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

Résumé

The Chibougamau pluton is a Neoarchean (~†2715-2718 Ma) multiphase intrusion genetically related to Cu-Au porphyry-style deposits. In Archean greenstone belts, porphyry-style mineralisation is marginal and poorly documented. Such deposits are however important in the Chibougamau area, where the main historical mining camp (Central Camp) is interpreted as a magmato-hydrothermal system. The mineralization has characteristics observed in porphyry systems, e.g. intermineral porphyritic dykes and hydrothermal breccia. The Chibougamau style of mineralisation is structurally controlled; it consists in NW-SE-striking sub-vertical sulphide-filled fractures, quartz veins and late iron carbonate-magnetite-sulphide veins. The main ore minerals are chalcopyrite, pyrrhotite, pyrite, and ±sphalerite-galena-molybdenite. Remobilisation of these deposits formed chalcopyrite veinlets and isolated gold nuggets. According to field and petrological descriptions, the Cu-Au porphyry-style deposits associated with the Chibougamau pluton may have formed in two main stages: 1) syn-magmatic stage, with possibly overprinting events (porphyry stage); and 2) syn-metamorphism remobilization. These processes are tested using pyrite chemistry. Quantitative laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses are performed and used as time-slice data sets, following the recent recommendations of Goucerol and coworkers. This powerful technic reveals the nature of the mineralizing magmatic fluid and documents the effect of remobilization within individual pyrite grains. Standard statistical methods (i.e. principal component analyses, PCA) are also used to assess element associations, which provides further insights into the nature of hydrothermal fluids.