Petrogenesis of the Fe–Ti–V–(P)-bearing Ferrogabbroic Big Mac Intrusion in the McFaulds Lake Greenstone Belt, Superior Province, Northern Ontario (Canada)

English Abstract:

The mafic-dominated Big Mac intrusion is one of numerous ferrogabbroic intrusions occurring in the McFaulds Lake area within the Meso- to Neoarchean eastern Oxford-Stull domain of the Superior Province. It forms a broadly layered subconcordant sill composed mainly of gabbro (sensu lato) with lesser anorthosite and rare pyroxenite. Several dm-m thick semi-massive to massive magnetite-ilmenite layers occur near the stratigraphic base of the intrusion. The gabbros, anorthosites, and pyroxenites are mostly composed of plagioclase and amphibole with rare preserved clinopyroxene, but still exhibit partially preserved cumulate textures with cumulus plagioclase. These lithologies can contain up to 35% Fe-Ti oxides with quite variable magnetite/ilmenite ratios. Oxides layers are composed mainly of large magnetite crystals with anhedral crystals of ilmenite and small ilmenite exsolution lamellae and can contain up to 76 wt.% FeO_t, 19 wt.% TiO₂, and 0.58 wt.% V₂O₅. The Big Mac intrusion extends over a strike length of ~60 km with an apparent thickness ranging from 1 to 4 km with an overall stratigraphic top to the northeast. Based on geochemistry and mineral chemistry, the northern part is more primitive whereas the southern part is more evolved. The northern part is characterized by the presence of mafic with rare ultramafic units, high magnetite/ilmenite ratios, more primitive geochemical signatures (Cr = 23−3830 ppm; P₂O₅ ≤0.19 wt.%), and the most primitive plagioclase and Fe-Ti oxides compositions, whereas the southern part is characterized by mafic units only containing up to 9% apatite, lower magnetite/ilmenite ratios, more evolved geochemical signatures (Cr = 15−134 ppm; P₂O₅ = 0.13−2.94 wt.%), and the most evolved mineral compositions. The presence of semi-massive to massive oxide layers with high Fe-Ti-V contents near the basal contact of the Big Mac intrusion clearly demonstrates its potential to host this style of orthomagmatic mineralization.