Tectono-hydrothermal evolution of the Neoarchean Abitibi greenstone belt, Canada

Abstract

The Timiskaming assemblage represents the youngest supracrustal assemblage of the Neoarchean Abitibi greenstone belt of Ontario and Quebec, Canada. The deposits of the Timiskaming assemblage include deep-water turbidites, alluvial-fluvial conglomerates and sandstones, as well as alkalic volcanic and intrusive rocks. The sedimentary and volcanic rocks of the Timiskaming assemblage record a special time in the greenstone belt evolution as they represent the first deposits in the Abitibi greenstone belt that were not exclusively deposited in a deep submarine setting. The Timiskaming assemblage is thought to have formed in response to a period of Neoarchean mountain building and crustal thickening. To better constrain the depositional setting of the Timiskaming assemblage, high-resolution lithofacies mapping was conducted on key outcrops. The mapping highlights the presence of graywacke turbidite deposits, alluvial-fluvial conglomerate-sandstone facies, near-vent pyroclastic surge and fallout deposits, and syenite intrusions within the Timiskaming assemblage. The clastic facies contain evidence for high rates of deposition and paleoseismic activity. The detrital material appears to be of varied provenance and may not only be derived from local sources. The abundance of near-vent pyroclastic deposits in Kirkland Lake is consistent with a subaerial depositional setting of parts of the Timiskaming assemblage and indicates that Timiskaming sedimentation was accompanied by at least local extension. The documented characteristics are consistent with deposition in a variety of tectonic settings, although they are most consistent with sedimentation in continental rift or strike slip basins. The alluvial-fluvial conglomerates and sandstones of the Timiskaming assemblage contain abundant quartz clasts and igneous clasts containing quartz veins. To better constrain the origin of these clasts, representative quartz samples were collected from exposures of the Timiskaming assemblage in the Timmins-Porcupine, Kirkland Lake, Duparquet, and Rouyn-Noranda mining camps. Microscopic investigations revealed that most quartz clasts have been affected by extensive recrystallization, hampering identification of the quartz origin. However, primary textural relationships, fluid inclusion inventories, and CL signatures are preserved in several of the sampled quartz clasts. Although some of the clasts probably originate from pegmatites, most of the clasts appear to have been sourced from hydrothermal veins, including veins comparable to those found in modern epithermal deposits or shallow orogenic deposits. The finding provides new important insights into the metallogenic evolution of the Abitibi greenstone belt. The presence of quartz vein material in the conglomerates and sandstones of the Timiskaming assemblage suggests that hydrothermal systems present in modern convergent plate settings already existed during the early phase of Neoarchean mountain building and crustal thickening.