MERCURY STABLE ISOTOPE GEOCHEMISTRY REVEALS DEPOSIT FORMATION PROCESSES IN ARCHEAN GOLD DEPOSITS: EXAMPLES FROM THE ABITIBI GREENSTONE BELT, CANADA

Mercury isotope geochemistry is a useful tool in distinguishing metal sources, and tracing concentration processes in nature. In hydrothermal systems, Hg shows both mass-dependent fractionation (MDF; δ²⁰²Hg and δ¹⁹⁸Hg), usually controlled by processes such as boiling, and mass-independent fractionation (MIF; Δ¹⁹⁹Hg and Δ²⁰¹Hg) due to compositional variation in the source of Hg or to secondary photochemical processes. The ability of Hg MDF and MIF to isotopically discriminate Hg in different rock mineral deposit types can help fingerprint potential sources of metals and ore forming processes. This study presents the first Hg isotope compositions in mineralized Archean magmatic and sedimentary rocks. It aims at better understanding Archean gold deposits and the cycle of Hg in these complex environments by studying its concentration and isotope composition in different deposit types and their corresponding host rocks from the Abitibi greenstone belt. Samples were prepared using a tube furnace pyrolysis technique and analyzed by MC-ICP-MS. Preliminary results are presented for 33 samples from four distinct gold deposits located along the Larder Lake-Cadillac fault zone: LaRonde Penna (LR: gold-rich VMS), Triangle (TR) and Ormaque (OM) (orogenic quartz-carbonate-tourmaline veins), and Lapa (LP: orogenic quartz-carbonate veins). Results indicate that δ²⁰²Hg varies between -1.13 to +0.84‰ (LR), -1.15 to +0.32‰ (TR), -0.71 to +0.72‰ (LP), and -0.62 to +0.88‰ (OM). Δ¹⁹⁹Hg ranges from -0.06 to +0.23‰ for LR; -0.07 to +0.05‰ for TR; -0.04 to +0.12‰ for LP; and -0.12 to +0.08‰ for OM. These isotopic variations strongly suggest that distinct origins and processes are associated with the formation of the hydrothermal systems responsible for the formation of these contrasting types of gold deposits (ie. synvolcanic versus orogenic). The fact that MIF values in the LR gold-rich VMS deposit samples, reaching +0.23‰, are significantly higher than the quartz-carbonate-tourmaline and quartz-carbonate vein systems (-0.12 to +0.12‰) suggests that Hg stable isotope compositions are reliable proxies for discriminating sources of Hg in Archean gold deposits. Other samples are currently processed to strengthen this conclusion and help further constrain the content and origin of Hg in the rocks and mineral deposits of the Abitibi greenstone belt.