The geochemistry and geomicrobiology of relict hydrothermal sulphide deposits

Abstract

The diagenetic re-mineralisation of seafloor-sulphide deposits and the role of microbes in the metal-exchange processes were investigated in metalliferous sediments from the Alvin relict hydrothermal zone in the TAG area at 26º8'N (Mid-Atlantic Ridge). The solid-phase and concomitant pore water concentrations of AI, Si,Ca, C_tot, C_org, S, Fe, Mn, Cu, Zn, P, V, Co, U, Mo, Au, Ag and REEs were measured in a 230 cm long gravity core from the southern periphery of the relict vent field. These measurements were complemented by detailed analysis of bacterial abundance and specific activity. The altered sulphidic sediments are capped with a ~30cm thick layer of carbonate-rich (~60% CaCO₃), Fe-stained sediments. Two distinct sulphide layers, interbedded with Fe-oxysilicates, and overlain by a thin layerof Fe/Mn oxyhydroxides, were found in this core. The dominant mineral-phase in both sulphide layers, which originate from mass-wasting of mound sediments, is pyrite with some goethite. Reaction of the exposed metal-sulphides in the upper sulphide layer with seawater has produced a thin layer of secondary atacamite, which is enriched in Au. Primary sphalerite is dissolved in the upper sulphide layer and re-precipitation as secondary sphalerite directly above and below. U continues to be scavenged from the porewater, producing marked enrichments on oxidised sulphide rims. The re-mineralisation processes identified in this core are in close analogy to the large-scale zone-refining that has been described for the active TAG mound and ancient ore-deposits.
REE/Fe ratios clearly distinguish between plume derived sediments in the carbonate cap and slumped material from the hydrothermal mound. The REE signature of bulk sediments and clay phases imply multiple stages of alteration by diffuse fluids in the upper sulphide layer and intermediate layer, whereas the lower sulphide layer is not affected. Alteration by reactive low-temperature hydrothermal fluids is also inferred to be responsible for the observed diagenetic overprinting of trace-metal distributions in the upper sulphide layer. The intermediate layer is rich in nontronite, which has been precipitated in situ from diffuse fluids.
The presence of Mn- and Fe-reducing bacteria coincide with elevated pore water concentrations of Mn and Fe, indicating direct involvement of bacteria in the cycling of these metals. Total counts of viable cells and general activity measurements show that although bacterial populations are relatively small, they are healthy and well adapted to this potentially toxic environment. The existence of active microbial communities in metalliferous sediments may therefore provide a continuum of bacterial populations between high and low temperature hydrothermal systems, thus representing an important transitional stage in the hydrothermal ecosystem. Microbial reduction and oxidation of S was observed throughout the core, indicating that microorganisms are particularly active in terms of S-cycling. For deep-sea sediments extremely high sulphate reduction rates (67 nmol/cm³/d) were measured in the iron-stained carbonate cap. In the absence of significant organic carbon (~0.2 %) this strongly suggests the synthesis of alternative electron-donors by chemolithotrophic bacteria to support the observed high rates of heterothrophic activity in these sediments.

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