Overview of Results from the Greenstone ... - Geology Ontario

Upper Komatiitic Horizon
The Upper Komatiitic Horizon is characterized by komatiitic ultramafic metavolcanic rocks that exhibit
pale orange-brown or bone-white weathered surfaces and dark green or pale green fresh surfaces, quite
similar to the ultramafic rocks observed within the Lower Komatiitic Horizon.
The rocks in the lower part of the Upper Komatiitic Horizon are primarily massive komatiitic dunites
and peridotites, but some olivine spinifex-textured komatiites are also present. In some areas, the ultramafic
rocks are less magnesian komatiitic basalts and exhibit pyroxene spinifex textures and pillow morphologies.
The rocks in the upper part of the Upper Komatiitic Horizon include both olivine spinifex-textured
komatiites and massive komatiitic peridotites. They display a wider range of primary structures
(polyhedral jointing, flow breccias, spinifex-textured veins and/or sills) and textures (cumulus and
spinifex textures). The thicknesses of the flows decreases upwards through the stratigraphy from
approximately 30 to approximately 2 m.
Nickel Sulphide Deposits
Numerous occurrences of nickel, copper and platinum-group element mineralization associated with
intrusive and extrusive komatiites have been identified in the Shaw Dome area (Hall, McDonald and
Dinel 2003; Hall, Houlé and Tremblay 2004; Houlé, Hall and Tremblay 2004; Houlé and Guilmette
2005). Of the most significant occurrences, the Galata prospect and possibly the McWatters deposit occur
in the Lower Komatiitic Horizon, whereas the Langmuir #1 and #2 mines, the Redstone mine, and the
Hart prospect occur in the Upper Komatiitic Horizon. All of these deposits are associated with very
magnesian, olivine-rich units, which have been interpreted as channelized lava flows or sills. All are also
closely related spatially to sulphide-bearing iron formation, and all are localized in some type of footwall
embayment (e.g., Pyke 1975; Green and Naldrett 1981; Robinson and Hutchinson 1982; Jiricka 1984;
Schuster 1995). Sproule et al. (2002, in press) concluded that all of the host rocks were undersaturated in
sulphide prior to emplacement. As noted above, these features are all critical in the formation of
magmatic Ni-Cu-(PGE) mineralization.
Thus, the combination of a fertile komatiitic magma (metal source), proximity to sulphide-bearing
iron formations (sulphur source), abundant olivine cumulate rocks indicating high magma flux (heat
source and dynamic environment), and embayments (localization) provide an excellent environment for
forming komatiite-associated Ni-Cu-(PGE) deposits in the Shaw Dome area.
Implications for Komatiite-Associated Deposits
A review of chalcophile element data (PGE, Ni, Cu, Co) of barren komatiitic rocks throughout the Abitibi
greenstone belt reveals that all komatiitic rocks, regardless of age, display the same overall unfractionated
trends (Sproule et al., in press). Thus, the komatiitic magmas that generated these rocks were sulphideundersaturated
when they were erupted and did not lose any sulphide mineral phases or PGEs during
ascent through the crust. Thus, all komatiitic rocks, regardless of assemblage, represent favourable
magma sources for Ni-Cu-PGE mineralization.
Several researchers have recognized the importance of volcanological processes in the genesis and
localization of Ni-Cu-(PGE) mineralization associated with komatiites. Volcanic facies may be established
by detailed mapping at the outcrop scale, stratigraphic mapping at the property and camp scale, and by
inference on a larger scale. Volcanic facies that have been identified include vent pyroclastic facies,
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