Thermochronology of the Camflo gold deposit, Malartic, Quebec; implications for magmatic underplating and the formation of gold-bearing quartz veins

Abstract

The dominant ore host at Camflo is a small monzonite pluton cut by main-stage gold-bearing quartz veins containing hydrothermal titanite and K feldspar. Main-stage veins are spatially and temporally related to brittle-ductile faults. Emplaced into most faults are younger transitional-stage quartz-pyrite lodes with sericite-rutile alteration halos. Titanite in main-stage gold quartz veins is replaced by rutile-calcite-quartz where overprinted by transitional-stage sericitic alteration.A U-Pb zircon age from the Camflo monzonite indicates that the stock was emplaced at 2685 + or - 10 Ma. A U-Pb age of 2621 + or - 4 Ma for hydrothermal titanite and a Pb-Pb isochron age of 2621 + or - 7 Ma defined by seven titanite and two K feldspar samples indicate that the gold quartz veins formed approximately 60 m.y. later which precludes a direct genetic link between these two events. Rutile analyses plot below the titanite-feldspar Pb-Pb isochron and so support field and petrographic evidence suggesting rutile did not precipitate from the same fluid that formed the titanite and K feldspar.Incremental heating and total fusion 40 Ar- 39 Ar ages of hydrothermal micas related to gold quartz veins and quartz-pyrite lodes are 70 to 170 m.y. younger than the hydrothermal U-Pb ages. In addition, 40 Ar- 39 Ar ages of micas on the 1650 level are approximately 10 to 15 m.y. older than like micas on the 3475 level. The younger 40 Ar- 39 Ar ages are interpreted as cooling, and not recrystallization or reset, ages. 40 Ar- 39 Ar ages indicate cooling rates of approximately 1 degrees C/m.y. for 2600 to 2550 Ma (rutile-vein muscovite), approximately 0.6 degrees C/m.y. for 2550 to 2511 Ma (vein muscovite-wall-rock muscovite), and approximately 0.3 degrees C/m.y. for 2507 to 2452 Ma (large biotite-small biotite). An apparent uplift rate of approximately 0.04 km/m.y. for 2520 to 2478 Ma is calculated by comparing biotite and white mica ages from the 1650 and 3475 levels.The U-Pb and 40 Ar- 39 Ar thermochronology is consistent with an accretionary tectonic setting for Camflo during the terminal stages of tectonic construction of the Superior province. A critical feature of this setting is large volumes of mantle-derived basalt underplating the lower crust during peak granulite formation at approximately 2648 to 2640 Ma. CO 2 -rich fluids liberated by solidification of ponded mafic magma after peak granulite formation interacted with other fluids, magmas, and rock either at their source or during migration in shear zones to upper crustal levels where they deposited significant quantities of gold at Camflo at approximately 2621 Ma. Slow cooling and slow apparent uplift at Camflo ensued for >150 m.y. after magmatic under-plating, resulting in the young 40 Ar- 39 Ar cooling ages recorded by hydrothermal minerals. This model is consistent with ages for granulite facies metamorphism in the Kapuskasing structural zone, postpeak metamorphic ages for CO 2 -H 2 O fluid inclusions in regional granulite terranes, and geochronological data from other gold deposits and may have widespread implications for the formation of gold deposits in the southern Superior province.