Abstract

Pb-isotope ratios of whole-rock specimens and mineral separates from the gneisses surrounding the Namew Lake Ni–Cu deposit, south of Flin Flon, Manitoba, are compared with the Pb-isotope ratios in the ore minerals and with recently reported zircon U/Pb analyses of samples from the gneisses and the ultramafic host of the ore. The Pb/Pb age from the gneisses is 1864 ± 8 Ma, and there is no evidence for any differences in slopes or initial ratios between individual samples. Thus, there is good agreement between the youngest zircon ages for the gneisses (1862 ± 3 Ma) and Pb/Pb ages for the same rocks. A suite of analyses from a late pegmatite, which cuts the gneisses and the ultramafic rock, yields a well-defined age of 1824 ± 5 Ma, consistent with the geologic requirement that it be younger than the ultramafic host to the orebody (zircon age of 1847 ± 6 Ma) and similar in age to late intrusive bodies in the Flin Flon area to the north.All these data are in marked contrast to apparent Pb/Pb ages for the sulfphides of the ore deposit. Pb/Pb isochron ages on the ore minerals themselves are significantly older than the ages obtained for the gneisses and the ultramafic host. The Pb/Pb data show evidence for multi-stage evolution, with most leach-residue sets yielding en echelon lines with an apparent age of 1948 ± 13 Ma, whereas the residues indicate an older age of 1996 ± 19 Ma. The apparent ages can be explained as being the result of a three-stage process whereby U/Pb ratios were reduced by a factor of 1.6–2.0 at the time of supergene enrichment of the upper part of the ore deposit. This presumably occurred during the formation of the late Precambrian peneplane in the area. One sulphide sample from near the Precambrian–Paleozoic unconformity indicates resetting of the U/Pb system at about 1121 Ma, consistent with what would be expected from the supergene enrichment process.There is some indication of retrograde metamorphic formation of epidote in the gneisses at 1728 ± Ma but no direct evidence for late Hudsonian (1700–1750 Ma) effects on the ore.

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