Hybridized ultramafic rocks in the Black Label hybrid zone of the Black Thor intrusive complex, McFaulds Lake greenstone belt, Ontario

Abstract

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.

3D Geological Modeling of the Double Eagle – Black Thor Intrusive Complexes, McFaulds Lake Greenstone Belt, Ontario, Canada

“…• A study on the implications of the Late Websterite Intrusion on the Black Thor and Black Label chromite horizons (Spath, 2017). (Schulte et al, 2010).…”

“…Associated chromitite occurs in a variety of mineralization styles (e.g. very thinly laminated to very thickly bedded, lightly to heavily disseminated, and interbedded massive to semi-massive chromitite) typically within intervals of dunite and peridotite, less commonly in the pyroxenite (Carson et al, 2015;Spath, 2017). In contrast, the overlying mafic sequence consists of mela-, meso-and leucogabbro with lesser anorthosite that have undergone minor chlorite alteration ( Fig.…”

“…In addition to the metasediments and metavolcanic rocks of the greenstone belt, these ultramafic-mafic intrusions are hosted by a medium-grained granodiorite-tonalite that has undergone minor sericiteepidote-chlorite alteration. Generally, the host rock lithologies exhibit mineral assemblages typical of greenschist facies metamorphism (Spath, 2017).…”

“…After emplacement but before complete crystallization, rocks of the lower and middle ultramafic series of the BTIC were cut by the extensive Late Websterite Intrusion (LWI; Spath et al, 2015;Spath, 2017). Composed predominantly of websterite and lesser feldspathic websterite, the LWI is localized mostly within the feeder zone and basal peridotite while also intruding the lower dunitic zones and the lower BLCZ (Spath et al, 2013).…”

“…Composed predominantly of websterite and lesser feldspathic websterite, the LWI is localized mostly within the feeder zone and basal peridotite while also intruding the lower dunitic zones and the lower BLCZ (Spath et al, 2013). Emplacement of the LWI caused local brecciation of the BLCZ, resulting in a variety of hybridized rock types termed the Black Label hybrid zone (BLHZ) (Carson et al, 2015;Spath et al, 2015). These hybrid rock types include pyroxene peridotite, pyroxene oikocrystic peridotite, olivine pyroxenite, pyroxene oikocrystic chromitite and angular to amoeboidal chromitite breccias from the thermomechanical erosion of preexisting BTIC units (Spath et al, 2013).…”