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

Shaw Dome Area
GEOLOGICAL SETTING
The Shaw Dome area (Pyke 1974) is outlined by an outward-facing stratigraphic contact between the
Deloro and Tisdale assemblages, and comprises three packages. “Package A” (Deloro assemblage): The
core of the domal structure is composed dominantly of massive and pillowed calc-alkalic intermediate to
mafic metavolcanic lava flows with lesser intermediate volcaniclastic rocks and ultramafic intrusive rocks
(dikes, sills and plutons). Several horizons of iron formations occur at different stratigraphic levels within
this sequence, but dominantly within the upper part. “Package B” (Tisdale assemblage): The middle part
of the sequence is composed of intermediate to felsic fragmental metavolcanic rocks (with lesser massive
and pillow lavas), iron formations, and ultramafic flows and intrusions. Several horizons of iron
formations occur at different stratigraphic levels within this sequence. “Package C” (Tisdale assemblage):
The peripheral part of the sequence is composed of massive and pillowed tholeiitic mafic metavolcanic
rocks intercalated with massive and pillowed komatiitic basalt and komatiite metavolcanic rocks.
The metavolcanic and metasedimentary rocks in the Shaw Dome have been metamorphosed to
lowermost greenschist to lowermost amphibolite facies, depending on proximity to later intrusions.
Deformation is strongly partitioned and many rocks exhibit well-preserved primary structures (e.g.,
pillows, polyhedral jointing) and textures (e.g., cumulate and spinifex textures). Within ultramafic rocks
in the area, olivine is normally serpentinized whereas pyroxene and plagioclase (in gabbroic facies) are
frequently preserved. Some rocks have been intensely and pervasively carbonated (calcite, magnesite, and
siderite), which has destroyed most primary textures. Nevertheless, cumulate and spinifex textures are
often preserved even in these highly altered rock.
KOMATIITIC ROCKS
The ultramafic rocks in the Shaw Dome area consist of intimately associated flows and high-level
intrusive components. Determination of the intrusive versus extrusive origin of these rocks and the
implications of these emplacement mechanisms is one of the goals of the Ni-Cu metallogenic theme of
the collaborative subproject being conducted between OGS and the Discover Abitibi Initiative.
Distinguishing between an intrusive or extrusive origin is difficult because of the scarcity of large, wellpreserved
outcrops. Nonetheless, the ultramafic rocks in this area have been subdivided into two groups:
an intrusive “Lower Komatiitic Horizon” and an extrusive “Upper Komatiitic Horizon” (Muir 1975; Pyke
1982; Larson 1996; Stone and Stone 2000).
Lower Komatiitic Horizon
The Lower Komatiitic Horizon is composed of two types of intrusive ultramafic rocks: a) discordant
ultramafic bodies and b) subconcordant ultramafic bodies. However, both of these types have essentially
the same textural characteristics and exhibit pale orange-brown or bone-white weathered surfaces and
dark green or pale green fresh surfaces. Most exposures consist of komatiitic dunite to peridotite, but
some pyroxenitic and gabbroic units are locally present in association with the other more ultramafic
components.
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