| dc.description.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. | en_us |
