IDENTIFYING A MAGMATIC-HYDROTHERMAL CONTRIBUTION IN ARCHEAN AURIFEROUS MINERALIZATION USING PYRITE CHEMISTRY

Most auriferous deposits hosted in the Abitibi greenstone belt, Canada, are interpreted to be orogenic in origin, resulting exclusively from metamorphic fluids. However, a wide typology of gold mineralization has been described, including synvolcanic (Au-rich VMS and Cu-Au-Mo associated with TTG complexes) and syntectonic (quartz-veins, disseminated, intrusion-associated). A magmatic-hydrothermal fluid contribution is now well recognized and accepted in the synvolcanic type but remains debated in syntectonic deposits. Independent of the mineralization style, most syntectonic gold deposits are located along major structures (eg. Cadillac fault, Abitibi) and hosted in deformed and metamorphosed terranes, thus making the identification of a possible magmatic-hydrothermal fluid contribution difficult. Early detection of a magmatic input would have a significant impact on exploration strategies used for future gold deposit discoveries.

We investigated trace element signatures of ~1000 pyrite grains, using LA-ICP-MS, from 18 syntectonic deposits located in the Abitibi greenstone belt. Our case studies are representative of 1) classic orogenic quartz-vein-type mineralization, hosted both in intrusions (ex: Goldex) and volcanic rocks (ex: Detour Gold), 2) disseminated-style (ex: Lapa) and 3) intrusion-associated deposits related to intermediate (ex: Beattie) or felsic intrusions (ex: Bachelor) of alkaline affinity and where a magmatic-hydrothermal gold contribution has been proposed.

We developed a new multielement diagram in which pyrite data are normalized to the average Archean sedimentary pyrite, illustrating element gain and loss relative from a possible sedimentary source for gold. Pyrite from orogenic vein-type are depleted in most trace elements. Only the Au-Co-Ni content is similar to sedimentary pyrites. Disseminated pyrite associated with alkaline intrusions are characterized by an overall higher abundance of most trace elements with positive Au-Se-Te-Ag-W-Tl anomalies. Such a signature is almost identical to modern alkalic porphyry deposits (e.g. Ladolam, Cripple Creek) where the contribution of magmatic-hydrothermal fluids is indisputable. Thus, we propose that pyrite chemistry can be used to identify the source of the fluids and the conditions of precipitation.