A model calculation of quartz precipitation from hydrothermal solution by mixing is performed. This model considers that a high-salinity and high-temperature solution mixes with a low-salinity and low-temperature solution at the constant pressure (1000 bars or 500 bars) or vapor-liquid equilibrium pressures. The temperature of the high-salinity solution is assigned to be 600°C (at 1000 bars), 450°C (at 500 bars), or 300°C (at vapor-saturation). The low-salinity solution is 200°C at the three pressure conditions. The salinity of the high-temperature solutions is 20 or 10 NaCl wt%, and that of the low-temperature solution is 1 NaCl wt%. Both temperature and NaCl concentration of the mixed solution are lowered by stepwise increase in the weight fraction of the low-salinity and low-temperature solution.
Calculations indicate that the amounts of precipitated quartz show the bell-shaped patterns against the temperature of the mixed solution. The model calculation is then applied to the cooling-and dilution-path of the hydrothermal solution for the Fujigatani-Kiwada deposits. By converting the solution temperature into the homogenization temperature, the quartz precipitation curve is recast onto the relation between homogenization temperature (and salinity) and the precipitated amount. The calculated results show the bell-shaped patterns observed for the frequency diagrams of homogenization temperatures and salinities of fluid inclusions.

View full abstract

The Tengumori deposit of the Kamaishi mine, Northeastern Japan, is a skarn-type copper ore deposit that develops near granitic intrusive rocks of Cretaceous age. Hydrothermal activity at the Tengumori deposit is divisible into three stages: 1) calc-silicate (divisible into clinopyroxene and garnet phases), 2) ore mineralization (chalcopyrite+pyrrhotite+quartz), and 3) calcite. The δ¹⁸O (SMOW) values were determined to be: +6.6to +7.8‰ for clinopyroxene (calc-silicate stage, 7 samples), +4.2 to +7.0‰ for garnet (calc-silicate stage, 14 samples), +12.0 to +13.7‰ for quartz (ore mineralization stage, 13 samples), and +10.6 to +12.8‰ for calcite (calcite stage, 19 samples). The oxygen isotopic temperatures calculated from the coexisting (but not contemporaneous) mineral pairs give consistent values of -330-460°C. However, they are higher than both of the pressure-corrected homogenization temperatures of fluid inclusions (-180-330°C) and the uppermost temperature of stability of chalcopyrite and hexagonal pyrrhotite (325°C). This temperature discrepancy is the result of changing δ¹⁸O_H2O value from -+7 to +9‰ in the cal-silicate stage, -+2 to +7‰ in the ore mineralization stage, and +1 to +5‰ in the calcite stage. The δ¹⁸O values of the calc-silicate stage solution can be interpreted as the unexchanged magmatic water or water in equilibrium with the host rocks. The hydrothermal solution of the ore mineralization stage is either magmatic water partially reequilibrated with the cooling pluton, water in equilibrium with the country rocks, or mixed solution with externally-derived water, e.g., local meteoric water. The δ¹³C (PDB) values of calcite crystals (calcite stage) range from -5.44 to -0.37‰. The calculated δ¹³C_H2CO3 values of the hydrothermal solution range from -5.8 to +1.4‰ and suggest that carbon was derived from both the granitic magma and the host limestone. Contributions of magmatic fluids to the calcite stage solution were recognized from the high salinity values (up to 23 wt. % equivalent NaCl) and the δ¹³C values of the calcite crystals.

View full abstract

Electron microprobe analyses of sphalerites from sixteen Kieslager-type deposits, mainly at Besshi and its vicinity, Hitachi, and Shimokawa, revealed that the Mn/Zn and Co/Zn atomic ratios may have markedly increased during contact metamorphism, while the Cd/Zn ratios remained unchanged. Regional metamorphism may have increased the Mn/Zn ratios to a limited extent.
Except for Shimokawa, the Mn/Zn and Co/Zn ratios of sphalerite precipitated from solutions are found to be both mainly lower than 2×10^-3. At Shimokawa, the Mn/Zn and Co/Zn ratios are highly variable, and high Co/Zn ratios occur in sphalerite of Cu-rich ores. The Cd/Zn ratios are mostly within 2×10^-3 at Shimokawa and Hitachi. The ratios display a wide variation in the deposits at Besshi and its vicinity but are mainly constrained within 8×10^-3.
Based on the equilibrium fractionation model between hydrothermal solution and sphalerite, the ΣmCd/ΣmZn and ΣmMn/ΣmZn ratios in the initial solutions responsible for the mineralizations at Besshi and its vicinity are estimated to be 2-3×10^-3 and lower than 20-30 at 300-350°C, respectively; they are quite similar to the ratios of high-temperature solutions at EPR21°N. Low Σm_Cd/Σm_Zn and Σm_CoΣm_Zn ratios characterize the solutions for the deposits at Hitachi. At Shimokawa, the hydrothermal solution related with Cu-mineralization was high in the Σm_Co/Σm_Zn ratio and contributed to the formation of Co-rich sphalerite in the Cu-rich ores.

