TECTONO-HYDROTHERMAL EVOLUTION OF THE NEOARCHEAN ABITIBI GREENSTONE BELT, CANADA: NEW INSIGHTS FROM TIMISKAMING ASSEMBLAGE QUARTZ PEBBLES

The Timiskaming assemblage (<2679–2669 Ma) represents an important host to the world-class orogenic gold deposits of the Neoarchean Abitibi greenstone belt of Ontario and Quebec. Sedimentary rocks of this assemblage formed in response to a period of crustal thickening and mountain building. Most notable are alluvial/fluvial polymict conglomerates that largely consist of locally derived igneous material. Many conglomerates also contain minor, albeit conspicuous, amounts of quartz pebbles and cobbles. To provide additional constraints on the origin of these quartz clasts, texturally diverse clasts were sampled from Timiskaming assemblage conglomerate outcrops in the Timmins-Porcupine, Kirkland Lake, Duparquet, and Rouyn-Noranda mining camps. The collected clasts range from those entirely composed of white milky quartz to apparent amethyst and even to clasts of igneous rocks that are crosscut by quartz veins truncated at the clast margins. Microscopic investigations indicate that the quartz in these clasts has typically been affected by extensive recrystallization and fluid overprint under metamorphic conditions. However, some clasts contain relict chalcedonic quartz, crustiform textures, and prismatic quartz crystals. The prismatic quartz grains show short-lived blue to yellow optical cathodoluminescence colors, oscillatory growth zonings, and contain low-temperature primary fluid inclusions. These properties are consistent with the quartz having formed in shallow subaerial hydrothermal veins, not unlike quartz in epithermal deposits or shallow orogenic deposits. Hydrothermal activity leading to quartz vein formation must have been broadly contemporaneous with deposition of the Timiskaming assemblage. Shallow subaerial hydrothermal systems have not previously been documented in the Neoarchean Abitibi greenstone belt.