Fluid inclusion

Fluid inclusions from Kuroko deposits were studied first by Tokunaga and Honma (1974) who showed that Kuroko deposits formed in a range of 200-260°C for the siliceous 

Marutani and Takenouchi (1978) clarified the variations in homogenization temperature and salinity of inclusion fluids in quartz from stockwork siliceous orebodies at the Kosaka mine (Fig. 1.35 Urabe, 1978). They showed that the temperature decreases stratigraphically upwards from stockwork ore zone (280-320°C) to bedded ore zone (260-310°C). Pisutha-Arnond and Ohmoto (1983) carried out fluid inclusion studies of the stockwork siliceous ores from five Kuroko deposits (Kosaka, Fukazawa, Furutobe, Shakanai, and Matsumine) and revealed that black ore minerals (sphalerite, galena, barite) and yellow ore minerals (chalcopyrite, quartz) formed at 200-330°C and 330 50°C, respectively, and salinities of the ore fluids remained fairly constant at about 3.5-6 equivalent wt% NaCl. They analyzed fluids extracted from sulfides and quartz Na = 0.60 0.16 (mol/kg H2O), K = 0.08 0.05, Ca = 0.06 0.05, Mg = 0.013 0.008, Cl = 0.82 0.32, C (as CO2) = 0.20 0.15 and less than 6 ppm each for Cu, Pb, Zn and Fe. 

Two hypotheses of seafloor depth at the time of mineralization have been proposed based on foraminiferal data, ca. 3500 m (Guber and Ohmoto, 1978 Guber and Merrill, 1983) and 1500 m (Kitazato, 1979). Considering seafloor depth of present-day ore formation at back-arc basins and fluid inclusion data mentioned above, shallow seafloor depth hypothesis (Kitazato, 1979) seems more likely. If the pressm e-temperature condition of Kuroko ore fluids was close to the boiling curve, the depth could be estimated to be 1,000-1,500 m, which is similar to that for present-day back-arc mineralization such as Okinawa Trough. 

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The quantitative imaging capability of the NMP is one of the major strengtiis of the teclmique. The advanced state of the databases available for PIXE [21, 22 and 23] allows also for the analysis of layered samples as, for example, in studying non-destmctively the elemental composition of fluid inclusions in geological samples. 

Fig. 1.15. Diagram showing the homogenization temperature of fluid inclusions vs. the iron content of the host sphalerite growth zone for sample locality NJP-X on the OH vein. The line shows the predicted iron content of the sphalerite if the sulfur fugacity of the system had been buffered by the triple point — Fe-chlorite (daphnite), pyrite, hematite (Hayba et al., 1985).

A large number of geochemical studies on Kuroko deposits (fluid inclusions, gas fugacities, chemical and isotopic compositions of ore fluids etc.) have been carried out. These are summarized below. 

Carbon dioxide fugacity fcOi- CO2 fugacity (/coa) of ore fluids is estimated based on CO2 concentration of fluid inclusions analyzed. By using equilibrium constant of the reaction, C02(g) + H2O = H2CO3, and assuming uh20 to be unity, /CO2 can be estimated. 

Since temperature of formation of carbonates can be estimated from homogenization temperature of fluid inclusions in carbonates, we can place a limit of CO2 from the above equilibrium relationships. The estimated CO2 range is 1-0.01 mol/kgH20. 

ES in ore fluids is generally in a range of 10 -10 mol/kg H2O based on ES in present-day geothermal waters and fluid inclusion analytical data (Shikazono, 1972a). /ci- is represented as a function of ionic strength and temperature. Ionic strength is related to salinity which can be approximated as Cl concentration. Cl concentration can be estimated from fluid inclusion study. 

Because of uncertainties of equilibrium constants, ES, pH, temperature, /02 and other parameters (activity coefficient, ionic strength, activity of water, pressure), the estimated values of concentrations may have uncertainties of 1 in logarithmic unit. However, it can be concluded from the thermochemical calculations and fluid inclusion data that the Kuroko ore fluids have the following chemical features. 

D and 8 0. 5D and of the Kuroko ore fluids were estimated based on analyses of fluid inclusions, Kuroko-forming minerals and hydrothermal alteration minerals (e.g., Pisutha-Arnond and Ohmoto, 1983). Estimated SD and of Kuroko ore fluids are plotted on 5D-5 0 diagram (Fig. 1.40). 

Some mechanisms of anhydrite deposition in Kuroko deposits. Shikazono et al. (1983) considered the depositional mechanism of anhydrite based on the mode of occurrence, texture, Sr content, nature of the contained fluid inclusions and isotopic composition of Sr, S and O in anhydrite together with the mineralogy of the sekko ore, combined with their experimental study on the patitioning of Sr between coexisting anhydrite and aqueous solution. The following is their discussion on the depositional mechanism of anhydrite. 

