Anomalously high concentrations of metals in fluid of the Semenov modern hydrothermal system (Mid-Atlantic Ridge, 13°31′ N): LA-ICP-MS study of fluid inclusions in minerals

Abstract

How and how much do hydrothermal fluids trans� port metals from the area of their involvement in the oreforming process to the zone of their precipitation? What should the composition of fluids, which may transport metals in the proportions necessary for the formation of mineral deposits, be? These problems have been studied by more than one generation of researchers in the field of ore deposits. An important role in the solution of this problem was played by the experimental and thermodynamic studies of mineral solubility in aqueous fluids at high temperatures. In spite of significant achievements in these areas, we cannot assume that the obtained results reflect the chemical composition of the natural oreforming fluid adequately, since modeling of metal behavior in it is limited by our knowledge of the thermodynamic con� stants of minerals and components of fluid. Significant achievements in improving our knowledge on the chemistry of mineralforming fluids were gained dur� ing the study of modern hydrothermal systems and fluid inclusions in minerals of hydrothermal deposits (1). A huge volume of data was obtained after the dis� covery of modern hydrothermal sources precipitating sulfide ores on the floor of the World Ocean (21° N) on the East Pacific Rise in 1978. During the past 35 years, more than 300 hydrothermal sites have been discov� ered and studied by submersibles. The major data on the chemistry of fluid and concentrations of metals in it were mostly based on the results of the analyses of its samples collected on the hydrothermal fields by sub� mersibles. However, study of fluid inclusions in miner� als from modern sulfide edifices has shown that the conditions of deposition, the evolution of salinity, and the temperature of fluids discharged on the sea floor were much more variable than followed from direct measurements (2-4). Development of new highpre� cision apparatus and analytical methods has provided completely different possibilities for the study of a wide spectrum of chemical elements in fluids trapped by fluid inclusions during mineral crystallization. Study of fluid inclusions in minerals from a number of ore deposits and analysis of the concentrations of met� als in fluids precipitating ores have been performed (5-7). Using these methods, it is very interesting to analyze the fluid captured by minerals during the for� mation of modern sulfide ores. For this purpose, we carried out investigations of fluid inclusions in miner� als from the Semenov modern ore cluster.