GSA 2020 Connects Online

Paper No. 28-5
Presentation Time: 6:30 PM

REDOX CONTROLS ON CU-PGE-AU MINERALIZATION IN THE POLARIS ALASKAN-TYPE INTRUSION, NORTH-CENTRAL BRITISH COLUMBIA


MILIDRAGOVIC, Dejan1, NIXON, Graham T.2, SCOATES, James S.3, NOTT, James3 and SPENCE, Dylan W.3, (1)Geological Survey of Canada, 1500 - 605 Robson Street, Vancouver, BC V6B 5J3, Canada; British Columbia Geological Survey, PO Box 9333 Stn Prov Govt, Victoria, BC V8W9N3, Canada, (2)BC Geological Survey, Ministry of Energy and Mines, P.O. Box 9320, Stn Prov Govt, Victoria, BC V8W9N3, Canada, (3)PCIGR, Dept. of Earth, Ocean and Atmospheric Sciences, University of British Columbia, 2020-2207 Main Mall, Vancouver, BC V6T-1Z4, Canada

The Early Jurassic Polaris intrusion in the Canadian Cordillera is an Alaskan-type mafic-ultramafic sill, whose lithological zonation reflects the combined effects of magma differentiation and crystal accumulation, punctuated by recharge and cumulate remobilization. The intrusion contains magmatic mineralization with petrologic and potential economic significance that records the evolution of an oxidized, H2O-rich ultramafic magma. Dunite-hosted platinum group element (PGE) mineralization with high IPGE/PPGE is hosted by thin layers and schlieren of chromitite. Discrete grains of platinum group minerals (laurite) are rare and the bulk of the PGE appear to be hosted within chromite ±olivine. The absence of Pt-Fe alloys and low Pt/Ir in dunite contradict the view that Pt-enrichment of ultramafic rocks is characteristic of Alaskan-type intrusions. This apparent discrepancy is consistent with the positive dependency of Pt solubility on the oxidation state of sulphide-undersaturated magmas. The earliest, high-temperature cumulates of the Polaris intrusion crystallized from strongly oxidized parental magma(s) (log fO2≥FMQ +2). Clinopyroxene- and hornblende-rich cumulates contain low abundances of disseminated pyrrhotite, chalcopyrite, pentlandite, ±bornite, ±pyrite. The sulphide-bearing rocks have primitive mantle (PM)-normalized depletions in IPGE and enrichments in Cu-PPGE-Au, similar to other Alaskan-type intrusions and parallel to PM-normalized concentrations in primitive arc lavas. The absolute abundances, and S-normalized whole-rock concentrations of chalcophile elements in sulphide-bearing rocks, are highest in olivine clinopyroxenites. Several olivine clinopyroxenites have high S-normalized concentrations of PPGE, Au, and Rh that are similar to those of the richest Ni-Cu-PGE deposits (e.g. Noril’sk). Sulphide saturation at the Polaris intrusion appears tied to the appearance of magnetite. A comparison with experimental data suggests that Polaris parental magmas were H2O-saturated, which promoted magnetite saturation at intermediate compositions (MgO ≥5 wt.%). Fractional crystallization of magnetite resulted in reduction of the residual magma to log fO2≤FMQ+1 and induced separation of an oxidized, Cu-rich immiscible sulphide melt.