotope data demonstrate that the source of the Thompson belt ores, including those orebodies hosted in ultramafic rocks and those hosted in pelitic rocks, was dominantly mantle derived. Establishing that Thompson belt magmatism is synchronous with the Molson dike swarm and the extensive Fox River sill has important new exploration implications and requires modification of existing concepts which have presumed that Thompson belt magmatism is significantly older than these other intrusive suites. Also, it would appear that other important Ni and Ni-Cu deposits elsewhere in the Circum-Superior belt and in the Trans-Hudson orogen formed at or near 1.88 Ga, and thus this age could be considered an important and widespread nickel mineralizing metallogenic interval during the Early Proterozoic.

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