The Heavy-Metal Fingerprint of the Irinovskoe Hydrothermal Sulfide Field, 13°20′ N, Mid-Atlantic Ridge

Abstract

A number of Cd-, Pb-, Ag- and Sb(±As)-bearing minerals are found in the Zn-rich smoker chimneys of the Irinovskoe hydrothermal sulfide field, 13°20′ N, Mid-Atlantic Ridge. Sulfide samples were studied using optical microscopy, SEM/EDS, XRD, EBDS, ICP-MS and thermodynamic approaches. The chimneys consist of major sphalerite (including Cd-bearing type with up to 41.38 wt% Cd) and wurtzite, subordinate opal, pyrite, chalcopyrite and Fe-oxyhydroxides, as well as accessory native sulfur, baryte, secondary copper sulfides, galena, CdS phase (most likely hawleyite), pyrrhotite, isocubanite, acanthite, Ag–Cu–Sb(±As)-bearing minerals, native gold, anglesite, gypsum, smectites, naumannite and lollingite. The main source of metals for the formation of Zn-rich sulfides was mafic rocks, with a subordinate role from ultramafic rocks. Crystallization of most accessory minerals at low temperatures (<120 °C) under acidic/reducing conditions and low S activity could be initiated by a magmatic input, which is supported by a negative S isotopic composition of bulk sulfide samples. The finding of Cd-rich sphalerite and a CdS phase in low-temperature mineral assemblage significantly expands the temperature limits of their possible formation. The high Cd contents of easily soluble sphalerite and the presence of the CdS phase should be taken into account in possible future mining and processing of seafloor hydrothermal sulfide fields.