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

Abstract

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 1.7wt.% and total precious metals range from 0 to 1.3ppm Pt+Pd+Au. In 100% sulfide the concentration of Ni ranges from 11 to 39wt.%, and the concentration of Pt+Pd+Au ranges from 8 to 27ppm. The intrusion is a steeply dipping, boudinaged, sill-like body with a strike length of approximately 1200m, an average width of 125m, and a depth extent of >1000m. The intrusion is composed of ~25% fresh dunite, ~50% serpentine-altered dunite and ~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.Representative drill core samples through the South Manasan Intrusion have been used to establish variations in whole rock geochemistry and mineralogy; these variations coupled with the stratigraphy of the host sedimentary rocks are consistent with magmatic differentiation in a sill-like body which was serpentinized and then tilted upright such that tops now face west. Intrusion of the original komatiitic magma and assimilation of sulfur from Ospwagan Group sedimentary rocks triggered sulfide saturation and formation of an immiscible sulfide melt. The early crystallization of olivine inhibited the sulfide melt from settling to the bottom of the magma column and, as a consequence, the sulfides now have a primary interstitial magmatic texture. Calculated Ni and PGE tenor values indicate an R factor of 500–2500. The current sulfide association dominated by pentlandite≫pyrite>chalcopyrite has a mineral paragenesis that is consistent with crystallization from a monosulfide solid solution which cooled down to pentlandite and pyrite. The subsequent processes of serpentinization, deformation and carbonate alteration modified the primary sulfide assemblages to pentlandite and pyrrhotite. The alteration of the dunite and serpentine to talc-carbonate occurred by open-system hydrothermal processes that produced a reduction in metal tenor through a dilution process accompanying the addition of carbonate and water.It is proposed that the enrichment of the sulfide assemblage at South Manasan in Ni and PGEs is a consequence of a primary magmatic process involving a high-R factor and that the effects of later overprinting processes (alteration and deformation) are not responsible for the high-tenor sulfide association. The study has important exploration implications, as it helps to establish the characteristics of a new type of Ni–PGE mineralization in the TNB, and thus provides a basis on which to better understand the formation of other low-grade ultramafic-hosted nickel deposits.