Hydrothermal vents jetting out water at 380 degrees +/- 30 degrees C have been discovered on the axis of the East Pacific Rise. The hottest waters issue from mineralized chimneys and are blackened by sulfide precipitates. These hydrothermal springs are the sites of actively forming massive sulfide mineral deposits. Cooler springs are clear to milky and support exotic benthic communities of giant tube worms, clams, and crabs similar to those found at the Galápagos spreading center. Four prototype geophysical experiments were successfully conducted in and near the vent area: seismic refraction measurements with both source (thumper) and receivers on the sea floor, on-bottom gravity measurements, in situ magnetic gradiometer measurements from the submersible Alvin over a sea-floor magnetic reversal boundary, and an active electrical sounding experiment. These high-resolution determinations of crustal properties along the spreading center were made to gain knowledge of the source of new oceanic crust and marine magnetic anomalies, the nature of the axial magma chamber, and the depth of hydrothermal circulation.

No abstract

Massive ore-grade zinc, copper and iron sulphide deposits have been found at the axis of the East Pacific Rise. Although their presence on the deep ocean-floor had been predicted rhere was no supporting observational euidence. The East Pacific Rise deposits represent a modern analogue of Cyprus-type sulphide ores associated with ophiolitic rocks on land. They contain at least 29% zinc meral and 6 % merallic copper. Their discovery will prouide a new focus for deep-sea exploration, leading to new assessmenrs of the concentration of metals in the upper layers of the oceanic crust.-THE area of the deposits of ore-grade zinc, copper and iron sulphide was explored and sampled in February-March 1978 by the manned diving saucer CYANA during the expedition CYAMEXi. The expedition, the only submersible diving programme that has so far been conducted on the East Pacific Rise (EPR), is part of the French-American-Mexican project RITA (Rivera-Tamayo), a 3-yr study devoted to detailed geological and geophysical investigations of the EPR crest. The ore deposits were sampled in water depths of close to 2,620 m a t two neighbouring sites near 20" 54' N 109" 03'W. (refs 2-5) about 9 0 km north of the Rivera transform fault and 240 km south of the Tamayo transform fault (Fig. 1). Three dives of Cyana (CY 78-06, 08 and 12) crossed the two sampling sites, and we collected samples during two of these dives (CY 78-08 and 12). However, during al1 dives in the EPR axial zone, signs of hydrothermal activity were seen, including colonies of dead giant clams, fields of pillow lavas with pronounced colourstaining at the base of pillows, and coloured deposits on exposed scarp surfaces of normal faults and open fissures'. Coral-like growths, possibly of native sulphur, occur in other locations, including a sedimented fault-scarp about 1.0 km to the West of where the sulphide ores were sampled. Sampling sites The two sites where the sulphides were sampled lie on the lightly sedimented flanks of steep-sided structural depressions, about 20-30 m deep, 20-30 m wide. and about 600-700 m west of the axis of the 'extrusion zone' where the youngest lavas occur. Whereas the extrusion zone is marked by a 50 m-high sedimentfree discontinuous ridge with n o fissures or faults, the structural *The authors are al1 members of the CYAMEX Scientific Team.

No abstract

The Cretaceous Samail ophiolite in Oman exposes an almost complete, 500‐km‐long, along‐axis section of oceanic crust, providing a unique opportunity to study the geometry, physical conditions, and effects of the hydrothermal circulation that fed the volcanic‐hosted massive‐sulfide deposits. These fossil discharge zones are rooted in the sheeted‐dike complex, down to the transition zone with the plutonic sequence. The sheeted‐dike complex as a whole was affected by greenschist‐facies metamorphism (albite, actinolite, chlorite, quartz, sphene). Diabase dikes are commonly altered into three major alteration end‐members termed spilite, mineralized spilite, and epidosite. These alteration patterns usually follow dikes, thus resulting in a well‐marked alongstrike vertical anisotropy. Their vertical distribution is also anisotropic: epidosites occur mainly in the basal sheeted‐dike complex, and mineralized spilites are usually limited to its top and to the transition zone with the volcanic extrusives. The alongstrike distribution of the different alteration patterns also shows a well‐marked correlation with the occurrence of massive‐sulfide deposits. The sheeted‐dike complex underlying these deposits is characterized by a sharp increase in the volume of epidosite and mineralized spilite. The stratigraphic position of the epidosite zones just above the magma chamber and in the lower sheeted‐dike complex, their attitude parallel to the margins of the dikes, the recorded high trapping temperatures, the high water/rock ratios, their textural reconstitution and base metal depletion suggest that they formed in a focused upflow portion of a vertical, along‐strike convecting, high‐temperature (subcritical) hydrothermal system a short time after emplacement of the dikes. The common occurrence of epidosite bands devoid of veins, and the absence of major fracturing or listric faulting, indicates that formation of epidosite does not first require tectonic extension.