Petrogenesis of the ferrogabbroic intrusions and associated Fe-Ti-V-P mineralization within the McFaulds Greenstone belt, Superior Province, Canada
Kuzmich, Benjamin Nick
Master of Science
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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.