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

Upper Part
The upper Blake River assemblage ranges in age from 2701 to 2696 Ma and consists predominantly of
flows of calc-alkaline basalt and andesite, locally with bimodal tholeiitic basalt and rhyolite. The Blake
River group occurs in the east-central part of the study area and is the most voluminous part of the
assemblage (see Figure 1). Goodwin (1979) identified this part of the Blake River group as the Misema
subgroup. New mapping and geochronology indicates that the Misema subgroup in Ben Nevis Township
(see “Ben Nevis Area Subprojects”) has a U/Pb age of 2696.6±1.3 Ma (Figure 4A). This is younger than
the pre-cauldron phase of the Noranda subgroup (2701±1 Ma: Mortensen 1993b), younger than the
Misema subgroup in Pontiac Township (2701±2 Ma: Corfu et al. 1989), and of the same age as the postcauldron
phase of the Noranda subgroup (the Reneault–Dufresnoy formation: 2697.9+1.3/–0.7 Ma
(Mortensen 1993b) and 2696±1.1 Ma (Lafrance, Moorhead and Davis 2003; and the Bousquet formation:
2698.6±1.5 Ma, 2698.0±1.5 Ma and 2694±2 Ma (Lafrance, Moorhead and Davis 2003). Thus, the Ben
Nevis–Clifford volcanic complex formed late in the Blake River group volcanic event.
The new mapping and geochronology in the Kamiskotia area has resulted in revision to the
stratigraphy proposed in this area by Ayer, Amelin et al. (2002). The volcanic rocks in the southern part
of this area are assigned to the Kamiskotia Volcanic Complex (KVC) (see “Kamiskotia Area
Subprojects”). The known VMS deposits in the study area occur within a restricted, east-facing
stratigraphic interval in the upper part of the KVC. New U/Pb ages for this interval, ranging from
2701.1±1.4 to 2698.6±1.3 Ma (Figures 3D, 3E and 3F) and, thus, indicate that it correlates better with the
upper Blake River assemblage, rather than the Tisdale assemblage (cf. Ayer, Amelin et al. 2002). The
VMS-bearing KVC unit is interpreted to unconformably overlie the upper part of the Kidd–Munro
assemblage to the south based on an age gap of about 12 my (see Figure 2). An as yet unreported age of
2700 Ma from a heterolithic volcanic unit in the hanging wall west of the Kidd Creek deposit (W.
Bleeker, Geological Survey of Canada, personal communication, 2005) suggests an interesting spatial
association between VMS deposits and the proposed unconformable contact between the upper Blake
River and the upper Kidd–Munro assemblages in both the Kamiskotia and Kidd Creek areas.
PORCUPINE ASSEMBLAGE
The Porcupine assemblage ranges in age from 2690 to about 2685 Ma and is, thus, similar to many of the
porphyries and syntectonic intrusions (discussed below). The assemblage consists predominantly of
wacke, siltstone and mudstone displaying Bouma sequence subdivisions indicating predominantly distal
deposition by turbidity currents, but locally also containing calc-alkaline felsic volcanic rocks,
conglomerates and iron formation. The age range has been revised from the previously published range of
2696 to 2692 Ma (Ayer, Amelin et al. 2002) based on a considerable amount of new geochronology over
the past few years. However, the absolute minimum age limit of the assemblage is difficult to determine
precisely because the uppermost portions of this assemblage are clastic sedimentary rocks in which the
zircons are of detrital origin and, thus, provide only the maximum age of deposition of the units.
Timmins Area
A number of distinctive sedimentary and volcanic units that are now included within the Porcupine
assemblage have been given stratigraphic names in the past including the Krist, Hoyle, Beatty and
Whitney formations (Born 1995).
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