Page 248 - Caldera Volcanism Analysis, Modelling and Response

Characterisation of Archean Subaqueous Calderas in Canada 223

mega-blocks and volcanic rubble breccias. The high porosity volcanic debris makes
it a prime site for metal-rich fluid discharge. Identification of the marginal annular
caldera wall in the ancient rock record is difficult but a predominance of chaotic
breccias, dyke intrusions and silica-filled fractures would be strong arguments. The
gold-rich VMS Sunrise deposit of the Myojin Knoll volcano, located at the foot
of the caldera wall at 1,210–1,360 m depth (Iizasa et al., 1999; Fiske et al., 2001),
underlines the importance of active margin faults. The volcanic apron zone, outside
of the major subsidence structures, displays a prevalence of volcaniclastic debris
with local effusive satellites. The massive sulphide deposits are probably smaller and
have lower temperature Zn-rich zones, as they are off axis from the principal
hydrothermal discharge sites. In the studied cases, the alteration and mineralisation
is of the replacement type and depths below the water–sediment–volcanic rock
interface are generally between 10 and 200 m (e.g. Galley et al., 1995; Doyle and
Allen, 2003). Our observations suggest sub-seafloor replacement close to or at the
rock–seawater interface.
The proposed model combines the physical geology of the Hunter Mine,
Normetal and Sturgeon Lake calderas. The model emphasises the volcanology of
effusive flow-dominated calderas, but considers the horst and graben structures
of piecemeal calderas. Hydrothermal alteration ensues with early silicification and
subsequent carbonate alteration. The studied calderas have an early massive to
locally network style silica cap either at depth (Lafrance, 2003) or close to the
surface (Mueller and Mortensen, 2002). Intense silicification has been identified at
Sturgeon Lake (Galley, 1993), at Snow Lake (Skirrow and Franklin, 1993), at Kidd
Creek (Hannington et al., 1999b), in the Noranda caldera (Gibson and Watkinson,
1990) and at the modern TAG deposit (You and Bickle, 1998). Silicification is
cherty in the volcaniclastic (Figure 13C, D) and background sedimentary
rocks (e.g. silicified shale; Figure 3A) and displays network veining structures
in basaltic breccias (Figure 13A, B). The silicified rocks are replaced by
hydrothermal carbonate (Figure 13C, D). Silica-rich fluids precipitate near
the seafloor (Figure 16A), but may form at depth as an impermeable barrier
(Figure 16B).
The semi-conformable carbonate alteration halo contains both a lateral and a
discordant (focused) carbonate zonation (Figure 16A, B). Detailed studies of the
Normetal and HMCs reveal a carbonate alteration assemblage that is pervasive
along the extent of the edifice and discordant along synvolcanic faults (Figure 15B).
The focused alteration next to the massive sulphide deposits displays a rather
consistent alteration assemblage of siderite–(sideroplesite)–Fe–ankerite, (Figure
16B) and is analogous to that of the Gemini area (Figure 1; Mueller et al., 2005).
The change from proximal siderite–Fe–ankerite to ankerite–Fe–dolomite is subtle.
The ankerite–Fe–dolomite assemblage may be kilometres from the mineralised
zone (Figure 15B) and the distal dolomite–calcite assemblage W10 km. There
appears to be a transitional hydrothermal carbonate from one zone to another, but
the overall pairings of siderite–Fe–ankerite to ankerite–Fe–dolomite and finally to
dolomite–calcite are representative of a proximal–distal carbonate alteration pattern.
The Kidd Creek alteration displays a different pattern as mafic to ultramafic rocks
favour an Mg-rich carbonate trend from dolomite to breunnerite to siderite