Page 236 - Caldera Volcanism Analysis, Modelling and Response

Characterisation of Archean Subaqueous Calderas in Canada 211

5.5. Normetal caldera evolution
The bimodal Normetal caldera is divided into 5 volcanic phases and a sedimentary
event (Figure 10A–F; Lafrance et al., 2000), with an overall geometry suggestive of a
piston caldera. The early phase I construction is a broad subaqueous shield volcano
composed of pillowed basalts and basaltic andesites associated with minor massive
dacite flows (Figure 10A). The shield volcano evolved into three structurally
controlled segments that formed volcanic centres with felsic subaqueous volcanism
characterised by effusive flows and autoclastic debris. The central caldera structure
formed during phase II and synvolcanic fault systems were well established
(Figure 10B). The principal events of volcanic construction with overlapping of
emission centres occurred during phase III. Volcanism was both andesitic and rhyolitic
with abundant volcaniclastic debris of both autoclastic and possibly pyroclastic origin
(Figure 10C). Phase IV is small-scale event, but important as indicated by the
emplacement of endogenous and exogenous domes and the geochemistry. Instead of
calc-alkaline rhyolites, distinct tholeiitic rhyolite volcanism occurred at this stage
(Lafrance et al., 2000; Figure 10D). After extensive volcanic construction, a period of
volcaniclastic sedimentation ensued. These deposits are volcaniclastic turbiditic
deposits and background sedimentary rocks that can be traced 30 along strike, and
therefore serve as the principal marker horizon for the Normetal Caldera complex
(Figure 10E). Phase V represents the Mine Sequence with volcaniclastic deposits
hosting the massive sulphides (Figure 10F). Subordinate (o20%) mafic and felsic
flows constitute the remaining portion of this last stage. The Normetal VMS deposit
and Normetmar mineralisation is constrained to the volcaniclastic lithofacies.

6. Sturgeon Lake Caldera, Wabigoon Subprovince

The 2,733–2,736 Ma Sturgeon Lake caldera of the Wabigoon Subprovince
(Figure 11A) is composed of a 2–4 km-thick, volcanic sequence (Hudak et al.,
2003; Franklin et al., 2005). The base of the caldera is intruded by high-level
2,734 Ma Beidelman Bay intrusive complex, a tonalite–diorite sill-like body at least
2 km-thick and traceable for 20 km along strike (Morton et al., 1991; Galley, 1993).
The 2–4 km-thick intracaldera succession contains six past producing VMS deposits
6
with a total of 18.7 10 tonnes at an ore grade of 8.5% Zn, 1.06% Cu, 0.91% Pb
and 119.7 g/tonne Ag. The 11.4 Mt Mattabi deposit was the largest. The Sturgeon
Lake VMS deposits are linked to the early and late phases of a 30 km wide
intracaldera volcano-tectonic depression (Figure 11B; Morton et al., 1991), which
is dominated by 1–2 km-thick volcaniclastic debris and subaqueous pyroclastic
density flow deposits. The sequence is briefly described as this caldera is a rare
Archean example of a subaqueous tephra dominated structure.

6.1. Sturgeon lake caldera volcanic succession
The Sturgeon Lake caldera is composed of an extensive mafic shield volcano and a
shoaling caldera-forming phase (Morton et al., 1991; Hudak et al., 2003), which is