Page 207 - Caldera Volcanism Analysis, Modelling and Response

182 W.U. Mueller et al.

7. The Link: Subaqueous Calderas with Chert–Iron Formation and Hydrothermal
Carbonates 214
7.1. Results of hydrothermal carbonate species 218
8. Discussion 220
8.1. Hydrothermal carbonate species distribution 220
8.2. Proposed Archean massive sulphide model in calderas 222
9. Conclusions 225
Acknowledgements 226
References 227

Abstract
Ancient and modern subaqueous calderas form in deep- to shallow-marine oceanic
settings and are primary sites for volcanogenic massive sulphide (VMS) deposits.
Caldera structures hosting VMS deposits are concentrated in the Archean Abitibi
greenstone belt and in the Wabigoon subprovince. The Hunter Mine and Normetal
calderas, two little-known effusive-dominated edifices, are emphasised and Sturgeon
Lake is the best known explosive-dominated caldera. Although VMS deposits
are linked to calderas, their 5–30 km diameter size, poses problems in pin-pointing
highly favourable sites for VMS exploration. The overall caldera geometry is
readily recognised based on regional-scale mapping but detailed volcanic facies
mapping is required to distinguish caldera subsettings in the Archean. Caldera
subenvironments include: (1) the caldera wall featuring (a) chaotic breccias,
(b) dyke intrusions, (c) synvolcanic faults and (d) pyroclastic debris; (2) the
intracaldera moat or floor with (a) horst and graben structures (synvolcanic faults),
(b) superposed dome-flow-hyaloclastite complexes, (c) extensive central dyke
swarm and (d) small explosive volcanic edifices formed by magmatic fountaining
eruptions and (3) caldera apron deposits showing (a) extensive volcaniclastic debris
of pyroclastic and autoclastic origin and (b) local dome-flow-hyaloclastite complexes.
All loci are possible sites for VMS formation, but the caldera wall favours large VMS
deposits.
Based on the studied calderas, a new hydrothermal alteration model is proposed that
helps explain chert—Fe carbonate assemblages in Archean volcanic sequences. The
early hydrothermal silica seals the volcaniclastic-dominated rocks (cap rock) at or near
the edifice–seawater interface. These silicified volcaniclastic turbidite deposits have
generally been referred to as chert or exhalites but this interpretation necessitates
reconsideration. Overprinting the silicification phase is a pervasive semi-conformable
carbonate hydrothermal alteration zone with a discordant focussed root zone along
faults. Three distinct carbonate pairings are observed: (1) proximal siderite (side-
roplesite) — Fe–ankerite next to the VMS-deposit, (2) an intermediate ankerite–Fe–
dolomite zone and (3) a distal calcite–dolomite zone. Transitions between zones are
subtle and changes are indicated mineral assemblage overlaps. Our results shed new
light on hydrothermal alteration patterns, but also resolve some of the problems
associated with chert–iron carbonate formations. The chert and hydrothermal
carbonates as well as the VMS deposits are of the replacement type, rather than
chemical precipitants and black smoker deposits.