Trace metal distribution in the Atlantis II Deep (Red Sea) sediments

• Re-examination of sediment layers in the Deep highlights key pathways of metal deposition.
• Abundant non-ferrous metals are deposited with poorly crystalline Si–Fe-(oxy)hydroxides.
• Diagenetic effects play a major role in the distribution of trace metals basin-wide.
• As, Au, and Mo are adsorbed from pore water by diagenetic pyrite.
• There is an unexpected inverse Cu/Zn zonation with respect to the location of the vents.

The Atlantis II Deep is one of the only locations on the modern seafloor where active formation of a brine pool-type stratiform ore deposit can be studied. The presence of the brine pool causes retention of the hydrothermally released metals within the brine covered area, resulting in the accumulation of 90 Mt of low-grade metalliferous sediment (2.06% Zn, 0.46% Cu, 41 g/t Ag, and 0.5 g/t Au; Guney et al., 1988). Almost all metals are derived from hydrothermal input, but some are also derived from seawater (e.g., Mo), pelagic phytoplankton (Ni) and detrital input (Cr). The hydrothermal fluid that is vented into the pool is rich in metals but relatively low in reduced sulfur compared to open ocean black smokers. Metals are deposited as sulfides from the cooling hydrothermal fluid but also by adsorption onto non-sulfidic “surface-active” particles (Si–Fe-OOH) in the brine pool. An unexpected increase in the Cu/Zn ratio of the sediments with distance from the vent source(s) may reflect pulses of higher-temperature venting and increased Cu fluxes to the brine pool, which are recorded as higher Cu/Zn ratios in the distal sediments or, alternatively, more efficient adsorption of Cu to Fe-OOH particles in the distal brine.During early diagenesis (a few thousand years) metals that are loosely bound to surface-active particles in the sediment apparently react with H2S to form sulfides. Proximal to the inferred vents, the ambient pore water is highly concentrated in trace metals such as Cd, Ag and Hg that are incorporated in diagenetic sulfides, including chalcopyrite and sphalerite. At greater distance from the vents, trace metals such as Mo, As, and Ga are taken up by framboidal pyrite. High concentrations of Au (up to 3 ppm) are found in both proximal and distal metalliferous sediments, indicating that both primary deposition with sulfides and adsorption by diagenetic pyrite are important depositional processes. Some of the inferred pathways for metal precipitation in the Atlantis II Deep sediments, especially adsorption onto surface-active particles and subsequent incorporation in sulfides during diagenesis, may have been important unrecognized processes for metal accumulation in ancient stratiform ore deposits thought to have formed in brine pools.

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