Alteration minerals

Distinctions between glauconite and celadonite may be vague in the area of approximately 0.2% tetrahedral Al. Celadonite is found as an alteration mineral in mafic volcanic rocks (127). 

Kaolin minerals (kaolinite, dickite, nacrite), pyrophyllite and mica-rich mica/smec-tite mixed layer mineral occur as envelopes around barite-sulfide ore bodies in the footwall alteration zones of the Minamishiraoi and Inarizawa deposits, northern part of Japan (south Hokkaido) (Marumo, 1989). Marumo (1989) considered from the phase relation in Al203-Si02-H20 system that the hydrothermal alteration minerals in these deposits formed at relatively lower temperature and farther from the heat source than larger sulfide-sulfate deposits in the Hokuroku district. 

Figure 1.20. Zoning map of alteration minerals in unit D3 around the Fukazawa deposits (Date et al., 1983).

Figure 1.22. Summary of alteration minerals and zoning around the Fukazawa deposits (Date et al., 1983). Legend unbroken line = present or formed in considerable quantity dashed line = pre.sent in small amounts, or uncertain formation definition of zones by Date et al. (1983).

Shikazono et al. (1998) found that carbonates are common alteration minerals in the Uwamuki mine area of Hokuroku district and carbonate alteration superimposed on chlorite alteration. They showed that the mode of occurrences and the Mg/(Mg -(-... 

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). 

The REE data, combined with alteration minerals and concentration of major elements in hydrothermally altered rocks, could be used to reconstruct the structure and evolution of a submarine geothermal system accompanied by Kuroko mineralization (Shikazono, 1999a). 

Among the epithermal vein-type deposits in Japan, four major types of hydrothermal alteration ean be diseriminated. They are (1) propylitie alteration, (2) potassic alteration, (3) intermediate argillic alteration, and (4) advaneed argillic alteration. The definitions of these types of alteration are mainly based on Meyer and Hemley (1967) and Rose and Burt (1979) who elassified the hydrothermal alteration in terms of alteration mineral assemblages. 

Representative propylitie alteration minerals inelude epidote, albite, earbonates, quartz, chlorite, sericite, and smectite. The less common minerals are mixed-layer elay minerals such as chlorite/smectite and sericite/smectite and zeolite minerals. 

Zeolite minerals (wairakite, laumontite etc.), mixed-layer clay minerals and sme-cite occur in the upper part of the propylitically altered rocks (e.g., Seigoshi, Fuke, Kushikino), but they are sometimes poor in amounts. Generally carbonates are more abundant in the mine area as in the Toyoha district. Temporal relationship between the formation of high temperature propylitic alteration minerals (epidote, actinolite, prehnite) and low temperature propylitic alteration minerals) (wairakite, laumontite, chlorite/smectite, smectite) in these areas (Seigoshi, Fuke, Kushikino) is uncertain. 

The Seigoshi and Toi deposits occur in the andesitic pyroclastic rocks of the upper horizon of the Yugashima Group and basic intrusive rocks. Distributions of the wallrock alteration minerals from underground in the Seigoshi mine and on the surface near the... 

In zone (1), quartz, K-feldspar, epidote, chlorite, prehnite and sphene are predominant alteration minerals. Epidote, prehnite and carbonate replace plagioclase phenocryst. Epidote often occurs as a veinlet with several millimeters wide, together with prehnite. K-feldspar, calcite and quartz tend to occur as a veinlet. Chlorite replaces pyroxene... 

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. 

Although a wide range of alteration minerals has been recognized in epithermal systems considered here, few of their chemical compositions have been determined. 

There are different approaches to the study of hydrothermal alteration. For instance, Shikazono (1978a) showed the relationship between chemical composition of hydrothermal solution in equilibrium with the alteration minerals and Cl concentration in hydrothermal solution. 

Giggenbach (1984) calculated the effect of temperature on the chemical composition of fluids buffered by alteration minerals. The causes for the hydrothermal alteration considered below are mainly based on the works by Shikazono (1978a) and Giggenbach (1984). The effect of the extent of water-rock interaction is not taken into account. 

Figure 1.86 illustrates the variations in the chemical composition of chloride-rich hydrothermal solution in equilibrium with common alteration minerals with temperature. Figure 1.86 demonstrates that (1) the chemical compositions of hydrothermal solution... 

