Physical Volcanology and Hydrothermal Alteration of the Footwall Rocks to the Archean Sturgeon Lake Massive Sulfide Deposit, Northwest Ontario

Abstract

The footwall rocks beneath the Archean Sturgeon Lake massive sulfide deposit consist of a 3 km thick sequence of steep north-dipping subaqueous volcanic and volcaniclastic rocks intruded by mafic dikes and sills. Preserved primary textures, fragment composition, and regional stratigraphic correlation allow for the volcanic and volcaniclastic rocks to be subdivided into seven distinct units. Mafic volcanic rocks comprise the lowermost units, consisting of massive, pillowed, and amygdaloidal flows, with minor interlayered felsic flows and pyroclastic deposits. These mafic units are overlain in the west half of the field area by a coarse heterolithic volcanic breccia, which grades up into, and is intercalated with quartz-phyric felsic pyroclastic flow deposits. This felsic unit is locally overlain by bedded volcanic debris flows and associated epiclastic sediments. These are in turn overlain by a thick sequence of massive to bedded quartz-phyric felsic pyroclastic flow deposits, the Mattabi series, with minor intercalated volcanic debris flows. These rocks are overlain by the host rocks to the orebody which consist of bedded to massive quartz- & plagioclase-phyric pyroclastic flow deposits. The succession is capped by mafic to intermediate massive and amygdaloidal flows, and was later intruded by two large dioritic sill-like bodies which dilate the stratigraphy. The east half of the field area is underlain by the lowermost mafic units which are overlain by fragment-poor ash-rich felsic pyroclastic flow deposits. A NE trending structure, interpreted to be a synvolcanic normal fault, provides the break between east and west, and may be a major bounding fault to a large caldera system. Convective circulation of hydrothermal fluids within the succession resulted in the formation of distinct alteration assemblages, which include a) silicified; b) Fechlorite; c) chloritoid; d) cordierite; e) aluminum silicate; f) sericite; and g) Mg-chlorite. Field and petrographic evidence suggest that two periods of hydrothermal activity affected the felsic volcanic rocks of the succession: 1) development of silicified rocks and Al-silicates from leaching of original constituents, and chloritoid/chlorite alteration by a process of Fe-enrichment, and 2) widespread sericitization from K enrichment, and cordierite alteration, by leaching of Fe and Mn, leaving the rocks enriched in Ca and Mg during the time of development of the Sturgeon Lake massive sulfide deposit. Subsequent regional deformation and metamorphism to upper greenschist facies produced a bedding-parallel foliation and development of metamorphic mineral phases, strongly dependent on pre-metamorphic altered compositions.