Three-stage formation of greenstone-hosted orogenic gold deposits in the Val-d'Or mining district, Abitibi, Canada: Evidence from pyrite and tourmaline

Orogenic gold deposits are the most widespread type of gold deposit worldwide, defining important exploration targets in Precambrian greenstone belts. Here, we refine the model for orogenic gold formation in the world-class Val-d'Or mining district (Quebec, Canada) using geochemical, isotopic and mineralogical data from quartz-tourmaline-carbonate (QTC) veins from several deposits across the district. Multi-element (Ag, As, Au, B, Ba, Co, Mg, Mo, Ni, and Pb) pyrite mapping, as well as major, trace, and rare earth element variations in tourmaline, define a three-stage paragenesis across several deposits. The first, reduced phase crystallized the quartz-carbonate-gold association in the veins; the second, oxidized phase favored tourmaline and barite crystallization (barite was not previously reported from this district); the third phase involved a reduced fluid and the crystallization of non-auriferous, As-Co-Ni-rich cubic pyrite, and tourmaline. Boron isotope ratios of tourmaline vary from -15.6 to -7.7 parts per thousand, which is attributed to the mixing of at least two fluids, one related to a deep, metamorphic source and the other to shallower, possibly relict formation water. A systematic regional pattern in B-isotope variation is found, with lighter values in the volcanic rocks and heavier ones in or close to intrusions, which suggests different mixing proportions in the vein field from NE to SW.