Abstract

U-Pb geochronology of the Giant Mascot Ni-Cu-PGE magmatic sulfide deposit and its host ultramafic rocks in southwestern British Columbia establishes a Late Cretaceous age for mineralization and constrains the temporal evolution of arc plutonism and Ni-sulfide ore formation in a convergent margin setting. The Giant Mascot intrusion is a crudely elliptical, 4-km2 plug composed of dunite, peridotite, pyroxenite, hornblende pyroxenite, and hornblendite that intruded diorites of the Spuzzum pluton. The ultramafic cumulates contain 28 subvertical mineralized conduits that are mostly hosted by olivine-rich rocks. In situ trace element ratios (e.g., Th/U, U/Yb) determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) distinguish Giant Mascot from Spuzzum zircon. Similar Ti-in-zircon temperatures are estimated for interstitial Giant Mascot zircon that crystallized late near the solidus (733°–641°C) and for early crystallized euhedral Spuzzum zircon (784°–658°C). Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology for individual zircon grains from the Giant Mascot ultramafic rocks yield high-precision 206Pb/238U crystallization dates at 93 Ma that are distinctly younger than the 95 Ma Spuzzum diorites. The CA-ID-TIMS results for samples from both intrusions preserve records of autocrystic zircon growth (150–1,100 k.y.) with rare older grains that were inherited from wall rocks. The youngest Giant Mascot ultramafic intrusive phases contain the Ni-sulfide mineralization that was generated by focused emplacement of repeated pulses of sulfide-bearing high-MgO magma through dynamic conduit systems. In contrast, LA-ICP-MS U-Pb geochronology, which permits spatial targeting of specific zones within zircon grains, results in a wide range of weighted mean 206Pb/238U dates (95–88 Ma) with higher associated uncertainties (±1–2 m.y.) and does not allow for effective discrimination of the Giant Mascot and Spuzzum ages. The crystallization ages establish that the Giant Mascot ultramafic rocks host the youngest dated Ni-Cu-PGE deposit worldwide and were generated in a continental arc tectonic setting as part of the regionally extensive Coast Plutonic Complex in the Pacific northwest (Washington state, British Columbia, Alaska). The Giant Mascot and Spuzzum intrusions are correlated with the 96 Ma Big Jim peridotite and nearby 96 to 91 Ma Mt. Stuart batholith in Washington state confirming ~170 km of dextral displacement along the Straight Creek-Fraser fault system. Direct dating of ultramafic rocks that contain Ni-sulfides and the ability to identify temporally discrete magmatic pulses in ultramafic-mafic rocks hosting Ni-Cu-PGE deposits provide a precise tool for determining the timing of ore-forming processes in convergent margins (e.g., Giant Mascot) and for Ni-Cu-PGE deposits in other tectonic settings.

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