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TitleRedox-controlled chalcophile element geochemistry of the Polaris Alaskan-type mafic-ultramafic complex, British Columbia, Canada
AuthorMilidragovic, DORCID logo; Nixon, G T; Scoates, J S; Nott, J A; Spence, D W
SourceCanadian Mineralogist vol. 59, no. 6, 2021 p. 1627-1660,
Alt SeriesNatural Resources Canada, Contribution Series 20200719
PublisherMineralogical Association of Canada
Mediapaper; digital; on-line
File formatpdf; html
ProvinceBritish Columbia
NTS94C/05; 94C/12
AreaAitken Lake
Lat/Long WENS-125.7500 -125.5000 56.5333 56.4167
Subjectseconomic geology; geochemistry; igneous and metamorphic petrology; Science and Technology; Nature and Environment; Lower Jurassic; tectonic setting; accretion; magmatism; intrusions; alaskan-type ultramafic complex; mineral deposits; chromite; copper; gold; ore mineral genesis; mineralization; ore controls; bedrock geology; lithology; igneous rocks; intrusive rocks; mafic rocks; ultramafic rocks; chromitites; dunites; gabbros; diorites; host rocks; sulphides; magmas; fluid dynamics; whole rock geochemistry; trace element geochemistry; major element geochemistry; mineral associations; alteration; models; Canadian Cordillera; Quesnellia Terrane; Polaris Intrusion; platinum group elements; Phanerozoic; Mesozoic; Jurassic
Illustrationsgeoscientific sketch maps; tables; photographs; photomicrographs; spectra; plots; schematic representations
ProgramTargeted Geoscience Initiative (TGI-6) Ore systems
Released2021 12 13
AbstractThe Early Jurassic Polaris Alaskan-type intrusion in the Quesnel accreted arc terrane of the North American Cordillera is a zoned, mafic-ultramafic intrusive body that contains two main styles of magmatic mineralization of petrologic and potential economic significance: (1) chromitite-associated platinum group element (PGE) mineralization hosted by dunite (±wehrlite); and (2) sulfide-associated Cu-PGE-Au mineralization hosted by olivine (±magnetite) clinopyroxenite, hornblendite, and gabbro-diorite. Dunite-hosted PGE mineralization is spatially associated with thin discontinuous layers and schlieren of chromitite and chromitiferous dunite and is characterized by marked enrichments in iridium-subgroup PGE (IPGE) relative to palladium-subgroup PGE (PPGE). Discrete grains of platinum group minerals (PGM) are exceedingly rare, and the bulk of the PGE are inferred to reside in solid solution within chromite±olivine. The absence of Pt-Fe alloys in dunite of the Polaris intrusion is atypical, as Pt-enrichment of dunite-hosted chromitite is widely regarded as a characteristic feature of Alaskan-type intrusions. This discrepancy appears to be consistent with the strong positive dependence of Pt solubility on the oxidation state of sulfide-undersaturated magmas. Through comparison with experimentally determined PGE solubilities, we infer that the earliest (highest temperature) olivine-chromite cumulates of the Polaris intrusion crystallized from a strongly oxidized ultramafic parental magma with an estimated log f(O2) > FMQ+2. Parental magmas with oxygen fugacities more typical of volcanic arc settings [log f(O2) ~ FMQ to ~ FMQ+2] are, in turn, considered more favorable for co-precipitation of Pt-Fe alloys with olivine and chromite. More evolved clinopyroxene- and hornblende-rich cumulates of the Polaris intrusion contain low abundances of disseminated magmatic sulfides, consisting of pyrrhotite and chalcopyrite with minor pentlandite, pyrite, and rare bornite (less than or equal to 12 wt.% total sulfides), which occur interstitially or as polyphase inclusions in silicates and oxides. The sulfide-bearing rocks are characterized by strong primitive mantle-normalized depletions in IPGE and enrichments in Cu-PPGE-Au, patterns that resemble those of other Alaskan-type intrusions and primitive arc lavas. The absolute abundances and sulfur-normalized whole-rock concentrations (Ci/S, serving as proxy for sulfide metal tenor) of chalcophile elements, including Cu/S, in sulfide-bearing rocks are highest in olivine clinopyroxenite. Sulfide saturation in the relatively evolved magmas of the Polaris intrusion, and Alaskan-type intrusions in general, appears to be intimately tied to the appearance of magnetite. Fractional crystallization of magnetite during the formation of olivine clinopyroxenite at Polaris resulted in reduction of the residual magma to log f(O2) less than or equal to FMQ+2, leading to segregation of an immiscible sulfide melt with high Cu/Fe and Cu/S, and high PGE and Au tenors. Continued fractionation resulted in sulfide melts that were progressively more depleted in precious and base chalcophile metals. The two styles of PGE mineralization in the Polaris Alaskan-type intrusion are interpreted to reflect the evolution of strongly oxidized, hydrous ultramafic parental magma(s) through intrinsic magmatic fractionation processes that potentially promote sulfide saturation in the absence of wallrock assimilation.
Summary(Plain Language Summary, not published)
Volcanic arc-related, Alaskan-type ultramafic-mafic intrusions are widespread in the North American Cordillera. They are recognized for the potential to host significant magmatic Pt-alloy and Ni-Co-PGE sulphide mineralization in their ultramafic portions. The mafic portions are increasingly recognized for their potential to host sulphides rich in Cu, PGE, and Au. The Polaris intrusion, in north-central British Columbia, comprises two types of mineralization: chromite-associated iridium-subgroup PGE mineralization and sulphide-hosted Cu-PGE-Au mineralization. A simple petrological model, relating mineralization to key intrinsic magma properties, water-content and oxidation state, accounts for the spectrum of mineralization styles in Alaskan-type intrusions.

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