Multiple fluid generations in the Sudbury igneous complex: fluid inclusion, Ar, O, H, Rb and Sr evidence

The elliptically zoned Sudbury Structure in northern Ontario is host to world-class nickel–copper–PGE deposits, which are generally regarded as the products of a single magmatic event. However, there is increasing evidence of multi-episodic fluid rock (including Ni–Cu sulphide) interactions in the Sudbury Structure that range from early remobilization of the cooling ores (ca. 1850 Ma), through metamorphism and deformation related to the Penokean Orogeny to neotectonic fracturing (ca. 5–13 Ma). Previous fluid inclusion studies have identified an H2O–NaCl±CO2±KCl fluid of highly variable phase ratios intimately associated with the ores, and a distinct population of primary two-phase fluid inclusions associated with post-intrusion metasomatic alteration. This study introduces two new distinct populations of fluid inclusions: (1) a population represented by two-phase CO2-bearing fluid inclusions hosted along healed-fracture planes in quartz that offset ore minerals, that was trapped between 280–340°C and 1750 to 3500 bars pressure. As the fluid inclusion trails offset and therefore post-date the ore, their genesis is consistent with a late Penokean (1.83–1.89 Ga) or later (ca. 1450) contractional event; (2) a two-phase highly-saline (>16 wt.% NaCl equivalent) fluid hosted within neotectonic (5–13 Ma Rb–Sr) galena–sphalerite–calcite–quartz–chlorite veins. Inclusions of this fluid were trapped at temperatures ranging from 60 to 135°C and maximum confining pressures of 950 bars. Stable isotope data (δO,δD) from biotite, amphibole and epidote from the alteration assemblages of remobilized veins within the Sudbury Structure show distinct evidence of a mixing trend of fluids originating within or near the `magmatic water boxʹ and trending towards a metamorphic fluid of isotopic composition similar to an evolved Sudbury Structure groundwater. Ar–Ar data from the amphiboles within Sudbury Intrusive Complex (SIC) footwall breccia from the Craig Mine are consistent with a mixed Ar–Ar spectrum. The step-heating Ar release spectrum yields ages ranging from ca. 1800 Ma at the low temperature end to approximately 2640 Ma for high temperature steps. These dates are in agreement with the ca. 1850 and 2647 Ma ages for the SIC and Levack gneisses, respectively, and are interpreted as the reinjection of country rocks partially melted by the SIC back into the Sudbury Structure. The isotopic evidence for mixed fluids derived from a combination of a magmatic fluid and a regional groundwater/metamorphic fluid suggests that the highly variable phase-ratio fluid inclusions associated with the ore may be divided into two end-member fluids represented by an exsolved SIC fluid and a regional groundwater/metamorphic fluid. This is consistent with at least five distinct fluids being present during the evolution of the Sudbury structure.