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 S-34 of the fault rocks provide evidence of a conduit for discharge of high-temperature hydrothermal fluids related to fluid circulation and mineralization. At the sea-floor, the fault rocks have been replaced by later Fe-dolomite + minor quartz, chlorite, and sulfides at temperatures of 90-120 C-degrees during waning hydrothermal activity.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.