TECTONIC SETTING AND MINERALOGICAL AND GEOCHEMICAL ZONATION IN THE SNAKE PIT SULFIDE DEPOSIT (MID-ATLANTIC RIDGE AT 23-DEGREES-N)

The Hydrosnake diving cruise with the submersible Nautile performed de tailed mapping and sampling of the Snake Pit hydrothermal field locate d at 23-degrees-N latitude on the Mid-Atlantic Ridge. The Snake Pit de posit consists of three coalescent east-west-trending mounds at the hi ghest point of a north-south neovolcanic ridge at the axis of the rift valley. The tectonic destruction of the mounds increases westward exp osing the core and stockwork of the deposit. At least two major hydrot hermal episodes separated by the formation of the axial graben are inf erred from chimneys and massive sulfide deposits forming the tectonize d western mound. Observation of lava tubes on fault scarps indicates t hat a volcanic event occurred between the two hydrothermal episodes. T he second hydrothermal episode is active on both sides of the graben. The first episode probably started along three parallel major fracture s in the incipient graben; the second episode is controlled by major f aults flanking the graben. Compared to deposits on fast-spreading ridg es, the more focused hydrothermal discharge at the Snake Pit site has resulted in bigger and more mature deposits. Four types of mineral ass ociations are distinguished: 1. The active top of the deposit is compo sed of sphalerite, pyrrhotite, and isocubanite chimneys associated wit h zinc-rich massive sulfide; this mineral association displays disequi librium and mineral replacements. 2. At the outer part of the mound, p yrite and marcasite are dominant in massive sulfides; equilibrium is b etter established between sulfides. 3. At the core of the deposit, cha lcopyrite and pyrite are dominant. 4. In the stockwork zone, isocubani te is the major sulfide mineral. Samples collected at the surface, fro m the central part, and the stockwork reveal horizontal and vertical g eochemical zonations of the deposit. Chimneys at the top of the mounds are Zn rich; associated minor elements are Cd, Pb, Ag, Sb, and Au. Ir on is the major component at the surface of the mound away from the ac tive chimneys; minor elements associated with Fe are As, Mo, and Au. T he 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 ma ssive sulfides, and 0.2 in the stockwork. Native gold grains (up to 12 mum) are associated with secondary digenite in old altered Cu-rich ch imneys. However, the high silver content and the occurrence of gold gr ains within primary bornite may indicate the formation of primary nati ve gold. The association of high gold values with pyrrhotite is unusua l and is better explained by a specific cooling history related to the morphology of the vents.