Petrogenesis of the ferrogabbroic intrusions and associated Fe-Ti-V-P mineralization within the McFaulds Greenstone belt, Superior Province, Canada

Abstract

The McFaulds Lake area (commonly known as the Ring of Fire), has been the site of much recent exploration within northern Ontario. The area represents a recently discovered Archean greenstone belt which is host to world class chromite deposits along with significant Cu-­‐Zn VMS, magmatic Ni-­‐Cu-­‐PGE and Fe-­‐Ti-­‐V occurrences. Much emphasis has been placed on the chromite mineralized ultramafic intrusions with little attention focused on the Fe-­‐Ti-­‐V mineralized ferrogabbroic intrusions. The Butler and Thunderbird intrusions represent the best described intrusions within the volumetrically significant ferrogabbroic suite within the McFaulds Lake area. These intrusions are characterized by a suite of well layered magnetite-­‐ilmenite rich rocks which are dominantly composed of gabbroic to anorthositic units with lesser stratigraphically conformable units composed of pure magnetite-­‐ilmenite. The Fe-­‐Ti oxide rich layers contain variable vanadium mineralization and low chromium contents within magnetite (up to 2.45 V2O5 wt. %, 0.99 % Cr2O3 wt. %) and ilmenite (up to 0.57 V2O5 wt. %). The massive and semi-­‐ massive oxide layers occur as basal members of repeated cycles characterized by sharp lower contacts which grade upwards into oxide-­‐rich pyroxenite, followed by oxide-­‐bearing leucogabbros and/or anorthosites. The layers are believed to be caused dominantly by magmatic convection currents within a system which is at least partially open to oxygen. No evidence has been found to suggest multiple pulses of magma. Oxide-­‐silicate liquid immiscibility is thought to only occur within the evolved, apatite-­‐bearing margins of the Thunderbird intrusion; however, additional drilling may reveal further apatite mineralization. The ferrogabbroic intrusions are thought to have originated from a shallow depleted mantle source, possibly related to a plume event. The ferrogabbros have likely undergone a two stage differentiation to account for the extreme iron enrichments. The first stage is characterized by an anhydrous, tholeiitic melt, within the upper mantle (above the garnet stability field, <110 Km) which underwent Fe-­‐Ti enrichment due to the crystallization of Fe-­‐ poor phases (e.g., olivine, plagioclase, etc.) within a system closed to oxygen. The second stage is considered to be a very shallow intrusion within the McFaulds Lake mafic-­‐felsic volcanic rocks. This final stage is characterized by a system which was at least partially open to oxygen from an originally reduced magma (<QFM buffer). These magmas initially crystallized Cr-­‐V-­‐rich magnetite-­‐ilmenite horizons and gradationally evolved into Cr-­‐V-­‐poor, apatite-­‐bearing ferrogabbros. These ferrogabbros likely share a parental magma with the coeval Cr-­‐Ni-­‐PGE-­‐ bearing ultramafic intrusions of the McFaulds Lake greenstone belt. Additionally, spatial and geochronological evidence suggests that abundant VMS-­‐style mineralization within the McFaulds Lake area may be a result of a thinned lithosphere during plume tectonics.