Normal faulting and evolution of fluid discharge in a Jurassic seafloor ultramafic-hosted hydrothermal system

We document a normal fault that lies at a high angle to an oceanic detachment that exposed peridotite on the Jurassic seafloor. Faults such as this are inferred to be discharge zones for ultramafic-hosted hydrothermal systems, but have not yet been sampled from the modern seafloor. The fault comprises 0.5–2-m-thick zones of sheared talc + sulfide within serpentinite, and ends upward in carbonated serpentinite, massive sulfide, and pillow basalts. Talc alteration and enrichment in metals, light rare earth elements, and 34S of the fault rocks provide evidence of a conduit for discharge of high-temperature hydrothermal fluids related to fluid circulation and mineralization. At the seafloor, the fault rocks have been replaced by later Fe-dolomite + minor quartz, chlorite, and sulfides at temperatures of 90–120 °C during waning hydrothermal activity. This is the first view of the subsurface discharge zone of a seafloor ultramafic-hosted hydrothermal system, showing that normal faults provide pathways to focus fluid flow and reaction. An important new result is field and geochemical evidence that high-temperature hydrothermal systems with black smoker–type venting, sulfide mineralization, and talc alteration can evolve to lower-temperature Lost City–type venting and carbonate mineralization.