Description: Sulphide-rich scales precipitated in wells of the seawater-dominated Reykjanes geothermal system on the Mid-Atlantic Ridge (MAR), Iceland are directly comparable to mineralisation in active seafloor hydrothermal systems. Geochemical profiles from 2.7 km depth to low-temperature silica-rich surface discharge show consistent temperature-dependent enrichment and depletion trends under well-constrained conditions. Copper, Zn, Cd, Co, Te, Ni, Mo, Sn, Fe and S are enriched at higher pressures and temperatures in the deepest scales, and Zn, Bi, Cu, Pb, Ag, As, Sb, Ga, Hg, and Tl are enriched at lower temperature and pressures near surface. Cobalt, Se, Cd, Zn, Cu, and Au have bimodal distributions and are hosted by different mineralogical assemblages at higher- and lower-temperature and pressures. Boiling and destabilisation of metal-bearing aqueous complexes are the dominant controls on sub-seafloor deposition of most metals (particularly Au), although some (e.g., Cu and Se) may be transported in the vapour phase. At least three quarters of the total Reykjanes metal budget is precipitated downhole and indicates a significant proportion of metals will be precipitated in the sub-seafloor of other boiling systems in the oceans. Extreme enrichment of Au, Ag and Pb further highlights potential metal accumulation and enrichment in the deep geothermal reservoirs.

Description: Highlights • Predictable trace element enrichments and depletions in the Reykjanes system. • Boiling exerts a major influence on the enrichment of metals. • High concentrations of Au and Ag and Pb indicate accumulation in reservoir fluids. • Three quarters of the metal budget is deposited at depth or in the upflow zone. Abstract Mineral precipitation in the seawater-dominated Reykjanes geothermal system on the Mid-Atlantic Ridge, Iceland is caused by abrupt, artificially induced, pressure and temperature changes as deep high-temperature liquids are drawn from reservoir rocks up through the geothermal wells. Sulfide scales within these wells represent a complete profile of mineral precipitation through a seafloor hydrothermal system, from the deep reservoir to the low-temperature silica-rich surface discharge. Mineral scales have formed under a range of conditions from high pressures and temperatures at depth (〉2 km) to boiling conditions in the upflow zone and at the surface. Consistent trace element enrichments, similar to those in black smoker chimneys, are documented: Cu, Zn, Cd, Co, Te, V, Ni, Mo, W, Sn, Fe and S are enriched at higher pressures and temperatures in the deepest scales, Zn and Cu, Bi, Pb, Ag, As, Sb, Ga, Hg, Tl, U, and Th are enriched at lower temperatures and pressures nearer to the surface. A number of elements (e.g., Co, Se, Cd, Zn, Cu, and Au) are deposited in both high- and low-pressure scales, but are hosted by distinctly different minerals. Other trace elements, such as Pb, Ag, and Ga, are strongly partitioned into low-temperature minerals, such as galena (Pb, Ag) and clays (Ga). Boiling and destabilization of metal-bearing aqueous complexes are the dominant control on the deposition of most metals (particularly Au). Other metals (e.g., Cu and Se) may also have been transported in the vapor phase. Very large enrichments of Au, Ag and Pb in the scales (e.g., 948 ppm Au, 23,200 ppm Ag, and 18.8 wt.% Pb) versus average concentrations in black smoker chimneys likely reflect that some elements are preferentially deposited in boiling systems. A mass accumulation of 5.7 t/yr of massive sulfide was calculated for one high-temperature production well, equating to metal fluxes of 1.7 t/yr Zn, 0.3 t/yr Cu, 23 kg/yr Pb, 4.1 kg/yr Ag, and 0.5 kg/yr Au. At least three quarters of the major and trace element load is precipitated within the well before reaching the surface. We suggest that a similar proportion of metals may be deposited below the seafloor in submarine hydrothermal systems where significant boiling has occurred. Mass accumulation estimations over the lifetime of the Reykjanes system may indicate significant enrichment of Zn, Pb, Au, and Ag relative to both modern and ancient mafic-dominated seafloor massive sulfide deposits, and highlights the potential for metal enrichment and accumulation in the deep parts of geothermal systems.