Komatiitic Sills and Multigenerational Peperite at Dundonald Beach, Abitibi Greenstone Belt, Ontario: Volcanic Architecture and Nickel Sulfide Distribution
Résumé
The density and the tectonic stresses in the deep crust and the physical properties of komatiitic magmas determine the level to which they will rise, but once in the near-surface environment, the density and rheology of the near-surface rocks (consolidated and dense lava flows and sedimentary rocks versus unconsolidated sedimentary or volcaniclastic deposits) govern whether they will be emplaced as lava flows, invasive flows, or sills. Where near-surface strata are competent (i.e., flow-dominated or consolidated sedimentary and/or volcaniclastic successions), komatiitic magma erupts as flows that form extensive lava shields (e.g., Kambalda). However, where near-surface strata are not competent (i.e., unconsolidated volcaniclastic- and/or sediment-dominated successions), komatiitic magmas typically are emplaced as high-level sills that increase the bulk density of the volcano-sedimentary pile and eventually allow the eruption of lava and the construction of complex sub-volcanic-volcanic lava shields (e.g., Dundonald and Shaw dome, Abitibi belt; Raglan, Cape Smith belt; Pechenga, Kola Peninsula; Thompson Ni belt, Manitoba). The latter environment is illustrated in the volcano-sedimentary succession in Dundonald Township, which is only weakly metamorphosed and deformed and superbly exposed in glacially polished outcrops. The volcano-sedimentary succession in this area comprises (from base to top) (1) the McIntosh formation, composed of a succession of pillowed and massive intermediate volcanic flows; (2) the Dundonald formation, composed of a lower section of komatiite sills, argillites, and felsic volcaniclastic deposits and an upper section of komatiite flows, komatiitic sills, and pillowed intermediate volcanic flows; and (3) the Frederick House Lake formation, composed of massive and pillowed mafic flows. The distribution and thickness of argillites and felsic volcaniclastic rocks define a synvolcanic graben in which the Dundonald South and Alexo Ni-Cu-(PGE) deposits occur within the center and the margin, respectively. Sills and peperites in the lower komatiitic succession at Dundonald Beach exhibit a multigenerational emplacement history recording progressive lithification and increases in the bulk density and rheological strength of the unconsolidated argillites, which ultimately permitted the eruption of lavas at Alexo. Importantly, the nature of the near-surface rocks also influences the localization of Ni-Cu-(PGE) deposits. In lava shields (e.g., Kambalda), the initial eruptions are typically most voluminous and, if erupted at sufficient flow rates, form channelized flows conducive to thermomechanical erosion of sulfur-rich footwall rocks. In sub-volcanic-volcanic lava shields, however, channelized units may occur within the subvolcanic plumbing system and/or within overlying lavas. Where only sills are channelized (e.g., Thompson), mineralization will occur only within the subvolcanic environment; where only the flows are channelized (e.g., Damba-Silwane, Zimbabwe), mineralization will occur only within the volcanic environment. Where both sills and lava flows are channelized (e.g., Dundonald, Shaw dome, Raglan), the distribution of the mineralization is more diverse and may occur as subsea- or sea-floor Ni-Cu-(PGE) deposits.