Hole 504B of the Ocean Drilling Program is dedicated to the study of crustal structure and hydrothermal processes in 5.9-m.y.-old oceanic basement. Continuing the work of previous legs, hole 504B was extended 212.3 m to a total depth of 1562.3 m below seafloor (bsf) during leg 111 in 1986. Quartz-sulfide veins occur at a depth of 1369--1388 m bsf in basalts of the sheeted dike complex. The ore minerals are predominantly pyrite, less chalcopyrite, rare Co-rich Cu-Fe-S phases, and a thiospinel (linnaeite/carrollite). Microphobe analyses yield a high Co content in Zoned vein pyrites (>8 wt %) as well as in the Cu-Fe-S phases (>5 wt %). UP to 35.8 wt % Co was detected in the thiospinel. A Co/Ni ratio of >100 distinguishes the vein pyrite from pyrite in the basaltic wall rock and from pyrite formed as an alteration product of olivine (Co/Ni <6). The Co/Ni ratios correlate positively with Cu and negatively with As. Co-rich, nonstoichiometric Cu-Fe-S sulfides in chalcopyrite are interpreted as metastable phases which have been quenched at a high temperature and prohibited from exsolution of the stable products chalcopyrite and pyrite. Fluid inclusions in quartz from the quartz-sulfide veins are two-phase and vary from liquid- to vapor-dominated. Their salinities range from 4.2 to 7.2 wt % equivalent NaCl and average 5.5 %. Pressure (360 bars) corrected average filling temperatures vary from 271¿ to 408 ¿C with a maximum of 486 ¿C.

This is consistent with calculated quartz formation temperatures for a single quartz separate (+4.2%0 Δ18O) using oxygen isotope thermometry. The Δ18O value of the hydrothermal fluid was determined to be +1.7%. The temperature data indicate fluid alteration of the sheeted dikes at about 350¿ to 500 ¿C. The maximum homogenization temperatures intersect the liquid/vapor two-phase boundary above the critical point of seawater. Thus phase separation could have occurred before or during the formation of the mineralized veins and the alteration of the sheeted dike sequence. ¿ American Geophysical Union 1990