The formation of volcanic massive sulfide deposits, including Kuroko deposits and seafloor hydrothermal deposits, is caused by the mixing of circulating hydrothermal fluids originating from magmatic heat with seawater that has been introduced under the seafloor. The placement of intrusive rocks contributed by magma creates high-temperature reacting semiconformable alteration zones in the associated surrounding rocks. The directionality of the volcanic sequences and faults that resulted in the intrusive activity was structurally dominated by the main stress stretching system in the seafloor geosynclines or grabens. As a reaction between hydrothermal water and rock, circulating hydrothermal water altered in the rock below the seafloor surface, forming footwall alteration zones, which developed vertically in the higher permeable host rock. Ore precipitates composed of pyrite, chalcopyrite, sphalerite, galena, barite, etc. are deposited at and below the seafloor surface. Comparison of seafloor mounds against network and veined stockwork zones in footwalls shows different temperature dependence of the solubility of chalcopyrite, sphalerite and galena. The vertical zoning from the copper-rich bottom to the zinc-rich top is due to the solubility of chalcopyrite and sphalerite during the cooling process and rapid sulfur saturation by bacterial activity. Distal products, which are plumes from the seafloor around volcanogenic massive sulfide deposits, have a protective function by covering the deposit and are also important in ore mineralogy because of their chemical characteristics (Eu, Tl, Mn, P, Ag, etc.) that indicate the halo of high-temperature hydrothermal plumes. Within the Wilson cycle, volcanic massive sulfide deposit formation is favored in environments of continental splitting (rifting) of oceanic and continental plates, oceanic expansion and subduction orogenic movements.
In some exploration cases, outcrops of Kuroko-bearing conglomerate volcanic clastic rocks were interpreted as distal products and aimed at finding the main body of volcanic massive sulfide deposits. In another case, observations of the mineralization of several hanging-wall alteration zones were used to identify the activities of the original volcanic massive sulfide deposit during its ore-forming phase. In the third case study, the precipitation environment of copper and zinc is discussed as a difference between mineralization on the volcanic front and on the back arc side in seafloor hydrothermal deposits in the Okinawa Trough area. As a final case, the mining districts of volcanic massive sulfide deposits globally are well known, however, like the South African case, it is expected that rifting, oceanic expansion and subduction stages exist outside the mining districts of volcanic massive sulfide deposits, where the formation system of mineralization is expected to arise.
