DIRECT DATING OF ULTRAMAFIC SILLS AND MAFIC INTRUSIONS ASSOCIATED WITH Ni-SULFIDE MINERALIZATION IN THE THOMPSON NICKEL BELT, MANITOBA, CANADA

摘要

The Paleoproterozoic Thompson nickel belt, a segment of the northwest margin of the Archean Superior craton in Manitoba, is one of the largest nickel-producing regions in the world and the second largest Ni-Cu-PGE (platinum group elements) mining camp in Canada after Sudbury. The nickel sulfide ores are hosted in or associated with ultramafic sills of komatiitic affinity that were emplaced into sulfur-rich metasedimentary rocks of the Ospwagan Group and subsequently strongly deformed and metamorphosed (amphibolite to granulite facies) during the Trans-Hudson orogeny. A U-Pb zircon age of 1880.2 +/- 1.4 Ma for a metaperidotite (olivine orthopyroxenite) from the upper part of a boudinaged ultramafic sill at the Pipe 2 deposit is interpreted as the age of crystallization of the intrusion, the first age from an ultramafic sill containing mined nickel sulfide in the Thompson nickel belt. This age confirms a direct petrogenetic relationship between Ni-Cu-PGE mineralization and widespread mafic-ultramafic magmatic activity (Molson igneous events) at ca. 1.88 Ga along the Superior province craton margin (e.g., Thompson nickel belt, Fox River belt, Cape Smith belt, New Quebec orogen). At the south pit of the Thompson mine, a U-Pb zircon age of 1860.7 +/- 0.7 Ma for a composite mafic intrusion (garnet amphibolite to layered metagabbro) identifies a new age of mafic magmatic activity in the Thompson nickel belt that is unrelated to nickel sulfide mineralization. The thermal effects of the Trans-Hudson orogeny are recorded by a range of zircon dates and ca. 1760 Ma metamorphic titanite from the South pit intrusion and are consistent with interpretations that regional metamorphism occurred as a single progressive event. The results of this study open up the possibility of using the high-precision U-Pb geochronology of mineralized ultramafic rocks to precisely constrain the duration of mafic-ultramafic magmatism in the Thompson nickel belt and assess potential age differences between ultramafic sills across and along strike of this world-class nickel metallotect.