Page 208 - Caldera Volcanism Analysis, Modelling and Response

Characterisation of Archean Subaqueous Calderas in Canada 183

1. Introduction

Calderas are volcano-tectonic collapse structures between ca. 2 and 100 km in
diameter resulting from paroxysmal explosions and effusive evacuation of high-level
magma chambers (Smith and Bailey, 1968; Bailey et al., 1976, Tilling and Dvorak,
1993; Lipman, 2000; Acocella, 2007). According to White et al. (2003) ca. 85% of
volcanism occurs underwater so that arc calderas should be a common feature (e.g.
Yuasa and Kano, 2003). Fiske et al. (1998, 2001) and Gamble and Wright (1999)
identified subaqueous calderas and noticed their explosivity, yet surprisingly the first
detailed documentation of subaqueous explosive deposits originated from ancient
sequences (e.g. Fiske and Matsuda, 1964; Niem, 1977). Silicic submarine calderas
were recognised as favourable volcanogenic massive sulphide (VMS) sites early on
(Ohmoto, 1978, 1996), but only later did Stix et al. (2003) allude to the importance
of caldera ring faults in VMS genesis. The latter is corroborated by the present
6
9 10 tonne caldera wall Sunrise deposit in the Myojin Knoll (Iizasa et al., 1999;
Glasby et al., 2000), as well as VMS mineralisation within the active submarine
Brothers caldera (Wright et al., 1998). The Izu-Bonin arc with boninites (high Mg-
andesites; Crawford et al., 1989), moderate to deep-water calderas and hydro-
thermal massive sulphide mineralisation (Ishibashi and Urabe, 1995; Iizasa et al.,
1999) is considered an analogue to the Archean Abitibi greenstone belt. As our
knowledge of seafloor calderas is restricted to submersible dives, ancient dissected
submarine calderas are prime sites to identify edifice geometry, construction and
collapse.
The Abitibi greenstone belt is world class VMS metallotect with a production
and reserves in excess of 480 Mt for polymetallic massive sulphide and gold-rich
massive sulphides, including the past Horne and current Bousquet-LaRonde mines,
Blake River caldera complex (Figure 1; Chartrand and Cattalani, 1990; Gibson and
Watkinson, 1990; Hannington et al., 1999a). Our study is divided into: (1) a segment
concerning the physical volcanology and geometry of the little known subaqueous
Abitibi Hunter Mine and Normetal calderas, and the Sturgeon Lake caldera,
Wabigoon Subprovince, and (2) a segment addressing the poorly documented
hydrothermal carbonate alteration. The felsic 2,728–2,734 Ma Hunter Mine caldera
is effusive dominated, as is the bimodal 2,728–2,730 Ma Normetal caldera, whereas
the felsic 2,733–2,736 Ma Sturgeon Lake caldera is explosive dominated. A new
volcanic hydrothermal alteration model for submarine Archean calderas is proposed,
and exemplifies how detailed volcanic facies and alteration mapping with basic
petrography are important tools in reconstructing volcanic edifices and VMS
exploration.

2. Abitibi Greenstone Belt Geology

The 300 700 km Abitibi greenstone belt (Figure 1) is the best-studied
greenstone belt of oceanic arc affinity in the world because of its vast economic
potential. Arc-related volcanism continued for 65 m.y. (ca. 2,735–2,670 Ma), and