Résumé
Ultramafic-hosted seafloor massive sulfide deposits have been reported in present-day oceanic settings over the past thirty years. These deposits and their fossil on-land analogs represent potential important resources for Cu, Co, Ni, Zn. However, developing comprehensive genetic models for deep-seatedhydrothermal processes is hindered by limited seafloor observations and reliance on large-scale geophysical studies. To overcome this issue, one can examine fossil systems, where hydrothermal plumbing architecture may be preserved. The Platta nappe (Swiss Alps) preserves a Jurassic hydrothermal system (the Marmorera-Cotschen Hydrothermal System; MCHS), where Cu-Fe-Co-Zn-Ni mineralization is associated with coeval Fe-Ca silicates (ilvaite, hydro-garnet, and diopside). The MCHS is hosted in serpentinites, which have been exhumed along a Jurassic paleo-detachment —a fossil analog ofa large-scale exhumation fault that forms oceanic core complexes at slow-spreading ridges— juxtaposing basalts onto serpentinites. The serpentinized footwall has numerous mafic intrusions, and the mafic-ultramafic rock contacts served as pathways for ore-forming fluids. Petrographic analyses and thermodynamic modeling indicate that Fe-Ca metasomatism occurred between 300‒360°C and at relatively low fO2 (from Magnetite-Native Fe to Fayalite-Magnetite-Quartz buffers), likely coeval with early-stage serpentinization. The composition of Fe-Ca silicates (Co, Ni, and REE contents, measured by in situ LA-ICP-MS) points to heterogeneous geochemical reservoirs involved during fluid-rock interaction. The geochemical compositions of Fe-Ca silicates in barren assemblages indicate an ultramafic-dominated system, while Fe-Ca silicates in mineralized assemblages point to an open-system, involving fluids derived from both mafic and ultramafic rocks. Mineralogical and geochemical signatures of Fe-Ca silicates in the MCHS do not support genetic relationships with common rodingitization. Our results suggest that Fe-Ca metasomatism may be a widespread deep-seated alteration along mafic-ultramafic rock contacts or in mantle rocks modified through melt-rock interaction accompanying mantle exhumation. We acknowledge JdC Fellowship (FJC2021-047190-I), funded by MCIN/AEI/10.13039/501100011033 with EU Next Generation Funds and funding from project PID2022-136471NB-C21 “The role of ultramafic rocks and related rocks in the sulfur and water cycles and their implications for the redox state of subduction zones (RUSTED)”, funded byMCIN/AEI/10.13039/501100011033 and by the ESF+.