Petrogenesis of the Sunday Lake Intrusion, Jacques Township, Ontario, Canada

Abstract

The Sunday Lake Intrusion (SLI) is an early-phase (1109.0±1.3 Ma) mafic-ultramafic intrusion associated with the Midcontinent Rift System. It was emplaced along the Crock Lake Fault, a splay of the regional Quetico fault. The intrusion is a layered funnel/tabular-shaped intrusion divided into Gabbro, Upper Ultramafic, Lower Ultramafic and Marginal zones. The intrusion hosts significant Ni-Cu-Platinum group-elements (PGE) mineralization within the Marginal Zone, which contains up to 2.11 g/t platinum, 0.95 g/t palladium, 0.16 g/t gold, 0.26% copper and 0.11% nickel. In total, this contact-type deposit hosts an estimated 20.4 Mt at an average grade of 2.5 g/t combined Pt+Pd+Au. Mineralization reflects late-stage exsolution of PGM from sulfide melt, including maslovite, michenerite, sperrylite and native silver, platinum and palladium. The SLI comprises wehrlite, olivine clinopyroxenite, feldspathic olivine clinopyroxenite, melagabbro, gabbro, leucogabbro, quartz monzonite and quartz gabbro. Trace element and radiogenic isotope data support a mantle plume origin, with patterns resembling ocean island basalts (OIB) and likely tied to the Keweenaw Plume. Mass-balance calculations yield a calculated parental magma composition of ~11.15 wt. % FeO and ~19.5 wt. % MgO, consistent with a high-Mg tholeiitic basaltic magma. Compositional variations in olivine and whole-rock MgO (wt. %) suggest the SLI was formed from two discrete magma injections: the first formed the Lower Ultramafic Zone and Marginal Zone, and the second formed the Upper Ultramafic Zone. Radiogenic isotopes values (ɛNd and 87Sr/86Sr) are mostly mantle-like, though early-pulse samples show negative Nb anomalies from limited interaction with the subcontinental lithospheric mantle beneath the lithosphere. A later injection of purely primitive, plume-derived magma appears to have flushed the staging chamber, depleting the subcontinental lithosphere mantle (SCLM) signature and resetting the sulfur isotope system to near-mantle values. This two-stage model explains the combination of mantle-like isotopes with localized Nb depletion. Some radiogenic samples from the Gabbro and Marginal zones record isotopic and trace element evidence for assimilation of Quetico metasedimentary rocks. Negative ɛNd values and Nb-Th anomalies, together with decreased Fo in olivine at the Lower Ultramafic Zone-Marginal Zone contact, suggest localized interaction of magma with country rocks. Although this process may have introduced a crustal sulfur signature into the system, it was largely diluted or reset by the later primitive recharge, leaving the overall sulfur isotope system dominated by mantle values.