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

The Blake River Group is divided into 3 subgroups: the Garrison, the Misema and the Noranda. The
volcanic rocks of Katrine, Ben Nevis and Clifford townships belong to the Misema subgroup. The
Misema subgroup in Ben Nevis Township has a U/Pb age of 2696.6±1.3 Ma. 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 post-cauldron
phase of the Noranda subgroup (the Reneault–Dufresnoy formation: 2697.9+1.3/–0.7 (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 Noranda cauldron sequence of the Noranda subgroup hosts the majority of VMS deposits in the
Blake River Group (Gibson, Watkinson and Comba 1989; Gibson and Watkinson 1990). The volcanism
in the Noranda cauldron is flow dominated (de Rosen-Spence 1976; Gibson, Watkinson and Comba 1989;
Paradis 1990), as is the pre-cauldron volcanism (Péloquin, Verpaelst and Gaulin 1989a; Péloquin et al.
1989b; Péloquin 2000). However, pyroclastic rocks form an important part of the post-cauldron
volcanism (Trudel 1978, 1979; Goutier 1997; Lafrance, Moorhead and Davis 2003).
The Ben Nevis–Clifford volcanic centre has a large pyroclastic component. Subaqueous andesite
flows are the dominant rock type in the study area. In Katrine Township, pyroclastic rocks are rare, and
felsic volcanic rocks even rarer. In Ben Nevis and Clifford townships, there is a spectrum of subaqueous
volcanic rocks from basalt to rhyolite. Both flow and pyroclastic facies occur in all rock types, but the
felsic volcanic rocks are dominantly pyroclastic with rare flow or dome facies. The large pyroclastic
component of the Ben Nevis–Clifford volcanic centre suggests that it may be a subaqueous composite
stratovolcano constructed on a mafic to intermediate volcanic “floor” represented by the Misema
subgroup mafic to intermediate volcanic rocks in Katrine Township. The increase in pyroclastic rocks,
and the pumiceous and scoriaceous nature of some of the fragments, indicates shallow depth of
emplacement for the Ben Nevis–Clifford volcano.
The geochemistry of the Ben Nevis–Clifford volcanic centre also differs from the Noranda
volcanism. Noranda has long been known to be bimodal (andesite–rhyolite) and to have 2 end-member
andesite affinities: tholeiitic and calc-alkalic (or LREE-enriched) (e.g., Goodwin 1977; Gélinas et al.
1977; Laflèche, Dupuy and Bougault 1992; Péloquin 2000; Péloquin et al. 2001). In contrast, the Misema
subgroup in the study area is unimodal (andesite dominant) with no silica gap, and there are no true
tholeiites in the area.
Volcanogenic massive sulphide (VMS)-style alteration and mineralization occur in the study area
indicating that a synvolcanic hydrothermal system existed. However, the mineralization is concentrated in
synvolcanic dikes as veins or in the multiple injection margins, suggesting that the fluids were
channelized within the dikes. This indicates that the present exposure may represent the feeder zone of the
hydrothermal system. The area is structurally complex and the influence of faults and deformation zones
on the stratigraphy of the area must be determined, as should the effect of possible folding.
High-level synvolcanic dikes, which can only be distinguished from extrusive rocks by their
crosscutting relationships, occur in all rock types throughout the study area, as do mafic to intermediate
dikes and irregular masses, which are comagmatic with the Blake River Group volcanism.
The differences between the Ben Nevis–Clifford volcanic centre and the Noranda cauldron indicate
that mineral exploration in Ben Nevis and Clifford townships should not be limited strictly to Norandatype
Cu-Zn VMS deposits. The presence of VMS-style alteration and mineralization in the area indicates
that a synvolcanic hydrothermal system existed. However, the apparent concentration of the
mineralization in the synvolcanic dikes, as veins or in the multiple injection margins, suggests that the
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