MASSIVE SULFIDE DEPOSITS AT MODERN SEDIMENTED OCEANIC RIFTS: GEOLOGICAL SETTING AND GENETIC PROCESSES

Sedimented oceanic rifts represent an important setting for seafloor-hydrothermal massive sulfide deposits. Modern examples of hydrothermally active sedimented rifts include Middle Valley, Juan de Fuca Ridge, Escanaba Trough, Gorda Ridge, and Guaymas Basin, Gulf of California. Sedimented rifts display the morphology of slow-spreading rifts with a flat valley floor up to 20 km wide and bounding uplifted fault blocks that tilt away from the rift axis. Because of this morphology and proximity to continents, the rifts are typically filled with hemipelagic and turbiditic sediments that are intruded by mafic sills. Massive sulfide mounds up to 35 m high and 150 m across occur on top and/or near the margins of sediment hills that interrupt the flat seafloor.

The internal architecture, as discerned from ODP drilling of the Bent Hill-ODP deposits, is characterized by stacked massive sulfide lenses of mostly vent complexes underlain by a sulfide stringer zone and overlain by chimneys, sulfide rubble and clastic sulfides shed off of mounds. The Bent Hill deposit is at least 104 m thick and contains approximately 15-20 million tonnes of massive sulfides. The vent complexes consist of a high-temperature pyrrhotite-sphalerite-isocubanite/chalcopyrite assemblage that is variably replaced by pyrite, marcasite, magnetite, sphalerite and Cu-Fe-sulfides. The sulfide stringer zone cuts silicified and chloritized sediments and is rooted in a laterally extensive stratabound copper zone composed of pyrrhotite-isocubanite-chalcopyrite that infills and replaces turbiditic sediments 170-200 m below the seafloor.

The Middle Valley and Escanaba hydrothermal systems have evolved from high-temperature (>300°C), metal-rich fluids that formed the massive sulfide deposits to moderate-temperature (<276°C), metal-poor fluids that are actively venting in both areas. Sr, C and S isotope compositions indicate sediment contamination of MORB-equilibrated fluids. The sediments have played an important role in capping the hydrothermal system and focusing fluid discharge at off-axis and long-lived vent sites in the case of the Bent Hill-ODP deposits. This probably explains why deposits formed in sedimented rifts are generally fewer in number but significantly larger on average than those formed at sediment-bare oceanic ridges.