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