If sulfur isotopic equilibrium between coexisting sulfates and sulfides was attained, using average values of sulfates and sulfides, -i-22%c and +5%c, respectively, we could estimate temperature using the equation by Ohmoto and Rye (1979). This temperature seems too high compared with temperature estimated from fluid inclusions and mineral assemblages (section 1.3.3). That means that sulfates and sulfides precipitated under the condition far from equilibrium. 

These predictions are generally in agreement with the observations homogenization temperatures of fluid inclusions in quartz from siliceous ore zone and in barite from black ore zone in the Kuroko deposits is relatively high, ranging from 350 to 250°C, and low, ranging from 250 to 150°C, respectively. 

Origin of ore fluids is constrained by (1) chemical compositions of ore fluids estimated by thermochemical calculations (section 1.3.2) and by fluid inclusion analyses, (2) isotopic compositions of ore fluids estimated by the analyses of minerals and fluid inclusions (section 1.3.3), (3) seawater-rock interaction experiments, (4) computer calculations on the seawater-rock interaction, and (5) comparison of chemical features of Kuroko ore fluids with those of present-day hydrothermal solutions venting from seafloor (section 2.3). 

Isotopic compositions of minerals and fluid inclusions can be used to estimate those of Kuroko ore fluids. Estimated isotopic compositions of Kuroko ore fluids are given in Table 1.10. All these data indicate that the isotopic compositions lie between seawater value and igneous value. For instance, Sr/ Sr of ore fluids responsible for barite and anhydrite precipitations is 0.7069-0.7087, and 0.7082-0.7087, respectively which are between present-day. seawater value (0.7091) and igneous value (0.704-0.705). From these data, Shikazono et al. (1983), Farrell and Holland (1983) and Kusakabe and Chiba (1983) thought that barite and anhydrite precipitated by the mixing of hydrothermal solution with low Sr/ Sr and seawater with high Sr/ Sr. 

In summarizing the fluid inclusion studies and stability of zeolite minerals, the most likely temperature range of zone (1), (2) and (3) is estimated to be 250-280°C, 150-230°C and < 150°C, respectively. Boiling of fluids for zones (2) and (3) suggests that the depth for the zeolite zone is probably less than 500 m from the surface and the epidote zone is more than 500 m. 

The fluid inclusions can be divided into two types vapor- and liquid-rich fluid inclusions. The filling degree of fluid inclusions from some samples from the silicified and alunite zones is variable and homogenization temperatures vary widely. This indicates... 

Based on the hydrothermal alteration mineral assemblages and the fluid inclusion, the probable range of gas fugacities (/s2, /o2 /H2S) and temperature can be seen in Figs. 1.81 and 1.82 these estimated fugaeities are quite different from those of the propylitic alteration. 

Numerous geochemical data (fluid inclusions, stable isotopes, minor elements) on the epithermal vein-type deposits in Japan are available and these data can be used to constrain geochemical environment of ore deposition (gas fugacity, temperature, chemical compositions of ore fluids, etc.) and origin of ore deposits. 

Figure 1.87. Summary of filling temperatures of fluid inclusions from Neogene vein-type deposits in Japan. Solid circle represents average filling temperatures of fluid inclusions for individual deposits (Shikazono, 1985b).

Filling temperature and NaCl eq. concentration of fluid inclusions from epithermal gold-silver and base-metal vein-type deposits (Shikazono and Shimizu, 1992)... 

Sulfur fugacity (/s ) As will be mentioned in section 2.4.3, fs2 can be estimated based on the Ag content of electram coexisting with argentite (or acanthite), the FeS content of sphalerite coexisting with pyrite and temperature estimated from homogenization temperatures of fluid inclusions. 

Carbon dioxide fugacity (fc02h The /CO2 values can be estimated from (1) gangue mineral assemblages including carbonates and (2) fluid inclusion analyses. 

Figure 1.99. Estimated /CO2-temperature ranges from anaytical data on fluid inclusions and homogenization temperatures (Shikazono, 1986). T Taishu (Pb, Zn), O Ohizumi (Cu, Pb, Zn), Y Yatani (Pb, Zn), Os Osaiizawa (Cu, Pb, Zn), H Hosokura (Pb, Zn), C Chitose (Au, Ag), S Seigoshi (Au, Ag).

Figure 1.100. Typical /coj-temperature ranges for Au-Ag-rich, Pb-Zn-Mn-rich, and Cu-Pb-Zn-rich vein-type deposits estimated from gangue mineral assemblages, homogenization temperatures of fluid inclusions, and thermochemical calculations (Shikazono, 1985b).

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