SR, ND AND PB ISOTOPE CONSTRAINTS ON THE FORMATION OF THE METALLIFEROUS SEDIMENTS IN THE NEREUS DEEP, RED-SEA

Previous chemical analyses of sediments from Nereus Deep core 412, loc ated in the northern part of the Red Sea at about 23-degrees-20'N, hav e demonstrated the occurrence of V-rich authigenic magnetites associat ed with Fe, Zn and Cu sulphides mostly located at the bottom of the co re. The evolutionary stage of Nereus Deep is intermediate between that of the immature Shaban Deep and the more evolved Atlantis II Deep. Pr evious studies have proposed either MORB basalts (by analogy with East Pacific Rise sulphide deposits) or sediments as possible sources of t he metals in the Red Sea metalliferous sediments. Sr, Nd and Pb isotop e systematics, observed at ten levels in Nereus core 412, lead us to d istinguish three types of sediments, (1) with predominantly basaltic f ragments associated with loose titanomagnetite crystals (0.70413 less- than-or-equal-to Sr-87/Sr-86 less-than-or-equal-to 0,70622; 0.51271 le ss-than-or-equal-to Nd-143/Nd-144 less-than-or-equal-to 0.51300; 18.61 8 less-than-or-equal-to Pb-206/Pb-204 less-than-or-equal-to 18.673), ( 2) detrital or biogenic sediments with a continental isotopic imprint (0.70938 less-than-or-equal-to Sr-87/Sr-86 less-than-or-equal-to 0.713 23; 0.51242 less-than-or-equal-to Nd-143/Nd-144 less-than-or-equal-to 0.51262; 18.848 less-than-or-equal-to Pb-206/Pb-204 less-than-or-equal -to 18.890), and (3) V-rich metalliferous sediments (0.70872 Sr-87/Sr- 86 less-than-or-equal-to 0.70895; 0.51260 less-than-or-equal-to Nd-143 /Nd-144 less-than-or-equal-to 0.51274; 18.717 less-than-or-equal-to Pb -206/Pb-204 less-than-or-equal-to 18.851). On Sr-Nd and Pb-Pb isotope diagrams this last group of sediments defines a field located between the fields of the first two groups. Our data suggest that the Sr, Nd a nd Pb in the studied metalliferous sediments were derived from overlyi ng sedimentary beds in which the two main constituent detrital compone nts show very distinct origins (Red Sea MORB and continental Arabo-Nub ian Shield). These data clearly support a model of convective circulat ion within the Nereus Deep sedimentary pile. This model involves leach ing two main components from the upper sedimentary layers of the core and a downward circulation of a hot fluid promoting the growth of V-ri ch magnetites and Fe, Zn and Cu sulphides when the fluid cooled by mix ing with the pore water of sediments from the lower part of the core. A driving force, represented by magmatic activity located near the 412 core in the Nereus Deep, is required to explain the presence of hot c onvective fluids. These data also suggest that, for sedimented ridges, the isotopic imprint of a basaltic source in metalliferous sediments or a-hydrothermal ore deposit may result by interaction of a hot fluid with detrital sediments that are enriched in basaltic clasts and loos e magmatic crystals.