Page 245 - Caldera Volcanism Analysis, Modelling and Response

220 W.U. Mueller et al.

8. Discussion

Subaqueous calderas are significant structures with varying geometries, complex
evolutionary histories and different types of processes responsible for volcano-tectonic
caldera subsidence. The three studied calderas give a spectrum of caldera types from
the effusive felsic Hunter Mine to explosive felsic Sturgeon Lake type. The effusive
but bimodal Normetal caldera adds to the complexity of subaqueous calderas, so that
it becomes very important to understand the physical volcanology of such systems.
Identifying an Archean caldera remains problematic as numerous observations must
be conducted to warrant such an interpretation. Prime criteria are recognising:
(1) synvolcanic fault systems, (2) volcanic facies or thickness changes across faults or
dykes, (3) felsic dyke swarms and (4) primary pyroclastic deposits, but also
considering (5) the map-scale geometry and (6) hydrothermal alteration patterns. The
numerous subaqueous emission centres in calderas can be located by tracing lateral
volcanic facies changes in mafic and felsic flows. Proximal flow segments are massive,
often columnar-jointed, that grade either into lobate (felsic) or pillowed portions
(mafic), and finally grade into brecciated flows that commonly terminate with
stratified hyaloclastite density flow deposits. Abundant flows and domes characterise
both Abitibi edifices. Extensive large-scale subaqueous pyroclastic deposits are lacking
in both study areas, so that caldera subsidence driven by the continuous extrusion of
thick lava flow units is compelling (Mueller et al., 2004). The process controlling
caldera subsidence is also similar to mafic summit calderas in which the magma
chamber evacuates and migrates along the rift zone (Tilling and Dvorak, 1993).
Fracture systems, filled with chert or jasper, are synvolcanic and related to
migration of hydrothermal/magmatic fluids during edifice construction. As shown
for the Normetal area, the geometry and distribution of volcanic rocks and their
facies organisation (Figures 7 and 8) is consistent with a subsidence structure
bordered by faults. The HMC contains a major dyke swarm, chaotic breccias near
faults (megablock slumps; e.g. Geshi et al., 2002), chert–jasper filled fractures and
small-volume fountaining eruption deposits. Incremental caldera collapse may be
due to low-volume eruptions (Skilling, 1993), but an absence of large-scale vertical
movement along synvolcanic faults precludes a piecemeal interpretation. Further
work is required, whereas the map geometry of the Normetal caldera supports a
classic piston type (Figures 6 and 10).
In contrast, the Sturgeon Lake caldera with three large explosive events during
the early and late caldera stage, displays the attributes of the explosive-dominated
Myojin Knoll silicic caldera (Fiske et al., 2001). The geometry of the volcanic
edifice and lithofacies distribution, the large high-level sill, the Beidelman Bay
intrusion, and a thick volcanic succession parcelled into blocks via synthetic and
antithetic faults, argue for a piecemeal caldera (Mueller et al., 2004; Figure 11).

8.1. Hydrothermal carbonate species distribution
The distribution of hydrothermal carbonate occurs in carbonate couplets, in which
the Ca-rich varieties are distal and the Fe-rich varieties are proximal to the massive