The Hydrosnake diving cruise with the submersible Nautile performed detailed mapping and sampling of the Snake Pit hydrothermal field located at 23-degrees-N latitude on the Mid-Atlantic Ridge. The Snake Pit deposit consists of three coalescent east-west-trending mounds at the highest point of a north-south neovolcanic ridge at the axis of the rift valley. The tectonic destruction of the mounds increases westward exposing the core and stockwork of the deposit. At least two major hydrothermal episodes separated by the formation of the axial graben are inferred from chimneys and massive sulfide deposits forming the tectonized western mound. Observation of lava tubes on fault scarps indicates that a volcanic event occurred between the two hydrothermal episodes. The second hydrothermal episode is active on both sides of the graben. The first episode probably started along three parallel major fractures in the incipient graben; the second episode is controlled by major faults flanking the graben. Compared to deposits on fast-spreading ridges, the more focused hydrothermal discharge at the Snake Pit site has resulted in bigger and more mature deposits. Four types of mineral associations are distinguished: 1. The active top of the deposit is composed of sphalerite, pyrrhotite, and isocubanite chimneys associated with zinc-rich massive sulfide; this mineral association displays disequilibrium and mineral replacements. 2. At the outer part of the mound, pyrite and marcasite are dominant in massive sulfides; equilibrium is better established between sulfides. 3. At the core of the deposit, chalcopyrite and pyrite are dominant. 4. In the stockwork zone, isocubanite is the major sulfide mineral. Samples collected at the surface, from the central part, and the stockwork reveal horizontal and vertical geochemical zonations of the deposit. Chimneys at the top of the mounds are Zn rich; associated minor elements are Cd, Pb, Ag, Sb, and Au. Iron is the major component at the surface of the mound away from the active chimneys; minor elements associated with Fe are As, Mo, and Au. The Cu-rich core of the deposit is enriched in Mo, In, Sn, As, and Au. The copper-rich stockwork is enriched in Co and Se. Mean values for Au are, respectively, 1.8 ppm in the chimney, 3.4 ppm in the massive Zn sulfides, 0.75 ppm in the massive Fe sulfides, 1.4 ppm in the Cu-Fe massive sulfides, and 0.2 in the stockwork. Native gold grains (up to 12 mum) are associated with secondary digenite in old altered Cu-rich chimneys. However, the high silver content and the occurrence of gold grains within primary bornite may indicate the formation of primary native gold. The association of high gold values with pyrrhotite is unusual and is better explained by a specific cooling history related to the morphology of the vents.
