230Th/U Dating of Seafloor Massive Sulfides from the Semenov-5 Hydrothermal Field, Mid-Atlantic Ridge

Abstract Dating of the modern seafloor massive sulfides (SMS) is a complex task largely due to the multistage deposition of ore-forming minerals and the influence of changing physicochemical conditions on the isotope geochemistry during the interaction of hydrothermal fluids with host rocks and near-bottom seawater. Based on data obtained for the sulfide ores of the Semenov-5 field at the Mid-Atlantic Ridge (MAR), the possibilities and limitations of the 230 Th/U method are demonstrated, particularly concerning the formation of the radiometric system in environment above and below the paleo-seafloor level. New ages obtained for surface ores range from ~60 to ~8 thousand years. However, dating of subsurface sulfides was impossible due to the minimal input of near-bottom seawater within mound and its influence on the deposition of the sulfides. The study indicates that massive sulfides formed beneath the seafloor are of metasomatic origin, suggesting that the sulfide mineralization is not limited only by paleo-seafloor but also extends to the greater depths. This finding has significant implications for other sulfide deposits within the MAR as well and may lead to the revision of their resource potential. The established long-term character of hydrothermal discharge within the Semenov-5 field is comparable with longevity of other hydrothermal fields within the Semenov ore cluster. Obtained data indicate a long and complex evolution of hydrothermal systems related to the ultramafic rocks of the ocean core complexes in slow-spreading mid-ocean ridges.