Late-alpine five-element mineralization in the Punta Corna vein system (Western Alps): evolution of methane-bearing crustal fluids and their role to arsenide precipitation and ore-deposit metallogeny

• The Punta Corna vein system is a five-element hydrothermal mineralization in the Western Alps. • Three hydrothermal stages occur leading to base-metal sulfides and Ni-Co arsenides. • Fluid inclusions revealed three different fluids precipitating ores during different stages. • Methane is the main a reducing agent triggering the formation of five-element mineralization. • Fluids mixing between reduced and oxidized sulfate-bearing brine triggered ore precipitation. The Punta Corna vein system (PC), located within the Western Alpine meta -ophiolites, is characterized by a five-element vein type mineralization including Fe-Co-Ni arsenides preceded and followed by a typical base-metal sulfide mineralization, comprising tetrahedrite, chalcopyrite, pyrite and galena. Three distinct hydrothermal stages were recognized: Sulfide stage I, Arsenide stage and Sulfide stage II. Microthermometric analysis of fluid inclusion assemblages from Sulfide stage I allowed to constrain fluid A (surface-derived, sulfate-bearing, ∼27.3 wt% total salinity and 140 °C homogenization temperature) and fluid B (deep-seated, methane-bearing, 18.8 wt% total average salinity and 163 °C homogenization temperature). The Arsenide stage is characterized by the presence of fluid C (deep-seated, ∼19 wt% total average salinity and 156 °C homogenization temperature) and fluid D (deep-seated, ∼13 wt% total average salinity and 230 °C homogenization temperature). Fluids B and C are inferred to represent the same fluid, with and without methane, respectively. The absence of methane in fluid C is interpreted as its consumption during arsenide formation by reduction. This detailed fluid inclusion study revealed evidence of pre-ore methane, which has been proposed as a reducing agent, important in the formation of five-element mineralization. This finding has two important implications: (i) it constrains the shift from a hydrothermal system precipitating base metal sulfides to a five-element one through the mixing of a metal-bearing fluid and a highly reduced methane-bearing fluid, and (ii) it records the presence of an oxidized sulfate-bearing brine and the reduced-metal bearing fluid in the crustal rocks, which mixed and thus formed the five-element mineralization. Crucial to this process is the role of late-Alpine brittle tectonics, which, through the development of two main fault systems, enhanced rock permeability allowing the input of different fluids in the active hydrothermal system.