High tenor Ni-PGE sulfide mineralization in the South Manasan ultramafic intrusion, Paleoproterozoic Thompson Nickel Belt, Manitoba, Canada

摘要

The South Manasan ultramafic intrusion, located in the Early Proterozoic Thompson Nickel Belt (TNB), contains Ni and platinum group element (PGE) mineralization in the form of disseminated sulfide. Whole-rock Ni values range from 0.3 to 1.7 wt.% and total precious metals range from 0 to 13 ppm Pt + Pd + Au. In 100% sulfide the concentration of Ni ranges from 11 to 39 wt.%, and the concentration of Pt + Pd + Au ranges from 8 to 27 ppm. The intrusion is a steeply dipping, boudinaged, sill-like body with a strike length of approximately 1200 m, an average width of 125 m, and a depth extent of >1000 m. The intrusion is composed of similar to 25% fresh dunite, similar to 50% serpentine-altered dunite and similar to 25% tectonized and carbonate altered dunite. The most intense alteration is found near the margin of the intrusion where it is in contact with Pipe Formation metasedimentary rocks of the Ospwagan Group. The sulfide assemblage in fresh dunite is interstitial between cumulus olivine and is composed predominantly of pentlandite with accessory pyrite with symplectic intergrowths of the two same sulfides. The interstitial sulfide assemblage displays a primary magmatic position between olivine grains, but the sulfides are penetrated by platy intergrowths of chlorite and serpentine. In contrast, the most intensely carbonate altered dunite contains irregular patches of pyrrhotite and pentlandite.