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Exploring Ultradeep Hydrothermal Vents In the Cayman Trough by ROV HyBIS ROV Finds, Images and Samples Massive Sulfide Deposits at 5,000 Meters Depth By Dr. Bramley J. Murton Senior Geologist Veit Hühnerbach Geologist National Oceanography Centre Southampton, England and Jo Garrard Design Engineer Hydro-Lek Ltd. Finchampstead, England T he 110-kilometer-long Mid-Cayman Spreading Center (MCSC), located within the Cayman Trough in the Ca- ribbean Sea, is the world's deepest seafloor spreading rift. Reaching depths beyond 6,000 meters, the MCSC hosts the deepest hydrothermal vents known. This ultradeep volcanic rift was explored by developing and operating the Hydraulic Benthic Interactive Sampler (HyBIS) ROV. The vehicle yield- ed the discovery of two hydrothermal vent fields on the rift floor: one at 5,000 meters depth and another in the central MCSC located on the side of a large seamount and forming the western flank of the rift. With their contrasting styles of fluid venting, sulfide min- eralization, geological setting and host-rock interaction, the differences between these two sites indicate that depth and basement rock type may significantly affect the metal con- tent of hydrothermal seafloor massive sulfide deposits. Exploration Strategy Ultradeep hydrothermal vents hold a key to understand- ing the subsurface formation of mineral deposits on land. In 2010, an expedition led by the U.K.'s National Oceanog- raphy Centre to the MCSC at 6,000 meters depth aimed to locate, film and sample ultradeep hydrothermal vents. These vents are considered to be the highest pressure, hottest and most copper-rich seafloor hydrothermal systems known to date. It was predicted that this study would answer some critical questions about the evolution of hydrothermal fauna nourished by the chemicals in the vents. The survey of the MCSC used a Simrad (Horten, Nor- way) EM 120 multibeam swath echosounder deployed from the RRS James Cook and the National Oceanography Centre's Towed Ocean Bottom Instrument (TOBI), a deep- The location of the Mid-Cayman Spreading Centre (MCSC) within the Caribbean Sea (a) and the EM 120 multibeam ba- thymetry map of the MCSC showing the location of two dif- ferent hydrothermal mounds (b). The 5,000-meter-deep Beeb site (c) and the 2,300-meter-deep Von Damm site (d) are also shown. towed 30-kilohertz side scan sonar equipped with an opti- cal plume sensor to detect hydrothermal plumes. CTD casts verified a number of potential plume signals, two of which were in the general vicinity of hydrothermal water-column plumes reported in 2009 by a U.S. team from the Woods Hole Oceanographic Institution. With two strong plume signals verified, a near-bottom www.sea-technology.com survey was conducted by the National Oceanography Cen- tre's Auto-Sub 6000 (AS6K), an AUV capable of diving to 6,000 meters and running a mission up to 48 hours at speeds of 7 knots. For the MCSC missions, the AS6K was deployed with a Simrad EM 2000 high-resolution multibeam swath SEPTEMBER 2012 / st 15