Stratigraphy and Hydrothermal Alteration of the Gagne Lake Prospect: An Occurrence of Volcanogenic-Type Massive Sulfides Near Mine Centre, Northwestem Ontario, Canada

Abstract

East-west trending, steeply dipping (85° +), overturned, Archean-age volcanic, sedimentary and intrusive rocks (sills) of predominantly greenschist-amphibolite transition facies grade are exposed in the Wabigoon Subprovince of northwestem Ontario, Canada approximately 1.5 km southeast of Gagne Lake. Volcanic rocks associated with the Gagne Lake prospect, a showing of volcanogenic massive sulfide-type mineralization (chalcopyrite, sphalerite, pyrite and galena), are primarily rhyolitic lava flows and pyroclastic (hydrovolcanic) rocks. The pyroclastic rocks serve as the host rock for the prospect. Mafic lava flows are interlayered with the felsic volcanics but constitute only a minor portion of the stratigraphy. Tonalitic and mafic sills comprise nearly 50% of the stratigraphy and range in time of emplacement from prior to formation of the Gagne Lake prospect to after intrusion of the Little Ottertail Lake Stock. Based on preserved primary textures and structures, the volcanic succession is thought to have formed under both subaerial and subaqueous conditions. Regional stratigraphic relationships suggest that the succession at one time formed part of an emergent volcanic island. Volcanic rocks and tonalitic sills underlying the prospect have been variably altered by hydrothermal solutions. Distribution and geochemistry of the altered rocks is such that four alteration assemblages can be defined: 1) least altered assemblage, 2) sericite (biotite)- chlorite-iron carbonate assemblage, 3) actinolite-chlorite-epidote assemblage and 4) dalmatianite (sericite, chlorite, iron carbonate). The alteration assemblages delimit a concentrically zoned alteration pipe below the prospect in which actinolite-rich rocks are enveloped by sericite or biotite-rich rocks and a stratigraphically semi-conformable zone of sericitc alteration within the hydrovolcanic rocks. A relatively small zone of dalmatianite (spotted alteration) envelopes the Gagne Lake prospect. Crosscutting relationships indicate that actinolite-rich rocks and dalmatianite formed at the expense of sericite and/or biotite-rich rocks. Alteration assemblages are believed to have formed by the circulation of hydrothermal solutions through the volcanic succession. Shallow circulating sea water reacted with felsic rocks and evolved into an acidic, potassium-rich brine. Reactions between this solution and felsic rocks in the field area produced the sericite/biotite-rich rocks by addition of potassium and magnesium to and leaching of calcium and sodium from the rocks. Deeper circulating solutions encountered mafic rocks at depth. Reactions between these fluids and the rocks produced a solution enriched in calcium, magnesium and iron. With ascent, this brine encountered sericite and biotite-rich rocks of the study area. The resulting reactions produced actinolite-rich rocks by addition of calcium, magnesium and iron to and leaching of potassium from the rocks. As this solution mixed with sea water near the water-rock interface, it became enriched in magnesium. Reactions between this magnesium-rich solution and sericite-rich rocks produced a chlorite-quartz alteration assemblage that became dalmatianite during prograde metamorphism.