Oxidation of H2S (reaction (1-35)) occurs under the near-surface environment. Oxygen may be supplied from oxygenated groundwater. These oxidation reactions liberate H+ ion, leading to a decrease in pH. Under low pH conditions intermediate argillic alteration minerals (e.g., kaolinite, sericite) are stable. 

This type of mixing could reasonably explain the occurrence of acidic alteration minerals such as kaolinite and alunite in the low-sulfidation epithermal gold vein district (e.g., Seta in northeast Hokkaido, Hishikari in southern Kyushu) (Yajima et al., 1997)... 

Figure 1.131. Plan of alteration mineral zonation of the volcanic rocks in the Hishikari area (Izawa et al., 1990).

Oxygen isotopic equilibrium between mixed fluid and alteration minerals is attained. 

The dependence of concentration of K+, Na+, Ca + and H4Si04 in equilibrium with common alteration minerals (K-feldspar, Na-feldspar, quartz) on temperature is shown in Fig. 1.140 (Shikazono, 1988b). This figure demonstrates that (1) chemical compositions of hydrothermal solution depend on alteration minerals, temperature and chloride concentration, and K" " and HaSiOa concentrations increase and Ca + concentration decrease with increasing of temperature. In this case, it is considered that potassic alteration adjacent to the gold-quartz veins occurs when hydrothermal solution initially in... 

Figure 1.140. The dependence of concentration of K+, Na, Ca + and HaSiOa in equilibrium with common alteration minerals (K-feldspar, Na-feldspar, quartz) with temperature (Shikazono, 1988b). Thermochemical data used for the calculations are from Helgeson (1969). Calculation method is given in Shikazono (1978a). Chloride concentration in hydrothermal solution is assumed to be 1 mol/kg H2O. A-B Na+ concentration in solution in equilibrium with low albite and adularia. C-D K+ concentration in solution in equilibrium with low albite and adularia. E-F H4Si04 concentration in solution in equilibrium with quartz. G-H Ca " " concentration in solution in equilibrium with low albite and anorthite.

Temperature of each reservoir was estimated from the assemblage of hydrothermal alteration minerals and temperature of alteration zone in active geothermal system (e.g., Hayashi, 1973 Takeno et al., 2000). 

However, in order to clarify the depositional mechanism of electrum and sulfides, more detailed description of alteration minerals, 8 0, 8D data and the salinity (Cl concentration)-enthalpy relationship are clearly required, and the two fluids mixing model has to be evaluated based on these data. 

Inome, X, Tomita, K., Yamamoto, M. and Oba, N. (1981) On alteration minerals in the Fuke mining district, Kagoshima Prefecture. J. Min. Soc. Jpn. Spec. Issue, 15, 116-132 (in Japanese with English abst.). 

Tamura, M. (1982) Alteration minerals and mineralization in the Shakanai Kuroko deposit, Akita Prefecture. Mining Geology, 32, 379-390. 

Chemical compositions of geothermal waters controlled by hydrothermal alteration mineral assemblage... 

Previous studies clearly indicated that the chemical compositions of geothermal waters are intimately related both to the hydrothermal alteration mineral assemblages of country rocks and to temperature. Shikazono (1976, 1978a) used a logarithmie cation-Cl concentration diagram to interpret the concentrations of alkali and alkaline earth elements and pH of geothermal waters based on thermochemical equilibrium between hydrothermal solution and alteration minerals. 

Alteration minerals at the greatest depth in areas of relatively high temperatures (200-300°C) are quartz, chlorite, mica, anhydrite, K-feldspar, calcite, pyrite, albite, wairakite, laumontite, and minor amounts of epidote and prehnite. 

The dominant alteration minerals at the deeper part of the well include anhydrite, epidote, sericite, chlorite, calcite, dolomite, rhodochrosite, kutnahorite, zeolites (mordenite, clinoptilorite), chlorite and sericite/smectite interstratified clay mineral with subordinate amounts of kaolinite in the shallower part (Imai et al., 1996). 

The generalized sequence of alteration minerals from shallower to deeper portions and/or from lower to higher temperatures in active geothermal systems, which is constructed mainly based on the work by Henley and Ellis (1983), is given in Fig. 2.25. It is shown in Fig. 2.25 that the change in alteration and gangue minerals largely depends on temperature as well as on the other physicochemical parameters such as /s2, /o2> /c02> and pH. 

Fig. 2.25. Generalized zonal. sequence of alteration minerals in active geothermal areas (Henley and Ellis, 1983 Shikazono, I985b).

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