Ore Mineralogy, Sulfide Geochemistry, and S Isotopes of Magmatic-Hydrothermal Au Mineralization Associated with the Mooshla Intrusive Complex, Doyon-Bousquet-LaRonde Mining Camp, Abitibi Greenstone Belt, Quebec

Abstract The Archean Mooshla Intrusive Complex is a polyphase magmatic body that is spatially, and possibly genetically, associated with numerous world-class Au-rich volcanogenic massive sulfide deposits and epizonal subseafloor intrusion-related (or porphyry-epithermal-style) Au-(Cu) vein systems in the Abitibi greenstone belt, Canada. To elucidate any genetic links between deposits and the Mooshla Intrusive Complex, mineralized samples from two intrusion-related Au-Cu vein deposits (Doyon and Grand Duc deposits) have been characterized using a combination of detailed petrography and trace element and sulfur isotope analyses of sulfide phases. The pyrite trace element and S isotope systematics indicate that numerous processes were operating to produce the different ore assemblages at Doyon and Grand Duc. However, the main Au precipitation mechanism for both the Doyon and lower-grade vein systems at Grand Duc is interpreted to be influenced by wall-rock sulfidation. Pyrite from high-grade veins at Grand Duc display significantly different trace element and δ34S patterns compared to those at Doyon and lower grade veins at Grand Duc. Pyrite from high-grade veins at Grand Duc also have elevated concentrations of epithermal-suite elements (i.e., Au-Ag-As-Te-Bi-Sb-Se) along pyrite rims and, combined with a drop in δ34S from the metal-poor core to the metal-rich rim in individual pyrite grains, suggest gold precipitation resulted from a magmatic-hydrothermal fluid boiling event. Although pyrite records a fluid boiling event, gold and tellurobismuthite occur synchronous with chlorite alteration, indicating gold and tellurides were likely a result of Au remobilization from pyrite during regional metamorphism. Although the trace element and ore mineral characteristics have been influenced by regional metamorphism, the δ34S compositions and patterns have retained their magmatic signature and importantly the Δ33S values are near zero within and between individual pyrite grains. Therefore, the combination of δ34S and Δ33S indicates a singular source of sulfur and rules out non-magmatic sulfur sources. Even though these deposits have been variably modified during regional metamorphism, the combination of methods applied here can identify primary magmatic-hydrothermal processes responsible for the development of these Au-rich deposits, potentially linking them to causative magmatism.