Publication Type:
Book ChapterSource:
Targeted Geoscience Initiative 4; Canadian nickel-copper-platinum group elements-chromium ore systems; fertility, pathfinders, new and revised models, Geological Survey of Canada, Calgary, AB, Canada, p.103-114 (2015)Keywords:
Archean, Canada, Canadian Shield, chemical composition, copper ores, eastern canada, gabbros, harzburgite, hybridization, igneous rocks, inclusions, intrusions, iron ores, magmas, metal ores, metals, mineral composition, Mineral exploration, Neoarchean, nickel ores, North America, Ontario, peridotites, petrography, platinum group, plutonic rocks, Precambrian, pyroxenite, sulfides, superior province, ultramafics, websteriteAbstract:
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.
Notes:
GeoRef, Copyright 2018, American Geological Institute.<br/>2016-108630<br/>Black Label hybrid zone<br/>Black Thor Complex<br/>McFaulds Lake greenstone belt
