The south Sturgeon Lake area of northwestern Ontario is underlain by an Archean submarine caldera complex which is host to six massive sulfide deposits. The complex is approximately 30 km in strike length and is partly filled by mesobreccias, subaqueous debris-flow deposits and other epiclastic rocks, as well as five major ash-flow tuff units. The most voluminous of these, the Mattabi ash-flow tuff, is up to 800 m in thickness and can be traced for more than 15 km across the caldera complex.Based on flow morphology and composition, the Mattabi ash-flow tuff is divisible into two distinct units: a bedded quartz crystal and pumice-rich unit (150-650 m thick), and a massive to poorly bedded ash unit (20-150 m thick). The bedded unit is host to the Mattabi massive sulfide deposit, as well as numerous other massive sulfide occurrences. In addition, this unit exhibits a pronounced stratigraphic and geochemical cyclicity. Individual beds (flow units) can be subdivided into basal quartz- and pumice-rich lower sections which are overlain by bedded ash tops; thicknesses of individual beds vary from 14 to 155 m. Each bed also exhibits a pronounced zonation of trace elements with high concentrations of Zr, Y, and Nb at and/or near the base gradually decreasing upward into the ash tops. The overlying massive ash unit contains no known massive sulfide mineralization and lacks any pronounced geochemical zoning.The bedded nature and geochemical zonation of individual flow units can be explained by periodic eruptions from a zoned or layered, recharged siliceous magma chamber. The difference in flow morphology between the bedded unit and the overlying massive ash-flow tuff most likely reflects the deeper submarine nature of the eruptive vents associated with the bedded deposits. The periodic and subaqueous nature of the Mattabi eruptions is indicated by the presence of exhalites and massive sulfide lenses between flow units. The ore lenses, which comprise the Mattabi deposit, formed between eruptive events from high temperature fluids whose origin may be related to processes within the magma chamber.