A currently popular model for the formation of volcanogenic massive sulfide deposits, based largely on isotopic studies and substantiated by water-rock experiments, implies that the ores were formed near the seafloor from hydrothermal solutions of seawater origin. Some of the major questions unanswered from this model have been: (1) the reason for the development of large scale seawater circulation systems through volcanic rocks in particular areas and during specific stages of submarine volcanism (e.g., ore formation after the eruptions of large volumes of volcanic materials), (2) the reason for the association with rapid-subsidence structures, (3) the mechanism for high temperature mineralization on or near the seafloor, and (4) the reason for metal and mineral zonings. Such questions can, however, be answered satisfactorily by a hypothesis that the hydrothermal processes which produced volcanogenic massive sulfide deposits occurred as a result of the creation of submarine calderas.
Geologic and petrochemical data are presented to suggest that the middle Miocene "Hokuroku Basin" in northern Japan, in which the largest concentration of the Kuroko ores occur, was a submarine resurgent caldera, and that the initiation and the termination of the mineralization were controlled, by the subsidence, and then by the resurgency of the caldera. Submarine calderas also appear to have played a major role in the formation of massive sulfide deposits in the other areas, such as the Bathurst district in Canada, the Rosebery-Mt. Lyell district in Tasmania, and in Cyprus.

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Ring dikes, normal faults, central uplift, and slumping structure are considered to be the key factors of the submarine resurgent cauldron. In the east Odate area, there occurs a ring distribution of post-Kuroko dacites and epithermal ore deposits around the central Otaki quartz diorite. Vein-type and Kuroko deposits are distributed almost along the half of the ring. There also occur submarine slumping structures. The estimated subsurface structure indicates the central uplifting and marginal depression in the inside of ring dacites. Those phenomena are interpreted as the fossil submarine resurgent cauldron in the Miocene age with the intermediate scale of about 8 kilometers in horizontal diameter, associated with the genesis of Kuroko deposits and epithermal veins. A structural model of the relationship between the processes of a cauldron formation and ore genesis is proposed.

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A preliminary interpretation on the depth of seawater during the Kuroko mineralization was made by examining the foraminiferal assemblages in the Middle Miocene sedimentary rocks in the Hokuroku district. In drill core samples of the M3 mudstone, M2 mudstone, and T2 tuff units from the Matsuki mine and from several other areas in the district, two fairly distinct benthic assemblages are recognized: a dominantly calcareous Melonis-Cibicidoides-Pullenia assemblage and a dominantly arenaceous Martinottiella-Spirosigmoilinella-Sigmoilopsis assemblage. The available evidence, such as the data from the Deep Sea Drilling Project, suggests that the species of the Melonis assemblage lived at lower bathyal depths (2, 500-4, 000 m) and above the carbonate com-pesation depth, while the species of the Martinottiella assemblage lived at abyssal depths near to or below the carbonate compensation depth (about 4, 000 m).
At least two cycles of alternation of the Melonis and Martinottiella assemblages were observed in the Matsuki cores. Such alternations may have been caused by fluctuations in the depth of seafloor, in the carbonate compensation depth, or in combination of both. The working hypothesis is advanced that the alternations of the foraminiferal assemblages were due to fluctuations in the depth of seafloor. The inferred paleobathymetric curve for the Hokuroku "basin" shows that the depth of seawater ranged from 3, 500 m to in excess of 4, 000 m during the m₃, M ₂, and T ₂ phases of sedimentation and during the Kuroko mineralization.

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Simple calculations are presented to illustrate the kinds of constraints hydrodynamic considerations can place on genetic hypotheses. It is tentatively shown that the rhyolite plugs commonly associated with Kuroko deposits are too small to be the cause of Kuroko mineralization. A larger intrusive heat (or magmatic fluid) source at depth is needed. Convective velocities in rocks of reasonable permeability are so much smaller than minimum ocean bottom current velocities that it is very unlikely Kuroko mineralization could have been precip-itated from solutions above the sea-sediment (or rock) interface. Hydrothermal solutions would be swept away as soon as they left the protective sediment or rock cover. Finally hydrothermal convection may be expected to promote ore slumping if the permeability of the precipitated ore is low. The "slumped" nature of some Kuroko ores may thus find a natural genetic explanation.

