Detecting magmatic-derived fluids using pyrite chemistry: Example of the Chibougamau area, Abitibi Subprovince, Québec

Abstract

Magmatic fluids are an important carrier of precious and base metals in many mineralized systems (porphyry deposits, intrusion-related gold systems – IRGS, volcanogenic massive sulfides – VMS). Recognizing magmatic inputs is essential for understanding metallogenic processes and for developing accurate exploration models, and the importance of fluids exsolved from magmas remains a source of much debate for some contexts, such as IRGS and orogenic gold deposits. This study aims to identify magmatic fluids in the Chibougamau area (northeastern corner of the Neoarchean Abitibi Subprovince, Canada), using pyrite chemistry. This is achieved by comparing the chemistry of pyrites recovered from the Cu-Au magmatic-hydrothermal (porphyry) systems of the Chibougamau area to “background” compositions, i.e., the chemistry of pyrites from weakly mineralized rocks from the same area. Analyses of quantitative laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) are processed in three distinct manners to better evaluate the trace elements controlled by silicates, oxides, sulfides, and sulfosalt inclusions and the trace elements that are structurally bound to pyrite. Detailed investigation of the trace element content of pyrite indicates that most elements are controlled by silicate inclusions, some reflect the composition of the host rock, others correlate with the mineralizing style, and only Te and Bi are discriminant of the porphyry systems of the Chibougamau area. Tellurium is structurally bound to pyrite or forms telluride nanoparticles in the study area, while the distribution of Bi is controlled by galena and chalcopyrite inclusions, and pyrite chemistry is interpreted using absolute Te values and Bi ratios (Bi/Pb and Bi/Cu). This study provides insight into the Cu-Au magmatic-hydrothermal systems of the Chibougamau area. It also has implications for the way pyrite chemistry can be used to probe for magmatic fluids in other mineralized contexts.