Lithium Isotope Distribution in Hydrothermal Alteration from the Cochenour Extension of the Red Lake Gold Mine, Ontario

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Goldcorp is one of the largest gold producers in the world and is actually developing its Cochenour mine, located on the old Cochenour-Willans mine site within the Red Lake district, Ontario, and connected to the Red Lake mine through an underground 5 km haulage drift. At Cochenour, two different types of mineralizations are identified: (1) replacement-style mineralization with/without arsenopyrite; and (2) quartz-actinolite vein-style mineralization with visible gold. The company-specific problem is that although the concentration of arsenic can act as a proxy for gold when arsenopyrite is present, no geochemical proxy has yet been identified for replacement-style mineralization without arsenopyrite or for vein-style mineralization. The objective of this research is to verify if the lithium isotope ratio (d7Li) of hydrothermally-altered rock surrounding the gold mineralization could act as a proxy for both styles of mineralization. It was previously demonstrated that the d7Li of hydrothermally altered rocks could correlate with gold at Kirkland Lake Gold Inc.'s Macassa Gold Mine (Nadeau, 2017) and it is likely that this correlation may also be present at the Cochenour deposit. The rationale behind the use of Li isotopes is that like Na and K, Li is a mobile alkaline element and is thus involved in hydrothermal alterations. However, because Li is present in rocks/fluids in trace concentrations, it follows a non-ideal Henrian behaviour and is thus controlled by partitioning between minerals and fluids. Furthermore, Li has 2 stable isotopes which fractionate readily via water-rock interactions so the Li concentration and the 7Li/6Li ratio of hydrothermally altered rocks should allow tracing the sources and pathways of hydrothermal fluids within hydrothermally-altered environments. The reason why partial correlation of d7Li and Au was identified at Kirkland Lake Gold Inc.'s Macassa mine has remained unclear and the present research aims to study a simpler system in a single type of lithology in order to shed light on the relations that exist between gold mineralization and lithium isotopic ratios.