Page 239 - Caldera Volcanism Analysis, Modelling and Response

214 W.U. Mueller et al.

(ii) an early, 650–1,300 m-thick, caldera stage composed of pyroclastic deposits that
contains the principal Mattabi VMS deposit, (iii) a 500–1,500 m-thick, late caldera
stage with andesite–dacite flows, endogenic domes, banded iron-formation and
volcaniclastic debris and (iv) the poorly correlated Lyon Lake Fault succession
composed of basaltic–andesitic flows and volcaniclastic rocks (Morton et al., 1991;
Hudak et al., 2003).
The precaldera shield volcano, referred to as the Darkwater succession (Figure 11),
was interpreted by Groves et al. (1988) as an emergent to subaqueous mafic sequence,
yet volcanism must be subaqueous considering the presence of massive sulphide
deposits up-section in prominent water-lain GB deposits. According to Groves et al.
(1988) this lowermost succession contains four units: (1) a 600–800 m-thick unit
composed of mafic massive and flow-top breccias, (2) 20–400m-thick felsic lava flow
units with individual massive to brecciated flows 3–80m-thick, (3) 40–100m-thick
breccias and 0.5–1.5 m-thick bedded tuff–lapilli tuffs deposited by turbidity currents
and (4) 100–800 m-thick bedded pyroclastic unit deposited by high- and low-
concentration density currents in a subaqueous setting.
The intracaldera succession has an early and late caldera phase, and Hudak et al.
(2003; Figure 12) have given a rigorous description of the pyroclastic rocks. The early
basal phase of the caldera features the inferred subaerial Jackpot Lake succession and
meso- to mega-breccias interpreted as caldera wall collapse deposits (Morton et al.,
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1991). Up-section the 16 km High Level Lake Tuff represents the first large-scale
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explosive phase and the voluminous 27 km Mattabi Tuff ends the early caldera phase.
The 80–300 m-thick High Level Lake tuff contains massive, graded to well-bedded
tuffs and lapilli tuff units that display marked lateral facies change along strike of
the intracaldera floor. The subaqueous Mattabi tuff pyroclastic units, collectively
15–650 m-thick (Figure 12), exhibit a vertical fining-upward sequence composed
of (a) a basal 10–155m-thick massive lapilli tuff, (b) a medial 6–48 massive to graded
lapilli tuff and (c) a normal to inverse graded medium bedded to laminated tuff up to
13 m-thick (Hudak et al., 2003), which Mueller et al. (2004) used to interpret as an
indicator of primary deposition in a subaqueous setting.
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The late caldera phase, with the exception of the explosive 7 km Middle L tuff
unit, features predominant subaqueous effusive volcanism. A thick pillowed andesitic
unit covers a large portion of the intracaldera floor and felsic lava domes are common.
The 15–150 m-thick Middle L tuff, thickest at the F-Group deposits, is composed of
(a) 3–60m-thick massive to normal graded quartz and feldspar-rich tuff and lapilli
tuff and (b) 0.1–15 m-thick massive graded and parallel laminated tuff locally rich in
quartz crystals. The Lyon Lake Fault sequence remains enigmatic. The massive
sulphide deposits are located in the porous pyroclastic deposits (Figure 12).

7. The Link: Subaqueous Calderas with Chert–Iron
Formation and Hydrothermal Carbonates

Calderas contain autoclastic and pyroclastic debris as well as caldera wall
collapse deposits that have a high porosity over kilometre distances, so that