The Marmorera-Cotschen hydrothermal system (Platta nappe, Switzerland): A Jurassic analogue to present-day oceanic ultramafic-hosted mineralized systems

Mid-Oceanic ridges are places of intense fluid-rock interactions. At (ultra)slow-spreading ridges where mantle rocks are exhumed along detachment faults, this notably leads to the formation of mineralized systems. They commonly form massive sulphides at the seafloor which are enriched in base (Cu, Zn, Ni), critical (Co) and precious (Au, Ag) metals. However, the limited conditions of observation at the seafloor lead to partial rather than integrative understanding of these hydrothermal systems, especially concerning deep hydrothermal processes. Alternatively, the study of fossil analogues preserved on-land offers the opportunity to study these systems in 3D and to access the deep hydrothermal plumbing system of such mineralizations. 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. As a rule, the hydrothermal system escaped strong Alpine overprint. In the Platta nappe, detachment faulting led to mantle exhumation against basalts. Associated HT fluid circulations led to the formation of mineralizations in the serpentinized footwall at the lithological interfaces with mafic intrusive rocks, suggesting the latter acted as preferential pathways for fluid flows. The Cu-Fe-Co-Zn-Ni mineralization forms massive, semi-massive sulphides and stockwork structures. It mainly consists of chalcopyrite, pyrrhotite, pentlandite, isocubanite and magnetite associated with Fe-Ca-silicates (ilvaite, hydro-andradite and Fe-diopside). Based on structural and petrographic features, the hydrothermal system of the Platta nappe is inferred to represent the root zone of present-day hydrothermal systems.