Abstract
The Thunderbird and Butler intrusions of the McFaulds Lake greenstone belt ("Ring of Fire") were studied to determine petrogenesis and associated Fe-Ti-V-(P) mineralization. These intrusions are characterized by variably well layered gabbro-anorthosite intrusions with abundant Fe-Ti oxides, broadly termed ferrogabbro. The layers are composed of partial to complete cycles that comprise basal massive oxide (magnetiteilmenite), which grade into semi-massive oxide units, followed by oxide-rich pyroxenite/melagabbro/gabbro, oxide-poor melagabbro/gabbro/leucogabbro, and topped with oxide-free leucogabbro/anorthosite. The cycles range from centimetres to metres in thickness and define the well layered portions of the intrusions that typically exhibit sharp upper and lower contacts with gradational internal contacts. Conversely, the intrusions contain broad intervals of disseminated magnetite-ilmenite (25%) hosted in melagabbro/gabbro/ leucogabbro/anorthosite, which range in thickness from metres to tens of metres. The layering and textures observed within the ferrogabbro units are thought to be dominantly produced by convection currents with intermittent periods of quiescence. The ferrogabbro intrusions are characterized by gently sloping LREE and flat HREE patterns. This geochemical signature most closely corresponds to an E-MORB source that is thought to have been the result of interaction of a mnalte plume with MORB-like mantle under the McFaulds Lake area. This plume-related magma is thought to have undergone differentiation, resulting in the abundant Cr-Ni-Cu-PGE-bearing ultramafic and evolved Fe-Ti-rich mafic suites in the McFaulds area. Additionally, the plume may have resulted in a thinned lithosphere and produced the coeval VMS occurrences. Also studied is the potential application of the TiO2/V2O5 ratio for the identification of prospective vanadium mineralization and to aid in the determination of magmatic stratigraphy.
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