Genesis of mafic-ultramafic inclusions in sublayer and inclusion quartz diorite and implications for the formation of associated NICU-PGE mineralization in the Sudbury igneous complex

Abstract

The lowermost, discontinuous units of the impact-generated Sudbury Igneous Complex (SIC), Sublayer, Footwall Breccia (FWBX), and Inclusion Quartz Diorite (IQD), are distinguished from overlying Main Mass norite rocks by the presence of abundant inclusions and Ni-Cu-PGE (PGE – platinum group element) mineralization. The majority of the felsic-mafic inclusions appear to be derived from exposed country rocks, but volumetrically important mafic-ultramafic inclusions have only rare equivalents in the surrounding country rocks and appear to be preferentially associated with sulfide mineralization. Establishing the petrogenesis of the mafic-ultramafic inclusions and the nature of their association with the Ni-Cu-PGE mineralization are therefore critical to understand the evolution of the impact melt, genesis of Sublayer, FWBX, and IQD, and the formation of one of the world’s largest accumulations of Ni-Cu-PGE mineralization. Petrographic, mineralogical, geochemical, and Sm-Nd and Re-Os isotopic data indicate three origins for the olivine-bearing mafic-ultramafic inclusions: (1) Anteliths, comprising olivine melanorites and olivine melagabbronorites in the Whistle and Levack embayments on the North Range, which are characterized by igneous textures, Zr/Y, Zr/Nb, Nb/U, and Zr/Hf ratios similar to igneous-textured Sublayer matrix (ITSM), unradiogenic εNd1850 Ma values (-8 to -5), and slightly unradiogenic to radiogenic γOs1850 Ma values (-8 to +94). They likely crystallized from a local mixture of SIC impact melt and a more mafic melt derived by melting of the widespread Huronian volcanic and subvolcanic units in the region. (2) Local xenoliths, comprising wehrlites and olivine clinopyroxenites in the Levack embayment and olivine melanorites in the Foy Offset on the North Range, which are characterized by shock mosaic and recrystallized textures, and trace element patterns (e.g., negative Th-U, Nb-Ta-(Ti), Sr, and Zr-Hf anomalies) similar to and Nb/U ratios overlapping with a layered mafic-ultramafic intrusion in the footwall of the Levack and Fraser deposits. They were likely derived from local mafic-ultramafic protoliths that were petrogenetically-related to the layered mafic-ultramafic intrusion in the footwall of the Levack and Fraser deposits. (3) Exotic xenoliths, comprising phlogopite/feldspar lherzolites in the Trill, Levack, and Bowell embayments and the Foy Offset dike on the North Range, which are characterized by variably igneous, tectonic-metamorphic, and shock-metamorphic textures, and orthopyroxene reaction rims against igneous-textured Sublayer matrix (ITSM), indicating disequilibrium with the impact melt. One composite inclusion exhibits igneous layering of feldspar lherzolite and olivine gabbro, suggesting derivation from an unexposed older layered mafic-ultramafic intrusion. The calculated parental magma for one particularly well-preserved feldspar lherzolite inclusion is similar to continental arc basalt formed by up to 5% partial melting of garnet peridotite. Ol-Cpx-Pl thermobarometry of several exotic inclusions indicate equilibration at 900o C – 1120o C and 210 – 300 MPa, suggesting emplacement into upper-middle crust (7.7 – 10.9 km), prior to being incorporated into the lower parts of the proto-SIC during impact excavation and/or thermomechanical erosion of target rocks. Most analyzed inclusions, ITSM, and Main Mass lithologies are enriched in highly incompatible elements with negative Nb-Ta-Ti anomalies, unradiogenic Nd, and radiogenic Os isotopic signatures. These features suggest that the impact sampled rocks that were derived from subduction-metasomatized mantle, including the widespread Huronian volcanic and intrusive rocks adjacent to the SIC. Melting of these volcanic and intrusive rocks and the underlying Neoarchean Superior Province upper-middle crustal rocks would produce the observed geochemical characteristics of the SIC lithologies and inclusions. The Main Mass has a very homogeneous Hf isotopic composition, indicating that the impact melt sheet was well mixed. However, Sublayer, IQD, and overlying basal Main Mass norites vary widely in terms of Pb-S-(Os) isotopic compositions. Most mafic-ultramafic inclusions, except for anteliths, contain no sulfides and exhibit no signatures of Ni-Cu-PGE depletion caused by prior sulfide saturation, which suggest that the association between mafic-ultramafic inclusions and NiCu-PGE sulfide mineralization is attributable to the hydrodynamic equivalence of less dense but larger silicate inclusions and denser but smaller sulfide melt droplets during transport and/or settling. Anteliths, locally-derived inclusions, and local variations in Pb-S-(Os) isotopes must have been generated in situ, requiring significant degrees of assimilation of footwall rocks via thermomechanical erosion, whereas most exotic inclusions other than shocked feldspar lherzolite were derived from deeper mafic-ultramafic protoliths, generated during impact excavation and/or thermomechanical erosion, and physically transported into their current locations. Thus, thermomechanical erosion played an important role in the generation of embayments, incorporation of xenoliths and sulfide xenomelts from the mineralized country rocks (e.g., EBLINipissing-Huronian), and formation of isotopic heterogeneity in the basal parts of the SIC. Convective- and gravity-driven mass flow contributed to the horizontal transportation of inclusions and sulfide xenomelts into the embayment when the impact melt contained <45% inclusions, but became less significant as proto-Sublayer incorporated more inclusions.