Page 251 - Caldera Volcanism Analysis, Modelling and Response

226 W.U. Mueller et al.

In contrast, at Sturgeon Lake caldera the shield structure grades rapidly into an
explosive succession with explosive and extensive ignimbrite-forming events similar
to the Valles Caldera. The three large pyroclastic events are only disrupted by minor
felsic dome-forming phases, but the major mafic pillowed flow event at the top
of the sequence attests to a submerged edifice. Identifying the type of caldera in
an Archean succession remains problematic, but from the apparent geometry,
the Hunter Mine caldera may be part of a nested caldera sequence, the Normetal
caldera has the salient attributes of a piston type and the Sturgeon Lake caldera best
qualifies as a piecemeal type.
With detailed volcanic facies and regional mapping, the overall structure of a
caldera sequence is readily identified, even in the Archean, but targeting VMS sites
remains problematic. Our model predicts mineralisation sites by using physical
volcanology and hydrothermal alteration processes. In Archean terranes, subaqu-
eous caldera subsettings require detailed volcanic facies mapping of intracaldera
settings so that individual domes or small explosive volcanic centres can be
identified. Volcanic massive sulphide sites are linked to synvolcanic fracture systems
and growth faults that form at the caldera wall and on the fragmented caldera floor.
In ancient systems, synvolcanic faults and associated blocks or segments may be
difficult to recognise, but they are extremely important. It is along these extensional
fractures that intracaldera subenvironments form. If synvolcanic faults cannot
be directly recognised in the subsettings, then indirect indictors such as numerous
dyke intrusions, abrupt volcano-sedimentary facies changes, synsedimentary or
synvolcanic deformation or chaotic breccia assemblages may be used to suggest
fault proximity. Intense hydrothermal alteration commonly overprints vent facies
assemblages.
Our carbonate alteration study shows a chronological alteration pattern from
proximal, focused, to distal, less focused, fluid movement with carbonate species
occurring in pairs. A systematic alteration process is recorded from proximal
siderite–Fe–ankerite to intermediate ankerite–Fe–dolomite to distal calcite–
dolomite. The type of carbonate alteration pattern varies according to host rock,
as felsic-dominated successions show the dolomite–ankerite–siderite trend, whereas
ultramafic rocks favour the dolomite–magnesite–siderite trend.

ACKNOWLEDGEMENTS

This study is a synthesis of observations and field mapping programmes conducted over many years.
Numerous students and companies have given their support, notably Noranda and Falconbridge
Exploration of Canada. Canadian funding agencies NSERC (Wulf Mueller) and FQRNT (John Stix)
as well Universite ´ du Que ´bec a ` Chicoutimi grants were instrumental in financing students and
supporting expensive mapping logistics. The senior author would like to thank University of Otago,
New Zealand and James White fellow co-author and friend for a great sabbatical, as well as
Jo Gottsmann and Joan Marti for an impeccable Tenerife Meeting in 2005. The Las Can ˜adas caldera
complex was a real eye-opener and helped the senior author put Archean complexes into a modern
perspective. Finally, we profited from succinct reviews by V. Acocella, D. Muira and A. Galley that
improved the quality of the manuscript significantly.