Major-oxides and trace-elements at DSDP Hole 78-543A

Four chemically distinct basalts were cored in 44 m of basement penetration at Deep Sea Drilling Project Site 543, in Upper Cretaceous crust just seaward of the deformation front of the Barbados Ridge and north of the Tiburon Rise. All four types are moderately fractionated abyssal tholeiites. The four types have different magnetic inclinations, all of reversed polarity, suggesting eruption at different times which recorded secular variation of the earth's magnetic field. Extensive replacement of Plagioclase by K-feldspar has occurred at the top of the basalts, giving analyses with K2O contents up to 5 %. The earliest stages of alteration were dominantly oxidative, resulting in fractures lined with celadonite and dioctahedral smectite, and pervasive replacement of olivine and most intersertal glass with iron hydroxides and green clay minerals. Latef, non-oxidative alteration resulted in formation of olive-green clays and pyrite veins in a portion of the rocks. Basalts affected by this alteration actually lost K2O (to abundances lower than in adjacent fresh basalt glasses), and gained MgO (to abundances higher than in the glasses). Finally, fractures and interpillow voids were lined with calcite, sealing in much fresh glass. Oxygen-isotope measurements on the calcite indicate that this occurred at 12 to 25C. Either altering fluids were warm or the basalts had become buried with a considerable thickness of sediments, such that temperatures increased until a conductive thermal gradient was established, when the veining occurred.

Chemical and isotopic compositions of rocks, ores, and hydrothermal minerals from the Rainbow and Logachev-2 hydrothermal fields

The aim of this paper is to analyze and compare mineralogy and geochemistry of copper-zinc sulfide ores from the Logachev-2 and Rainbow hydrothermal fields of the Mid-Atlantic Ridge (MAR) confined to serpentinite protrusions. It was found that Zn(Fe) and Cu, Fe(Zn) sulfides had been deposited in black smokers pipes almost simultaneously from intermittently flowing, nonequilibrium H2S-low solutions of different temperatures. Pb isotope composition confirmed that the deep oceanic crust had been a source of lead. The ores from the Rainbow field are 20-fold higher in Co than ores restricted to basalts and show a high ratio of Co/Ni=46. The ores from the Rainbow field are enriched in 34S isotope (aver. d34S=10 per mil) because of constant flow of cold sea water into the subsurface zone of the hydrothermal system. Ores from the Logachev-2 field are 8 times higher in gold compared to other MAR regions. Sulfide ores from the Rainbow and Logachev-2 fields have no analogues among MAR ore occurrences in terms of enrichment in valuable components (Zn, Cd, Co, and Au).