Page 241 - Caldera Volcanism Analysis, Modelling and Response

216 W.U. Mueller et al.

circulation of hydrothermal and metal-bearing fluids is readily accommodated. Silica
impregnation and carbonate alteration are important ingredients. Modern arc
analogues show calderas are excellent hydrothermal massive sulphide sites (Ishibashi
and Urabe, 1995; Iizasa et al., 1999), yet extensive carbonate alteration with VMS
appears to be an Archean and Paleoproterozoic phenomenon, although it has been
observed in the Paleozoic, Que River deposit, Mount Read Volcanics (Offler and
Whitford, 1992). Morton and Franklin (1987) initially defined two types of alteration
patterns related to hydrothermal fluid movement: (1) the Noranda type with a
discordant chlorite–sericite alteration pipe, and (2) a Mattabi type with semi-
conformable carbonate alteration zone. Numerous Archean silicic-dominated calderas
in the Archean Superior Province display up to 50km wide carbonate alteration
zones (Chown et al., 2000; Lafrance, 2003). Franklin (1993) and Galley (1993)
recognised the importance of carbonate alteration in VMS deposits but detailed
studies of hydrothermal carbonate remain scarce. Exceptions are the Normetal caldera
(Lafrance, 2003), the Ben Nevis area of the Blake River Group (Hannington et al.,
2003) and the Hemingway property, Kidd Creek (Schandl and Wicks, 1993).
Still controversial are banded iron-formations (BIF) and related chert, as both
have been generally interpreted as chemical precipitates rather than replacement
deposits (Maliva et al., 2005; Lascelles, 2007). They are commonly referred to as
exhalites (e.g. Tucker-Barrie et al., 2005), but the interpretation as exhalite is highly
misleading and often erroneous. Precambrian iron-formations have been correlated
with an early Earth CO 2 -enriched atmosphere (Trendall, 2002). The chert–iron
carbonate formation in the studied submarine volcanic centres is of hydrothermal
origin (e.g. Chown et al., 2000), and represents a subset of what was originally
referred to as Algoma type iron-formations or as proposed by Dimroth (1986),
pelagic iron-formations. They are distinct from classical chert–magnetite oxide iron
formations originally termed Superior type or platform iron-formations (Dimroth,
1986). Iron-formations of the pelagic iron-formation type and associated chert
horizons are key elements in VMS exploration.
Silicification associated with carbonates represents an early stage of hydro-
thermal alteration (Galley, 1993; Chown et al., 2000), and is located near or at the
surface (Skirrow and Franklin, 1994; this study) or at a depth of 1–2 km close to
synvolcanic plutons (Galley, 1993; Skirrow and Franklin, 1994; Lafrance, 2003).
The silica is derived from (1) low-temperature 50–1501C seawater rock inter-
actions (element leaching) of the volcanic rocks (Galley, 1993) and (2) deuteric
(autohydrothermal) fluids from felsic volcanics and synvolcanic plutons that
subsequently precipitated as amorphous silica in porous rocks of the discharge zone.
In contrast to the Normetal caldera silicification at depth, the HMC displays an
impermeable silica cap rock close to or at seafloor surface as suggested by silicified
shale (Figure 3A) and overlying unaltered basalt–komatiite sequence. The silica-
rich fluids migrated along synvolcanic fractures and dyke margins (Figure 3F).
Rather than being a silica gel deposited on the Archean seafloor or exhalative,
the laminated cherts are felsic turbiditic tuffs (Figure 3C, D) produced by a
combination of percolating silica-rich fluids along bed boundaries, bed compaction
and diagenesis of felsic glass shards. Silicification is also observed in basalt breccias
that show a silicified matrix (Figure 13A, B). The fragments were subsequently