Geology and genesis of hybridized ultramafic rocks in the black label hybrid zone of the black thor intrusive complex, McFaulds Lake Greenstone Belt, Ontario, Canada

Abstract

The ca. 2.7 Ga Black Thor Intrusive Complex (BTIC) is a komatiitic elongate layered intrusion composed primarily of dunite, lherzolite, olivine websterite, websterite, and chromitite overlain by lesser gabbro and anorthosite. After emplacement but before complete crystallization, a Late Websterite Intrusion (LWI) reactivated the feeder conduit and intruded the base and the core of the BTIC, including the Black Label Chromitite Zone. LWI is a discordant to semiconcordant intrusion that produced marginal zones of heterogeneous, interfingering hybrid matrix and clasts defined as the Black Label Hybrid Zone (BLHZ). The clasts range from 1 to 200 cm in size (rarely > 5m), exhibit amoeboidal to subangular shapes, with sharp to diffuse margins, and varied in compositions from dunite to lherzolite to chromitite. The nature of these clasts appear to have been controlled by the compositions of the lithologies, the thickness of layering, the nature of the contacts between layers, and the initial temperatures of the lithologies being incorporated. There are two types of hybrid groundmass: 1) hybrid harzburgite containing xenocrystic olivine within a websterite, and 2) hybrid chromite harzburgite containing xenocrystic chromite and olivine within a websterite. Both types of hybrid rocks are exceptionally well preserved in terms of mineralogy and textures. The genesis of the BLHZ is extremely complex and involved five interdependent assimilative processes: (1) mechanical disaggregation of clasts resulting in dispersal of xenocrysts; (2) grain-boundary melting (clinopyroxene + plagioclase) resulting in selective assimilation via partial melt mixing; (3) mineral-reaction relation resulting in the dissolution of high-Mg olivine xenocrysts and subsequent growth of intermediate-Mg hybrid orthopyroxene; (4) Mineral-melt re-equilibration between entrained chromite and olivine xenocrysts in LWI melt resulting in diffusive chemical exchange; and (5) complete dissolution of xenocrystic phases very locally formed clast-free rocks of intermediate composition. From map- and core-scale cross-cutting relationships a possible emplacement model can be made: 1) initial emplacement of the BTIC and primary cumulate layers; 2) LWI reactivation of the BTIC feeder, and the diking/silling, stoping and partial assimilation of BTIC wall rocks; 3) formation of heterolithic breccias, heterogeneously hybridized rocks (BLHZ), and associated sulfide mineralization; and 4) local late stage fractional crystallization of LWI magma. The intrusion of LWI magma into the BLCZ does not appear to have consumed any of the chromitite, but has locally reduced the grade of the mineralization through dilution and dispersal. Low-grade patchy disseminated to net-textured Fe-Ni-Cu-(PGE) sulfide mineralization locally occurs in clast-rich regions of the BLHZ and appears to have been generated during the hybridization process. This is the first occurrence known to us where mixing of a magma and cognate xenoliths has led to the formation magmatic sulfides. This style of mineralization is relatively restricted within the BTIC but is consistent with the limited solubility of S in mafic-ultramafic magmas and the small amounts of magma involved.