Constraints on sub-seafloor hydrothermal mineralising processes and metal fluxes using deep sulphide-rich scales from the Reykjanes geothermal system, Iceland

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 TI 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.