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The Kuroko-type deposits in the Hanaoka-Kosaka district are divided into three sub-types based on the ratio of major base metals in a single unit deposit. The ratio of copper to lead and zinc increases in order of the "B", "C" and "Y" sub-types. There is the north-southerly lateral zoning of these sub-type deposits in the Hanaoka and Kosaka districts.
δD values are distinctly high in the "B" sub-type deposits.δ34S values are distinctly high in the "C" sub-type deposits and iron contents of sphalerite are low in the "Y" sub-type deposits. The higher δD values of "B" sub-type deposits occurring in the central part of basins may indicate sea-water as the predominant source for ore fluids. Using iron contents of sphalerite and δ³⁴S values of pyrite the possible depositional environments of three sub-type deposits were estimated on fO ₂-pH diagram.
In conclusion, each ascending ore fluid responsible for different sub-type deposits was different in geochemical nature. This suggests that the chemical and physical changes of ore fluid circulating in a hydrothermal system are important for the cause of differences among three sub-types of the Kuroko-type deposits.

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A brief review of lead isotope studies in Japan is made with particular reference to "kuroko" ore deposits. The lead isotopic evidence favors a continental crustal derivation for the lead in these deposits with possibly a component from subducted pelagic sediments. Supporting information is presented from the nature of the "green tuff" belt (abundant dacite and sparse basalt) and lead-rich nature of the ore. The question remains, however, whether the metals are derived from the immediate volcanics or the underlying volcanics or basement. A plan is briefly presented of lead isotope investigations to determine how important the "green tuff" is as a source of ore relative to the underlying volcanics or basement as accessed by deep penetration of ocean-water fluids along transverse fault and fracture zones.

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The isotopic composition of strontium in sulphate minerals from the Fukasawa and Kosaka mines has been measured to evaluate the importance of seawater in the development of Kuroko deposits. The ⁸⁷Sr/⁸⁶Srr ratios in. samples of bedded gypsum and anhydrite fall in a narrow range (0.70827-0.70854) whose upper limit approaches that of Miocene seawater. The ⁸⁷Sr/⁸⁶Srr ratios of barites fall in a somewhat wider range (0.70677-0.70820) between the upper limit of the ⁸⁷Sr/⁸⁶Srr ratio of the Miocene volcanics and the lower limit of the ⁸⁷Sr/⁸⁶Srr ratio in the anhydrite and gypsum samples.
The ⁸⁷Sr/⁸⁶Srr data are most readily explained if the ore forming fluid was normal Miocene seawater which acquired isotopically lighter strontium from the underlying Miocene volcanics prior to precipitation of the sulphates. None of the ⁸⁷Sr/⁸⁶Srr ratios are higher than that of Miocene seawater suggesting that the contribution of isotopically heavy strontium from the Paleozoic basement rocks was minor. Mixing of the hydrothermal solution with seawater during deposition of the sulphate minerals appears to have been insignificant.

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Observations of doubly-polished, uncovered thin sections of Kuroko from Furutobe reveal a paragenesis of great complexity. Sphalerite crystals up to 2 mm across show finely detailed growth banding, and the crystals occur both as broken fragments in "moved" ore and as later fillings between clasts. The "moved" ore exhibits a variety of sphalerite types (including coarse crystals as well as fine-grained aggregates) as clasts showing that coarse crystals did exist prior to the synsedimentary slumping. Growth of large crystals seems incompatible with rapid deposition on the sea floor, therefore an alternative model is suggested. The initial deposits were derived from submarine hot springs and consisted of very fine-grained sulfides with silica and barite. With con-tinued supply of hot fluid the lower part of the sulfide deposit covering the vent became heated, and solution and reprecipitation occurred below an upper, less permeable "blanket." Deposits formed on slopes slumped repeatedly yielding graded beds with coarse sphalerite and other sulfides incorporated in a melange of finer ore and gangue fragments.
Much of the sphalerite exhibits a fine "dusting" of chalcopyrite that appears to have developed subsequent to the growth of the host sphalerite, probably by a process of replacement, not of exsolution.

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