Page 212 - Caldera Volcanism Analysis, Modelling and Response

Characterisation of Archean Subaqueous Calderas in Canada 187

evacuated magma and volume and magma composition. Piston, trapdoor piecemeal
calderas are well documented whereas down sag and funnel calderas are uncommon
(Lipman, 1997). Acocella (2007) gives an excellent caldera review.

4. Hunter Mine Caldera

The Hunter Mine caldera (HMC) in the NVZ (Figure 1) is a complex
subaqueous felsic-dominated, calc-alkaline arc edifice (Dostal and Mueller, 1996)
overlain conformably by a komatiite–basalt succession (Dostal and Mueller, 1997;
Mueller and Mortensen, 2002; Figure 2A, B). Initial depth of the HMC is W500 m
as suggested by: (a) a silicified mudstone horizon at the top of the HMC (Chown
et al., 2000; Figure 3A), (b) banded iron-formation (Figure 3B; BIF) and abundant
Bouma-cycled turbiditic tuffs (Figure 3C, D) throughout the stratigraphy, (c)
bedded felsic hyaloclastite deposits and (d) an upper depositional contact with
pillowed komatiite–basalt flows. Evidence of shallow-water wave-induced
structures or reworking is lacking.
The composite HMC stratigraphy (Figure 2B) displays a 5–6 km-thick
dominantly felsic sequence characterised by an extensive 5–7 km wide felsic feeder
dyke swarm (Mueller and Donaldson, 1992b). The caldera sequence is divided into
lower, middle and upper formational stages based on U–Pb age determinations and
lithology. The lower and upper stages are distinct caldera forming phases separated
by a major intrusive phase, the Roquemaure sill (Table 1). The central caldera
depression was at least 7 km in diameter. Numerous synvolcanic faults and fractures
with local vertical displacements of up to 20 m are not uncommon. Synvolcanic
structures include: (1) fluid escape structures in tuff turbidites (Figure 3E), (2)
precipitation of hydrothermal fluids within fractures/faults resulting in black and
white chert, chert–jasper–magnetite filling fractures and draping dyke margins
(Figure 3F), (3) discrete volcanic facies changes across faults and dykes and (4)
abrupt changes in the style and types of hydrothermal alteration. The high-level,
TTG (tonalite–trondhjemite–granodiorite), synvolcanic Poularies pluton is char-
acterised by widespread argillic to propylitic porphyry-type alteration associated
with numerous chalcopyrite–molybdenite showings (Chown et al., 2002). The
Poularies pluton and thick Roquemaure sill (Figure 2B) are considered the heat
sources for caldera-hosted hydrothermal convection, as required for VMS (Galley,
2003).

4.1. Lower formational stage (first caldera event)

The 3–4 km thick lower formational stage (Figure 2B) represents the first caldera
event with dome-flow deposits and abundant autoclastic breccias (70%), and
subaqueous pyroclastic deposits and reworked equivalents (30%). Three distinct
lithofacies were observed: (1) coherent and brecciated felsic lithofacies, (2)
volcaniclastic and iron-formation lithofacies and (3) a felsic dyke swarm best
exposed at this edifice level (Table 1).