Impact-Generated Hydrothermal Systems and Associated Mineralization: A Case Study at Manicouagan

English Abstract:

Impact-associated hydrothermal alteration is manifest in various locations within the Manicouagan structure of Quebec, Canada. Major hydrothermal systems are developed in the east of the structure, where associated mineralization is manifest as veins, vugs, and geodes. This work focuses on zeolitization (the predominant hydrothermal activity) and SiO₂-phase mineralization. Within the central uplift (comprised of anorthositic gneiss), thomsonite and natrolite (Thm-Ntr) have been deposited as veins (thickness of ~2-3 cm) in more intact host rocks, and as more pervasively distributed occurrences in more fractured rocks. Analcime is mainly associated with shock veins, which locally pervade the central uplift. Thm-Ntr is also commonly developed as veins within basal suevite (melt-bearing brecciated rock) as a result of labradorite alteration occurring beneath the impact melt sheet; IMS (i.e., between the IMS and footwall rocks). SiO₂ phases occur as colourless and purple (amethyst) varieties at different locations within the IMS. Fluid inclusion and oxygen isotope determinations reveal that mineralization of SiO₂ phases occurred at ~225 to 430°C. High salinity fluids (14-26 wt% NaCl eq.) in the absence of salt (NaCl) crystals suggests Ca²⁺-rich hydrothermal activity (with more limited Na⁺ mobilization). The δ¹⁸O values indicate that magmatic fluid was involved in the production of SiO₂ phases within the upper unit of the IMS (δ¹⁸O_fluid range from +0.81 to +7.67‰), while fluids involved in geode formation at the base of the IMS were derived from metamorphic origins (δ¹⁸O_fluid range from +17 to +23.17‰). Both formations appear to be closed systems, and are not connected; which suggests low permeability of the IMS body. Based on the δ¹⁸O results, there is no evidence of meteoric water being involved in the hydrothermal phases at Manicouagan.