A fossil, serpentinization-related hydrothermal vent, Ocean Drilling Program Leg 173, Site 1068 (Iberia Abyssal Plain): Some aspects of mineral and fluid chemistry

The basement at Site 1068, Ocean Drilling Program (ODP) Leg 173 (serpentini zed peridotite in fault contact with overlying amphibolite-clast-dominated sedimentary and tectonic breccias) is host to a hydrothermal system rooted in serpentinization reactions occurring at depth. The serpentinite grade's downward from cataclasites at the fault, through brecciated, recrystallized , tochilinite-bearing serpentinite, to awaruite-bearing massive, mesh-textu red serpentinite. Andradite is common throughout and is a major sink for ir on The breccias are similarly zoned, from tectonized rocks near the fault u pward into sedimentary breccias. Mg-silicate vein assemblages and rodingiti zed amphibolite clasts near the fault give way to calcite veins and nonperv asive albite-chlorite alteration upsection Marcasite (+/- pyrrhotite at the fault) is the sulfide phase and occurs only in the tectonic breccias. Fe o xides are magnetite near the fault and hematite and ferric oxyhydroxides up section The zonation reflects mixing of seawater with a fluid whose composi tion (low f(O2), f(S2), Si, CO2, high Ca, Fe, Ca/Mg, pH) is controlled by s erpentinization reactions. The deepest serpentinites have strongly reduced mineral assemblages that are unusual in a totally serpentinized peridotite. This probably reflects equilibration with a fluid derived from ongoing ser pentinization at depth. The upper serpentinites, on through the mineral seq uences seen in the breccias reflect increasing input from seawater upsectio n. Increased f(O2) and f(S2) stabilizes increasingly S- and O-rich assembla ges. Calcite (and ferric oxide) precipitation decreases pH, stabilizing mar casite. Relative to mid-ocean ridge hydrothermal systems, fluids in serpent inite-hosted hydrothermal system are poor in S and rich in Mg and are unlik ely to host large sulfide ore deposits.