Page 224 - Caldera Volcanism Analysis, Modelling and Response

Characterisation of Archean Subaqueous Calderas in Canada 199

4.3.2. Volcaniclastic lithofacies and iron-formation lithofacies
The volcaniclastic and iron-formation lithofacies is 2–20–m-thick and composed
of 2–50 cm-thick tuff–lapilli tuff (Figure 3C). The tuff–lapilli tuffs are graded T a
(S 3 -beds of Lowe, 1982), T ab , T abc , T ad and T abcde beds that are consistent with
high- and low-concentration density current deposition. Rip-up clasts of chert and
jasper are common. Pyroclast constituents include felsic shards, wispy vitric and
angular lithic volcanic fragments, pumice and broken and euhedral crystals, which
collectively argue for a pyroclastic deposit (Fisher and Schmincke, 1984), although
they could be syneruptive with limited reworking. The black T d of the tuff
turbidites and the T e mudstone represent a felsic vitric ash component (e.g. Fritz
and Vanko, 1992) and background sedimentation that settled through the water
column, respectively (Figure 3A). Synvolcanic faults with fluid movement
(Figure 3E), water escape structures, and small-scale load casts are well preserved.
The iron-carbonate horizons alternate with chert, which is either silicified tuff or
diagenetically recrystallised and compacted shard-rich tuff. Even the iron-formations
have been locally silicified, indicating numerous stages of hydrothermal alteration.

4.3.3. Mafic dykes and sills
Numerous sills and dykes intrude the upper formational stage. Tholeiitic columnar-
jointed dykes intruded N-trending, synvolcanic faults and fractures, with
hydrothermal black and white chert as well as laminated jasper deposited
along brecciated dyke margins (Figure 3F). The gabbro sills have a well-defined
subophitic texture and are massive or columnar-jointed. Columnar-jointed sills
were emplaced high in the sequence, where abundant seawater may have percolated
to cause rapid cooling and vesiculation due to devolatisation. In addition, aphanitic
basalt and dacite bodies intrude at a shallow angle subparallel to bedding. These sills
acted as local barriers for hydrothermal fluid movement, which resulted in sulphide
precipitation at the volcaniclastic sediment–sill interface.

4.4. Evolution of hunter mine caldera
The subaqueous HMC has a polyphase history (Figure 5A–E), in which a caldera,
dominated by effusive felsic volcanism evolved over ca. 6 m.y. The distribution of
lithofacies and synvolcanic faults, if only at the small scale, suggest either a piston or
incipient piecemeal caldera. The first caldera-forming event (lower formational
stage, Figure 5A, B) exhibits both incremental fragmentation of the caldera floor
into horst and graben structures and the development of a caldera margin along a
major ring fault. Early reverse and subsequent normal faults formed and facilitated
subsidence. Volcanism was dominated by thick dome-flow-hyaloclastite complexes
and some of the vesicular clast-rich breccias may have a low-energy explosive
(frothing) component, caused by devolatisation and violent water–magma
interaction. Energetic fountains driven by rapid devolitisation of magma developed
locally under deep-water hydrostatic pressures (Mueller and White, 1992). A period
of felsic quiescence ensued as major tholeiitic sills intruded high in the sequence.
The Roquemaure sill (Figure 5C) of the middle formational stage attests to a phase