Page 250 - Caldera Volcanism Analysis, Modelling and Response

Characterisation of Archean Subaqueous Calderas in Canada 225

(Figure 15B). Hydrothermal carbonates associated with Kidd Creek felsic rocks
have the Normetal dolomite–ankerite–siderite trend.
The pervasive nature of this type of alteration halo yields a semi-conformable
distribution but it is decidedly discordant along fluid pathways or discharge zones
(Figure 16B). As noted from numerous massive sulphide mining camps such as
Matagami and Rouyn-Noranda of the Abitibi greenstone belt, massive sulphide
deposits occur in clusters within a volcanic structure, and Figure 16B provides
a survey of favourable sites. The caldera wall favours large VMS deposits, whereas
the central moat and volcaniclastic apron subsettings generally contain smaller VMS
deposits probably 5 Mt and less.

9. Conclusions

Submarine calderas are highly favourable loci for volcanic massive sulphide
deposition, but recognising a VMS site on the intracaldera floor, the caldera margin,
and volcaniclastic apron requires a more systematic approach because caldera
dimensions are highly variable. This is especially true of ancient systems. Calderas
have a complex diachronous emplacement history with respect to dome-flow
complexes, dykes-sills, and subvolcanic intrusions. The overall geometry and
evolution of studied Archean calderas compares favourably with modern oceanic
arc and island arc analogues, such as Tenerife-Las Can ˜adas and the subaqueous Myojin
Knoll caldera, with a shield-building phase, a dome-building stage or stratovolcano
construction and a caldera volcano-tectonic collapse event. The overall evolution of
calderas requires approximately 6–13 m.y. as shown for the (1) Valles Caldera, (2)
Tenerife-Las Can ˜adas and (3) HMC sequences, whereby the most devastating events
generally occur in the last 1 m.y. This timeframe shows that calderas evolve and
require a prolonged construction period prior to catastrophic subsidence.
The basal shield-forming succession, a fundamental precaldera constructional
stage, involves a broad subaqueous to subaerial shield volcano, in which individual
shield volcanoes coalescence (e.g. Hawaii and Tenerife). The basal sequence in
ancient subaqueous successions comprises thick units of basaltic to andesitic lava
flows, breccias and stratified hyaloclastites (tuff, lapilli tuff and lapilli tuff breccia).
Minor felsic (dacite) dome-building volcanism may develop locally during the
shield-building event. Subsequently, numerous individual felsic constructional
phases develop on the shield volcano. The major subaqueous constructional phase
in the effusive dominated calderas features large and small felsic dome-flow-
hyaloclastite complexes formed along synvolcanic fault systems. Small explosive
fountaining eruptions develop locally. The caldera subsidence structure is controlled
by abundant effusive volcanism and possibly magma chamber migration. Mafic
volcanism can be contemporaneous with felsic volcanic activity, as in the Normetal
caldera and during the late Hunter Mine caldera event. Fragmental volcanic debris,
commonly heterolithic, is mainly of autoclastic origin but chaotic collapse breccias
are prevalent at the caldera wall. In the subaqueous setting, fragmental debris is far
more prominent in dome-building phases.