Page 247 - Caldera Volcanism Analysis, Modelling and Response

222 W.U. Mueller et al.

the proximal series has a range of 0–2 km, the ankerite–dolomite series is 2–5 km
and the calcite–dolomite zone is generally W3 km from the site of mineralisation at
Normetal. The proximal carbonate assemblage (0–1 km) at the HMC contains
siderite (sideroplesite) and Fe–ankerite (Figure 14B). Just as seafloor chimneys and
ore zones evolve due to increasing temperatures from sphalerite to pyrrhotite–
chalcopyrite (e.g. Ansil deposit; Galley et al., 1995), hydrothermal carbonate
alteration systems may evolve Fe–ankerite to siderite–sideroplesite as fluids become
more focused. Overprinting of carbonate alteration patterns (Mueller et al., 2005;
this study) as well as metal mineral species is common (Galley et al., 2000) and can
be explained by evolving hydrothermal systems. On the large-scale the alteration
halo is semi-concordant, but next to the discharge zone along synvolcanic faults
a discordant alteration pipe develops. It is interesting to note that known carbonate
alteration patterns have a precursor silicification stage that seal the system
sufficiently for carbonate precipitation and subsequent mineralisation. The sealing
permits temperature increase as sulphide bearing fluids are constrained and focused,
rather than dissipated throughout the complex. Hydrothermal massive sulphide
deposits and their alteration patterns are complex systems governed by high-level
plutons, their intrusive phases (Galley, 2003; Chown et al., 2002) and caldera
geometry (Roche et al., 2000; Kennedy et al., 2004), all of which control sub-
seafloor fluid convection.

8.2. Proposed Archean massive sulphide model in calderas

As calderas are large-scale volcano-tectonic subsidence structures, discovering the
location of VMS in ancient sequences is difficult. The combination of physical
volcanology and hydrothermal alteration indicators is important. Numerous
subenvironments are dispersed on the intracaldera floor (moat), the caldera margin
and volcaniclastic apron. Each of these can host massive sulphide deposits but all are
linked to synvolcanic faults and magma conduits. Pin-pointing these subsettings are
of economic importance. Favourable intracaldera sites are (1) dome-flow
complexes, and (2) small volume explosive fountaining edifices as both contain
abundant porous and permeable volcaniclastic debris of pyroclastic or autoclastic
origin (Figure 5B, D). The presence of fine-grained tuff should not deter
exploration as it can be proximal to the mineralised zone. The grain-size of
volcaniclastic debris is not a criterion for either proximal or distal within the moat
setting. It is the eruption and transport process that controls grain-size distribution.
Sound criteria for finding emission centres are: (1) dykes or dyke swarms as these
intrude synvolcanic faults, (2) tracing laterally flows and their facies changes, and
(3) hydrothermal alteration patterns. Smaller VMS deposits should develop in the
sub-surface of the intracaldera moat.
In contrast, the most voluminous massive sulphide deposits should form at the
caldera margin (e.g. 16 Mt Quemont Breccia; Gibson and Watkinson, 1990;50 Mt
Horne Mine, Kerr and Gibson, 1993; 62 Mt Flin Flon deposit; Syme and Bailes,
1993) between the outer normal faults and inner inverse faults as the highest heat
and fluid flow is registered here. The caldera margin, km-thick, is a tectonically
active area with no coherent stratigraphy that is generally composed of large