View full abstract

Many hydrothermal gold, silver, copper, and mercury deposits occur in the Kitami region, northeastern Hokkaido, related to Middle to Late Miocene back-arc volcanism of the Kuril arc. Most of these deposits are vein-type, with a small number of Kuroko and disseminated types. Veins in the region have a dominant ENE-WSW direction, and this predominance is independent of deposit type (epithermal, mesothermal), metal species (gold, copper, mercury), and host rocks (Cretaceous to Paleogene sedimentary rocks, Miocene sedimentary and volcanic rocks); this direction is also independent of local geological structures such as a caldera and graben.
A relation between age and predominant direction of veins in each deposit indicates three major trends: (1) a counterclockwise rotation from N50°W to S70°W, during the period from ca. 14Ma to 11 Ma, (2) a constant N70°E trend from 13 to 11 Ma, and (3) a trend between N30°E and N60°E from ca. 9 Ma to 5 Ma. The first trend corresponds to the estimated direction of movement of the Pacific plate, with 20°-30° counterclockwise offset, and is explained as the direction of right-lateral strike-slip shear related to the compressive stress that developed parallel to the subducting direction of the Pacific plate. The second trend, N70°E, is parallel to the right-lateral strike-slip faults of the Kamishiyubetsu tectonic zone; this transcurrent fault zone formed during 13-10 Ma along the volcanic front of the southwestern Kuril arc, due to oblique subduction of the Pacific plate. The third trend is related to secondary extensional fractures or a pull-apart vein system associated with a N30°E-striking right-lateral strike-slip fault movement, as deduced from an analysis of vein-hosting fractures in the Kitami region. This movement may be ascribed to conjugate shear faulting in a widening forearc sliver of the Kuril arc, formed to overcome a resistance at the leading edge of the sliver.
The formation of major hydrothermal deposits in the region was related mainly to fault movement along the Kamishiyubetsu tectonic zone, which shifted trenchward with time from ca. 13 Ma to 10 Ma, as obliquity of the subduction increased. Vein-type mineralization occurred where fault movement was associated with subsequent rhyolitic volcanism; volcanism also shifted southward, following the migration of faulting.

View full abstract

Reviewing approximately 200 original articles on submarine and sub- seafloor hydrothermal deposits, the present study provides a classification of these deposits from several points of criteria; including geological tectonics, mineralogy, and forms of ore bodies; and a worldwide distribution map based on this classification (Figs. 1 and 2).
The deposits were divided into three systems based on geological tectonics; mid-oceanic ridge system, arc-trench system, and hot spot system (Table 1), while the former two systems were sub-divided into the on-spreading and offspreading axis systems.
Leaving gangue materials out of consideration, the deposits were divided into the sulfide type, oxide type, and sulfideoxide mixed type (Table 3). The sulfide type was further divided into the Cu-Zn-Pb and As-Sb-Ag-Hg sub-types, and the oxide type into the Mn and Fe sub-types. The Cu-Zn- Pb sulfide type deposits, in most cases forming mixtures with other ore types, have been discovered in both the mid-oceanic ridge system and arc-trench system, in particular on the spreading axes of the both systems (Figs. 1 and 2). The distribution of Pb minerals (mainly galena) is not so wide as Cu and Zn minerals but is rather restricted in the deposits on spreading axes near continents. The As-Sb-Ag-Hg sulfide type deposits which generally form mixtures with other ore types have been discovered near continents, regardless of the system. The As-Sb-Ag-Hg sulfide end-member type is in close relation with shallow submarine hot spring activities near coasts.
The Mn oxide type deposits are widely distributed without showing clear relation to the geological setting. The Fe oxide type deposits widely occur as a rather minor constituent of mixtures with other types on the spreading axes of the midoceanic ridge system, and as an end-member in the volcanic front of the arc-trench system as well as in the hot spot system. It is probable that the volcanic exhalative Fe sediments are widely distributed along the recent plate boundaries, in particular along submarine volcanic belts in the arc-trench system of the earth.
The on-seafloor deposits appear in various forms of ore bodies, e.g., massive sulfides, chimneys, mounds, and metalliferous sediments, while the beneath-seafloor deposits appear in rather restricted forms, e.g., sulfide veins and dissemination (Table 4). The forms and ore types for the on-seafloor ones have lateral variation with distance from the vents; massive sulfides and/or sulfide chimneys on the vents, sulfide and/or oxide mounds around the vents, and Fe-Mn metalliferous sediments widespread on the seafloor. This lateral change in the form and ore type may be accounted for by plume diffusion and fractionation due to the difference of the metal solubilities and the bottom seawater currents (Fig. 3). Sulfide particles formed from discharging solutions accumulate on and near the vents because of their low solubilities forming massive ore bodies, chimneys or mounds. Fe and Mn hydroxide particles, on the other hand, hardly precipitate because of their long residence time in seawater. They are widely dispersed together with some sulfide particles by seawater flow, and accumulate on the seafloor to form metalliferous sediments.

View full abstract