Abstract
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 (sideroplesite) — 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.
Published Version
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