Subseafloor structure of a submarine hydrothermal system within volcaniclastic sediments: a modern analogue for 'Kuroko-type' VMS deposits

Deep drilling through an active hydrothermal field at the lheya North Knoll in the middle Okinawa Trough during IODP Expedition 331 provided a unique opportunity to directly access the subseafloor environment. Whereas sedimentation along the slopes of the knoll was dominated by volcanic clasts of tubular pumice, intense hydrothermal alteration was recognized in the vicinity of the hydrothermal centre even at shallow depth below the seafloor. Together with extremely high temperature gradients encountered, this prevalent alteration is attributed to laterally extensive infiltration of fluid within the sediment layer. Styles of sulfide-sulfate mineralization encountered are directly comparable to those found within economically important Kuroko-type volcanogenic massive sulfide (VMS)