Benjamin Kuzmich's MSc thesis 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 V₂I₅ 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 cause 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 intrusions; 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, <110km) 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.

A copy of the thesis can be downloaded here