Y. Fouquet D. J. Miller J.M. Bahr P.A. Baker T. Bjerkgard C.A. Brunner R.C. Duckworth R. Gable J. Gieskes W.D. Goodfellow H. M. Groschel-Becker G. Guerin J. Ishibashi G. Iturrino R.H. James K.S. Lackschewitz L.L. Marquez P. Nehlig J.M. Peter C.A. Rigsby P. Schultheiss Wayne C. Shanks III Bernd R. T. Simoneit M. Summit D.A.H. Teagle M. Urbat G.G. Zuffa R.A. Zierenberg 1998 Hydrothermal circulation at the crests of mid-ocean ridges plays an important role in transferring heat from the interior of the Earth. A consequence of this hydrothermal circulation is the formation of metallic ore bodies known as volcanic-associated massive sulphide deposits. Such deposits, preserved on land, were important sources of copper for ancient civilizations and continue to provide a significant source of base metals (for example, copper and zinc). Here we present results from Ocean Drilling Program Leg 169, which drilled through a massive sulphide deposit on the northern Juan de Fuca spreading centre and penetrated the hydrothermal feeder zone through which the metal-rich fluids reached the sea floor. We found that the style of feeder-zone 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 mineralization not previously observed below sea-floor deposits, and may provide new targets for land-based mineral exploration. application/pdf 10.1038/33126 en The deep structure of a sea-floor hydrothermal deposit article