Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The Muscocho Pluton is one of a number of syntectonic Neoarchean granodiorite intrusions formed during the main deformation phase in the Abitibi Subprovince that lead to stabilization of the Superior craton. Such magma locally are associated with magmatic-hydrothermal styles of mineralization, interpreted by some works to reflect intrusion-related gold systems (IRGS). Determining whether magmas can transport enough Au to fertilize the upper crust during this tectonomagmatic interval requires insights into physico-chemical parameters such as pressure, temperature, and oxygen fugacity (ƒO2). The ƒO2 parameter is particularly important because it affects S-speciation and S-solubility, which correlates with Au-solubility in magmas. Retrieving such parameters from the metamorphosed, locally hydrothermally altered, and generally deformed Neoarchean rocks is particularly challenging. As such, this contribution uses fresh to altered granodiorites of the Muscocho Pluton, a 16 km diameter granodiorite intrusion spatially associated with polymetallic mineralization, to evaluate whether physico-chemical parameters can be retrieved from ancient altered rocks based on amphibole, apatite, and zircon chemistry. This study also has implications for the origins of the Muscocho Pluton, which ascended slowly toward the upper crust as a single pulse of evolved magma and became emplaced at about 12 km depth. The magma had an ƒO2 above the Ni/NiO solid oxygen buffer (NNO + 1), which is not considered optimal for Au transportation. The magma also formed a deep intrusion unable to provide magmatic fluids to trigger an efficient hydrothermal system. This study provides the basis for a systematic evaluation of the physico-chemical conditions within which syntectonic magmas evolved in the upper crust. Knowledge of these physico-chemical conditions are necessary to better constrain the IRGS metallogenic model.