Classification of modern sulfide ore deposits at the ocean floor

Studies of tectonics, volcanism, and sedimentation in areas displaying hydr othermal ore-forming processes on the ocean floor has clarified the causes of the differences in function, abundance, composition, and properties of h ydrothermal systems and related hydrothermal ore deposits. Classifications of hydrothermal deposits consider these main factors. The frequency of occu rrence, duration of mineralization, and, correspondingly, the dimensions of the ore deposits in oceanic rifts depend on the spreading rate. Compositio n of the original hydrothermal solution produced by the interaction of seaw ater with crustal and upper mantle rocks is mostly controlled by the compos ition of the host rocks. In some geodynamic settings, the ascending origina l solution call become unstable and experience phase separation accompanied by fractionation of chemical elements and formation of subsurface hydrothe rmal deposits. Surficial hydrothermal deposits can be subdivided into three groups formed by the ore-bearing solutions on the basis of their compositi on and properties, depending on the environment of deposition: (1) deposits discharged to the ocean floor surface directly from the volcanic basement, (2) deposits that have migrated through the sedimentary sequence after dis charge from the basement, and (3) deposits that have been discharged into t he highly mineralized bottom brine.