PEPERITIC TEXTURES AND FACIES ARCHITECTURE OF A PALEOPROTEROZOIC BASALTIC ANDESITE INTRUSION, FLIN FLON, MANITOBA, CANADA: EVIDENCE FOR THE EMPLACEMENT OF A CRYPTOFLOW DURING THE GROWTH OF A DOMINANTLY BASALTIC VOLCANO

The Paleoproterozic 1920 unit occurs at the base of the Hidden formation and stratigraphically overlies the volcanogenic massive sulphide deposits at Flin Flon, Manitoba, Canada. The 1920 unit is an Fe-Ti-P-rich basaltic andesite (Fe₂O₃, 12 wt.%, TiO₂, 1.1 wt.% and P₂O₅, 0.30 wt.%) and, with its overlying volcaniclastic unit, crudely defines a localized fault-bounded subsidence structure. The presence of blocky and fluidal peperite along its upper contact, and lens-shaped inclusions of tuff along its basal contact, suggests the basaltic andesite was emplaced as a cryptoflow into wet, unconsolidated volcaniclastic sediments. Locally the basaltic andesite overran the wall of the graben into which it was emplaced, breaching surface and forming a pillowed flow. The abundance of synvolcanic basaltic dikes within the footwall to and cross-cutting the unit also indicate that magmatism, like that in modern back-arc environments, was controlled by fissure-fed eruptions during extension. The unique geochemistry of the 1920 unit suggests the presence of a high-level magma chamber. This heat source, combined with evidence of extension, provides an environment conducive to the formation volcanogenic massive sulphide deposits. The 1920 unit and its confining graben occur above the Callinan and Triple 7 ore bodies indicating that the synvolcanic structures that controlled magmatism, subsidence, and hydrothermal fluids during the emplacement of the 1920 unit were long-lived structures that likely also controlled these processes within the massive sulphide-hosting footwall succession. Therefore, the synvolcanic structures within the hanging wall, can be used to target massive sulphide mineralization at depth.