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High tenor NI-PGE sulfide mineralization of the south manasan ultramafic intrusion, Thompson Nickel Belt, Manitoba
Get PDFThe South Manasan ultramafic intrusion (ca. 1880 Ma) located in the Early Proterozoic
Thompson Nickel Belt (TNB) contains Ni and platinum group element (PGE) mineralization
hosted by disseminated sulfide. Whole-rock Ni values range from 0.3 to 1.7 wt. % and total
precious metals (TPMs) range from 0 to 1.3 ppm Pt + Pd + Au and equate to tenor values (i.e.,
metal in 100% sulfide) of 11-39 wt. % Ni and 8-27 ppm TPMs. The South Manasan intrusion is
a steeply dipping sill-like body with a boudinaged outline having a strike length of
approximately 1200 m, average width of 125 m and a minimum depth extent of 1000 m. The
intrusion is composed of approximately 25% fresh dunite, 50% serpentine altered dunite and
25% tectonized and carbonate altered dunite. The most intense alteration is found near the
intrusion’s margin where it is in contact with metasedimentary rocks of the Pipe Formation, part
of the surrounding Ospwagan Group. In fresh dunite the sulfide assemblage characterized by an
intercumulate texture is dominated by pentlandite with accessory pyrite; the latter having a
symplectic-like texture. The pentlandite-pyrite assemblage in the serpentinized dunite, although
still characterized overall by an intercumlate-texture, has well developed platy intergrowths with
chlorite and serpentine. In the most intensely modified unit (the carbonate altered dunite) the
sulfide assemblage consists primarily of pyrrhotite and pentlandite.
Whole-rock geochemical data (n=360), modal mineralogy and mineral chemistry
obtained on representative drill core throughout the South Manasan intrusion have been used to
establish a type section in order to evaluate the relative roles of primary magmatic versus
secondary (i.e., serpentinization, carbonate alteration and deformation) processes. These data
indicate that the primary silicate-sulfide assemblage was systematically modified during : serpentinization, carbonate alteration and deformation of the South Manasan intrusion such that a
sequence of primary versus secondary events can be established. Intrusion of the original
komatiitic magma and formation of the South Manasan intrusion took place at a shallow level
into consolidated Ospwagan Group sediments with subsequent contamination of this melt with
crustal S. This triggered sulfide saturation and generation of an immiscible sulfide melt.
Calculated Ni and TPM tenor values constrain the R factor to between 500 and 2500. 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. The
current sulfide association dominated by pentlandite>>pyrite>chalcopyrite has a mineral
paragenesis that is consistent with subsolidus re-equilibration of a primary pentlanditepyrrhotite-
chalcopyrite assemblage. The subsequent processes of serpentinization, deformation
and carbonate alteration resulted in modifying the primary sulfide assemblages and their textures
(i.e., to platy habits), but did not greatly alter the bulk composition, in particular metal contents,
except for addition of volatiles (H2O, CO2).
It is concluded therefore that the enrichment of the sulfide assemblage at South Manasan
in Ni and PGEs is a consequence of a primary magmatic process involving high R factor and that
the effects of later overprinting processes (alteration, deformation) are not responsible for the
presently observed high-tenor sulfide association