Abstract

Deep Sea Drilling Project (DSDP) hole 504B is the only hole drilled in the DSDP program that intersected both pillow and dike basaltic units and greenschist-facies alteration zones. Pillowed basaltic units constitute the upper portion of the basaltic pile. Lower in the section, a pillow–dike unit is transitional to a sheeted dike unit that constitutes the lowest unit of the hole. Hydrothermal alteration of the basaltic units consists of, from top to bottom, a low-temperature oxidized alteration zone; a low-temperature reduced alteration zone; a transition–stockwork zone; and a greenschist-facies alteration zone.Samples of the altered basaltic units were analyzed for gold content and δ18O. The initial, primary gold content of basalts of the core is believed to have been relatively uniform and between 0.5 and 1.0 ppb. Average results for the various alteration zones are 0.44 ± 0.07 for the low-temperature oxidized zone; 0.40 ± 0.08 for the low-temperature reduced zone; 0.66 ± 0.21 for the transition–stockwork zone; and 0.19 ± 0.03 for the greenschist-facies zone. The results indicate that gold has been locally mobilized in the low-temperature alteration zones, but no significant net loss or addition of gold has occurred. In the transition–stockwork zone, some local enrichment of gold has occurred, but overall the zone does not appear to have been substantially enriched in gold. In the greenschist-facies alteration zone, gold has been pervasively and uniformly depleted, resulting in the low average values for this zone and a small range of values.Gold leached during the greenschist-grade hydrothermal alteration event may contribute to the high gold contents recorded from some massive sulfide deposits associated with active sea-floor hydrothermal vents or may be incorporated into deep-sea Fe-rich cherts.

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