THE INTERNAL STRUCTURE OF AN ACTIVE SEA-FLOOR MASSIVE SULFIDE DEPOSIT

THE hydrothermal circulation of sea water through permeable ocean crus t results in rock-water interactions that lead to the formation of mas sive sulphide deposits. These are the modern analogues of many ancient ophiolite-hosted deposits(1-4), such as those exposed in Cyprus. Here we report results obtained from drilling a series of holes into an ac tively forming sulphide deposit on the Mid-Atlantic Ridge. A complex a ssemblage of sulphide-anhydrite-silica breccias provides striking evid ence that such hydrothermal mounds do not grow simply by the accumulat ion of sulphides on the sea floor. Indeed, the deposit grows largely a s an in situ breccia pile, as successive episodes of hydrothermal acti vity each form new hydrothermal precipitates and cement earlier deposi ts. During inactive periods, the collapse of sulphide chimneys, dissol ution of anhydrite, and disruption by faulting cause brecciation of th e deposit. The abundance of anhydrite beneath the present region of fo cused hydrothermal venting reflects the high temperatures (>150 degree s C) currently maintained within the mound, and implies substantial en trainment of cold sea water into the interior of the deposit. These ob servations demonstrate the important role of amhydrite in the growth o f massive sulphide deposits, despite its absence in those preserved on land.