Gold Mineralization, Hydrothermal Alteration, and Li Isotope Fractionation at Cochenour Orogenic Gold Deposit, Red Lake, Canada

Abstract

Abstract Lithium isotopes have been used to study subduction zones, continental weathering, and magmatic–hydrothermal transitions, but little is known about their behavior in metasomatic footprints surrounding hydrothermal ore deposits. Although minerals such as biotite, chlorite, and white micas may sometimes be characteristic of Au mineralization, they are not truly diagnostic given that they also usually occur outside the ore zones. Here, whole rock δ7Li compositions are measured in samples from drill core cutting through metasomatized basalts from the Cochenour orogenic Au deposit, Red Lake, Canada, to verify whether Au ore-related biotite, chlorite, white micas, and quartz have specific δ7Li signatures. Laser ablation ICP-MS maps show 1000–1100 ppm Li in biotite, 200–600 ppm in chlorite, and 20–100 ppm in white mica. In the ‘arsenopyrite-bearing replacement-style’ alteration and mineralization, Au is associated with arsenopyrite, pyrrhotite, and chalcopyrite in the ore zone, but is more concentrated in white mica than in sulfides outside the ore zone. In the cross-cutting ‘quartz-actinolite vein-style’ alteration and mineralization, Au is distributed with chalcopyrite and arsenopyrite in the selvages of the veinlets, but is associated with pyrite, calcite, and biotite outside the ore zone. Mass balance calculations suggest that Au ore-related biotite, chlorite, white mica, and quartz have δ7Li values of ∼+4‰, ∼0 to +2‰, ∼−2‰, and ∼+6 to +12‰, respectively. This is explained by Li isotopic fractionation occurring during the retrograde sequence of auriferous biotite-chlorite-white mica alteration and the late auriferous quartz vein event, identified in the present samples as well as in the Red Lake-Campbell mine complex and the Red Lake greenstone belt. In the ‘arsenopyrite-bearing replacement-style’ alteration, the intercorrelation of δ7Li and Au and their correlation over distance show greater amounts of Au in biotite-enriched rocks, where biotite has δ7Li values of ∼+4‰. Such values indicate a mantle source for the early biotite-related auriferous fluid at Red Lake.