The Sr isotope geochemistry of oceanic ultramafic-hosted mineralizations
Résumé
The source of metals involved in the formation of oceanic ultramafic-hosted hydrothermal Cu-Fe-Co-Zn-Ni mineralization remains poorly constrained. Here, we focus on a fossil ultramafic-hosted hydrothermal mineralized system preserved in the Platta nappe (SE Switzerland), where mantle rocks were exhumed along detachment faults to the seafloor during Jurassic rifting. The Cu-Fe-Co-Zn-Ni mineralization, associated with Fe-Ca-metasomatism (ilvaite-hydroandradite-diopside), represents an analogue of the root zone of present-day hydrothermal systems formed at mid-ocean ridges (e.g., Rainbow hydrothermal field at the Mid-Atlantic Ridge). We apply the Sr isotope geochemistry to Fe-Ca silicates and secondary, alteration products that include serpentinites, altered mafic and carbonated rocks to constrain the source(s) of metals and to characterize the plumbing system. The Fe-Ca silicates and carbonates have Sr isotope ratios close to that of Jurassic seawater, suggesting a near seafloor, seawater-dominated hydrothermal system with high fluid/rock ratios. The altered mafic rocks have 87Sr/86Sr ratios lower than those of Jurassic seawater. In contrast, serpentinites display a large range of Sr isotope ratios, including values higher than those of Jurassic seawater, indicating long lived fluid/rock interactions and multi-stage fluid infiltrations. These results suggest that hydrothermal activity started during final crustal thinning, when seawater percolated along normal faults through the hyper-thinned crust, acquiring high Sr isotope ratios before penetrating and initiating serpentinization of the subcontinental mantle rocks. At this early stage, the system was rock-dominated (i.e., low fluid/rock ratios), leading to high 87Sr/86Sr ratios in serpentinites. On their way-back to seafloor, the uprising, serpentinization-derived fluids mixed with seawater resulting in the precipitation of metal sulfides and Fe-Ca silicates, with subsequent carbonation in a fluid-dominated system. Our study shows that the Sr isotope geochemistry can be used to identify reservoirs involved in the formation of mineral deposits and for the characterization of the plumbing system of oceanic ultramafic-hosted mineralizations.
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