| Titre | Redox-controlled chalcophile element geochemistry of the Polaris Alaskan-type mafic-ultramafic complex, British Columbia, Canada |
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| Auteur | Milidragovic, D ;
Nixon, G T; Scoates, J S; Nott, J A; Spence, D W |
| Source | Canadian Mineralogist vol. 59, no. 6, 2021 p. 1627-1660, https://doi.org/10.3749/canmin.2100006 |
| Image |  |
| Année | 2021 |
| Séries alt. | Ressources naturelles Canada, Contribution externe 20200719 |
| Éditeur | Association minéralogique du Canada |
| Document | publication en série |
| Lang. | anglais |
| DOI | https://doi.org/10.3749/canmin.2100006 |
| Media | papier; numérique; en ligne |
| Formats | pdf; html |
| Province | Colombie-Britannique |
| SNRC | 94C/05; 94C/12 |
| Région | Aitken Lake |
| Lat/Long OENS | -125.7500 -125.5000 56.5333 56.4167 |
| Sujets | Jurassique inférieur; cadre tectonique; accretion; magmatisme; intrusions; complexe ultramafique Alaskien; gisements minéraux; chromite; cuivre; or; genèse des minerais; minéralisation; contrôles des
minerais; géologie du substratum rocheux; lithologie; roches ignées; roches intrusives; roches mafiques; roches ultramafiques; chromitites; dunites; gabbros; diorites; roches hôtes; sulfures; magmas; dynamique des fluides; géochimie des roches
totales; géochimie des éléments en trace; géochimie des éléments majeurs; associations de minéraux; altération; modèles; Cordillère canadienne; Terrane de Quesnellia ; éléments du groupe de platine; géologie économique; géochimie; pétrologie ignée et
métamorphique; Sciences et technologie; Nature et environnement; Phanérozoïque; Mésozoïque; Jurassique |
| Illustrations | cartes géolscientiques généralisées; tableaux; photographies; photomicrographies; spectres; graphiques; représentations schématiques |
| Programme | Initiative géoscientifique ciblée (IGC-6) Systèmes minéralisés |
| Diffusé | 2021 12 13 |
| Résumé | (disponible en anglais seulement)
The 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. |
| Sommaire | (Résumé en langage clair et simple, non publié)
Les intrusions ultramafiques-mafiques de type Alaska liées à l'arc volcanique sont répandues dans la Cordillère du Amérique du Nord. Les
portions ultramafiques de ces intrusions sont reconnues comes d'hôtes pour la minéralisation magmatique de sulfures Ni-Co-EGP et alliages de Pt. Les portions mafiques sont de plus en plus reconnues pour leur potentiel à héberger des sulfures riches
en Cu, EGP et Au. L'intrusion Polaris, dans le centre-nord de la Colombie-Britannique, comprend deux types de minéralisation: la minéralisation EGP, du sous-groupe iridium associé au chromite, et la minéralisation Cu-EGP-Au contenue dans des
sulfures. Un modèle pétrologique simple, reliant la minéralisation aux propriétés intrinsèques du magma, la teneur en eau et à l'état d'oxydation, est développé pour tenir compte du spectre des styles de minéralisation dans les intrusions de type
Alaska. |
| GEOSCAN ID | 328040 |
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