VOLCANIC STRATIGRAPHY AND GEOCHEMICAL CHARACTERISTICS OF THE PALEOPROTEROZOIC (~ 1878 MA) TROUT LAKE VOLCANOGENIC MASSIVE SULPHIDE (VMS) DEPOSIT, FLIN FLON, MANITOBA, CANADA

The Trout Lake deposit (ca. 1878 ± 1 Ma) is a rhyolite-hosted, bimodal-mafic, volcanogenic massive sulphide deposit (VMS). It is one of the largest VMS deposits in the Flin Flon mining district containing 23.6 Mt of ore grading 1.79% Cu, 4.96% Zn, 1.53 g/t Au, and 15.48 g/t Ag. The deposit underwent greenschist facies metamorphism and polyphase synvolcanic to postvolcanic alteration.

The Trout Lake deposit formed during a period of high-temperature magmatism in a rifted arc that evolved into a backarc basin. The volcanic stratigraphy the deposit occurs as an upright homoclinal sequence that dips steeply to the northeast. The deposit consists of several massive sulphide ore lenses hosted by rhyolitic volcaniclastic rocks within a localized felsic eruptive center composed of FIII-type tholeiitic rhyolite flows and sills. This eruptive center occurs lateral to high-Mg andesite (HMA; MgO > 5.0 wt%) and Nb-enriched andesite (NEA; Nb > 10 ppm) flows that are underlain by footwall basaltic flows with a low-Ti tholeiitic composition (LOTI; TiO₂ < 0.48 wt%). Postvolcanic gabbros with backarc basin basalt (BABB) composition intrude the volcanic rocks, which are unconformably covered by younger graphitic argillites deposited at < 1843 ± 9 Ma. The volcanic stratigraphy of the Trout Lake deposit was repeated and structurally interleaved with the graphitic argillites during west-directed thrusting.

The presence of LOTI indicates high-temperature melting of refractory mantle sources. In contrast, HMA and NEA were derived from enriched mantle sources moderately affected by slab derived melts. Zircon saturation thermometry indicates that the FIII-type rhyolites formed at temperatures > 900˚ C during partial melting of mafic crust. These petrologic characteristics suggest that the Trout Lake deposit formed in a hot, intraoceanic, extensional-arc setting. Asthenospheric upwelling, subduction of young and hot oceanic crust, and ridge subduction are all mechanisms that explain the high-temperature magmatism and the formation of the massive sulphide ore lenses. The BABB geochemical signatures of the postvolcanic gabbros suggest that the tectonic environment of the Trout Lake area evolved from an intra-arc rift during the deposition of the ore lenses, to an incipient backarc basin during the intrusion of the gabbros.