Element partitioning during hydrothermal alteration at ultramafic-hosted mineralized systems: insights from the fossil Marmorera-Cotschen hydrothermal system (Platta nappe, SE Switzerland)

Ultramafic-hosted mineralized systems commonly form massive sulphides at the seafloor which are enriched in base (Cu, Zn, Ni), critical (Co) and precious (Au, Ag) metals. In present-day settings, the limited conditions of observation at the seafloor prevents a complete understanding of these hydrothermal systems, especially concerning deep hydrothermal processes. A way to unravel deep hydrothermal processes that occur in these systems is to focus on fossil analogues preserved on-land which noticeably well crop out in mountain belts. We adopted this strategy here and focused on a mineralized system preserved in the Platta nappe (SE Switzerland), a remnant of the Jurassic opening of the Alpine Tethys Ocean. The geometry and petrographic assemblages of the hydrothermal system, previously established, served as a base for the present study. We performed a geochemical tracing both on whole rocks and in-situ metal-bearing phases (sulphides and oxides) sampled at three distinct structural positions of the hydrothermal system. Among the geochemical tracers, Co, Ni and Se appear as good proxies to constrain hydrothermal processes. Indeed, at given structural position, the Co/Ni ratio increases in the most mineralized and altered sample suggesting this ratio is linked to the intensity of hydrothermal alteration. Also, towards the top of the system, a general trend showing respective decrease and increase of the Co/Ni ratio and of the Se content in metal-bearing phases was observed. The evolution of these geochemical tracers together with petrographic evidences supports a genetic model for the Marmorera-Cotschen hydrothermal system involving hydrothermal fluid progressively mixing with seawater.