Origin of Fe-Ca-Metasomatism in Exhumed Mantle Rocks at the MARK Area (23°N, ODP Leg 153) and Implications on the Formation of Ultramafic-Hosted Seafloor Massive Sulfide Deposits

Abstract

At Mid-Ocean Ridges, hot, reduced, acidic, and metal-rich fluids are responsible for the formation of ultramafic-hosted seafloor massive sulfide deposits (UM-SMSs), where mantle exhumation efficiently operates. As UM-SMSs display great structural, mineralogical, and geochemical variabilities from site to site, a simple genetic model cannot be applied. Notably, fluid circulation and Fe-Ca metasomatism are reported in ultramafic-hosted hydrothermal deposits exposed in ophiolites, suggesting it might have genetic implications on the formation of mineralized systems. Similar Fe-Ca metasomatism was reported in drilled mantle rocks at the Mid Atlantic Ridge Kane (MARK) area, offering access to the vertical dimension beneath an exhumed oceanic core complex to provide an integrative study of the nature and geometry of deep magmatic and hydrothermal processes. At MARK, mantle rocks underwent complex processes of melt-rock and fluidrock interactions. Magma channeling and interactions with surrounding rocks enriched mantle silicates in Fe, Co, and Zn. There, subsequent hydrothermal alteration allowed to stabilize Fe-rich silicates. Mineralogy and geochemistry of hydrothermal phases at MARK suggest mineral crystallization under temperatures from similar to 830 degrees down to 350 degrees C during early mantle exhumation at a depth <6.5 km below seafloor, followed by serpentinization of the massif during progressive mantle denudation. Considering the lithological heterogeneity at (ultra)slow-spreading ridges, metal enrichment in deep mantle rocks during melt-rock interactions may be a widespread process. In ultramafic-dominated environments where extensional tectonics allow fluid flows to these deep zones, fluids may leach and transport metals to the surface, accounting for metal entrapment in UM-SMSs.