THE DEEP-STRUCTURE OF A SEA-FLOOR HYDROTHERMAL DEPOSIT

Hydrothermal circulation at the crests of mid-ocean ridges plays an im portant role in transferring heat from the interior of the Earth(1-3). A consequence of this hydrothermal circulation is the formation of me tallic ore bodies known as volcanic-associated massive sulphide deposi ts. Such deposits, preserved on land, were important sources of copper for ancient civilizations and continue to provide a significant sourc e of base metals (for example, copper and zinc)(4-6). Here we present results from Ocean Drilling Program Leg 169, which drilled through a m assive sulphide deposit on the northern Juan de Fuca spreading centre and penetrated the hydrothermal feeder zone through which the metal-ri ch fluids reached the sea floor. We found that the style of feeder-zon e mineralization changes with depth in response to changes in the pore pressure of the hydrothermal fluids and discovered a stratified zone of high-grade copper-rich replacement mineralization below the massive sulphide deposit. This copper-rich zone represents a type of minerali zation not previously observed below sea-floor deposits, and may provi de new targets for land-based mineral exploration.