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 wor ld-class nickel-copper-PGE deposits, which are generally regarded as the pr oducts of a single magmatic event. However, there is increasing evidence of multi-episodic fluid rock (including Ni-Cu sulphide) interactions in the S udbury 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 s tudies have identified an H2O-NaCl +/- CO2 +/- KCl fluid of highly variable phase ratios intimately associated with the ores, and a distinct populatio n of primary two-phase fluid inclusions associated with post-intrusion meta somatic alteration. This study introduces two new distinct populations of f luid inclusions: (1) a population represented by two-phase CO2-bearing flui d inclusions hosted along healed-fracture planes in quartz that offset ope minerals, that was trapped between 280-340 degrees 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 lat er (ca. 1450) contractional event; (2) a two-phase highly-saline (> 16 wt.% NaCl equivalent) fluid hosted within neotectonic (5-13 Ma Rb-Sr) galena-sp halerite-calcite-quartz-chlorite veins. Inclusions of this fluid were trapp ed at temperatures ranging from 60 to 135 degrees C and maximum confining p ressures of 950 bars. Stable isotope data (delta O,delta D) from biotite, a mphibole and epidote from the alteration assemblages of remobilized veins w ithin the Sudbury Structure show distinct evidence of a mixing trend of flu ids originating within or near the 'magmatic water box' and trending toward s a metamorphic fluid of isotopic composition similar to an evolved Sudbury Structure groundwater. At-Ar data from the amphiboles within Sudbury Intru sive Complex (SIC) footwall breccia from the Craig Mine an consistent with a mixed Ar-Ar spectrum. The step-heating Ar release spectrum yields ages ra nging 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 int erpreted as the reinjection of country rocks partially melted by the SIC ba ck into the Sudbury Structure. The isotopic evidence for mixed fluids deriv ed from a combination of a magmatic fluid and a regional groundwater/metamo rphic fluid suggests that the highly variable phase-ratio fluid inclusions associated with the ore may be divided into two end-member fluids represent ed by an exsolved SIC fluid and a regional groundwater/metamorphic fluid. T his is consistent with at least five distinct fluids being present during t he evolution of the Sudbury structure. (C) 1999 Elsevier Science B.V. All r ights reserved.