Although komatiite has been defined as an ultramafic volcanic rock characterised by spinifex texture, there is a growing recognition that similar textures form in high-level dykes and sills.Here we report the results of a petrological and geochemical investigation of a ~5m thick komatiite sill in Dundonald Township, Ontario. This unit forms part of a series of komatiites and komatiitic basalts, some of which clearly intruded unconsolidated sediments. The komatiite sill is differentiated into a spinifex-textured upper part and an olivine cumulate lower part. Features characteristic of the upper sections of lava flows, such as volcanic breccia and a thick glassy chilled margin, are absent and, instead, the upper margin of the sill is marked by a layer of relatively large (1-5 mm) solid, polyhedral olivine grains that grades downwards over a distance of only 2 cm into unusually large, centimetre-sized, skeletal hopper olivine grains. This is underlain by a ~1m thick zone of platy spinifex-textured olivine and coarse complex dendritic spinifex-textured olivine. The texture of the olivine cumulate zone in the overlying unit is uniform right down to the contact and a lower chilled margin, present at the base of all lava flows, is absent. The textures in the sill and the overlying unit are interpreted to indicate that the sill intruded the olivine cumulate zone of the overlying unit. Thermal modelling suggests that soon after intrusion, a narrow interval of the overlying cumulate partially melted and that the liquid in the upper part of the sill became undercooled. The range of olivine morphologies in the spinifex-textured part of the sill were controlled by nucleation and crystallization of olivine in these variably undercooled liquids.
The ca. 2.7 Ga Black Thor intrusive complex (BTIC) is an ultramafic to mafic, layered intrusion composed primarily of dunite, peridotite, pyroxenite, and chromitite overlain by lesser gabbroic rocks and rare anorthosite. After emplacement but before complete
crystallization, a late websterite intrusion (LWI) reactivated the feeder conduit and transected the basal part of the BTIC, including the Black Label chromitite zone (BLCZ). All rocks have been metamorphosed to lower greenschist facies, but igneous minerals are preserved in some parts (particularly
in the LWI) and relict igneous textures are well preserved in most parts. Logging of selected parts of 39 drill cores shows that semi-concordant intrusion of LWI magma and incorporation of inclusions produced a 1 to 10 m thick marginal zone of heterogeneous, interfingering brecciation defined as the
Black Label hybrid zone (BLHZ). The BLHZ contains variably sized (1-50 cm) dunite/lherzolite/ chromitite inclusions with subangular to amoeboidal geometries, sharp to diffuse contacts, and locally significant amounts of patchy disseminated to patchy net-textured Fe-Ni-Cu-(PGE) sulphide
mineralization. The core of the LWI is typically an inclusion-free, medium-grained, orthopyroxene-rich adcumulate with accessory chromite or olivine; however, inclusion-rich intervals of the LWI contain more olivine and chromite produced by disaggregation and partial assimilation of BTIC ultramafic
rocks. There are two types of hybrid groundmass: one containing xenocrystic olivine and one containing xenocrystic chromite and olivine in varying proportions. Geochemical signatures of the hybrid rocks reflect the partial assimilation and brecciation of chromitite/lherzolite/dunite sequences.
Similar Th-U-Nb-Ta-light rare earth element LREE patterns suggest that the LWI is related to the BTIC, presumably representing a more fractionated magma from deeper in the system. Further characterization of the hybrid rocks and inclusion variability is in progress and will help to establish the
range and variability of processes within the BTIC, and their influence on the genesis of associated Fe-Ni-Cu-PGE sulphide mineralization in the BLHZ.
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