Hydrothermal vents at 5000m on the Mid-Cayman Rise: Where basement lithology and depth of venting controls sulphide deposit composition

This contribution describes the geological setting of hydrothermal activity within the Mid-Cayman Rise (MCR) using data acquired during RRS James Cook cruise JC44 (MAR-APR 2010) from the deep-towed sidescan sonar TOBI, 6000m-diving AUV Autosub6000 and the Robotic Underwater Vehicle (RUV) HyBIS. The 110 km-long Mid-Cayman Rise (MCR), located within the Cayman Trough, Caribbean Sea, is the deepest seafloor-spreading centre in the world reaching to ~7000m. Its morphology is consistent with ultra-slow-spreading and magnetic anomalies confirm its divergence rate of only ~15 mm yr^-1, with active spreading since ~49 Ma. Hence the MCR poses an end-member of extreme depth for oceanic accretion and hydrothermal circulation. Our sonar imagery revealed accretion of new volcanic crust is focused within two ridge segments, to the north and south of a centrally located massif of peridotite and gabbro. Following earlier indications of hydrothermal plumes [1], we discovered two unique high-temperature hydrothermal system within the MCR: one at a depth of 5000m in the neovolcanic zone of the northern segment of the MCR, and another at 2200m on the flanks of the MCR coincident with the peridotite massif. These sites show contrasting styles of fluid venting, mineralisation, geological setting and host rock interaction enabling us to propose that depth and basement lithology plays an important role in controlling the metal tenor of submarine hydrothermal deposits.