Hydrothermal Alterations

In zones of hydrothermal alteration it is apparent that the formation of dioctahedral montmorillonites is limited by temperature. They almost never occur in the innermost zone of alteration, typically that of sericitization (hydro-mica or illite), but are the most frequent phase in the argillic-prophylitic zones which succeed one another outward from the zone where the hydrothermal fluid is introduced in the rock. Typically, the fully expandable mineral is preceded by a mixed layered phase (Schoen and White, 1965 Lowell and Guilbert, 1970 Fournier, 1965 Tomita, et al., 1969 Sudo, 1963 Meyer and Hemley, 1959 Bundy and Murray, 1959 Bonorino, 1959). However, temperature is possibly not the only control of expandable clay mineral occurrence, the composition of the solutions and the rock upon which they act might also be important. It is possible that high magnesium concentrations could form chlorite, for example, instead of expandable minerals. 

Bass (1971) has reported a montmorillonite which forms a significant portion of a,meteorite sample. This is again a rare occurrence and can be ignored as far as it would have any far-reaching consequences on the mineralogy of sediments. 

Several studies identified the hydrothermal alteration halo in the dacitic rocks surrounding Kuroko deposits (Fig. 1.20) (Shirozu, 1974 Utada et al., 1974, 1981 Utada, 1980 Ishikawa et al., 1976 Izawa et al., 1978 Date et al., 1983 Urabe et al., 1983 Ishikawa, 1988 Marumo, 1989 Inoue and Utada, 1991 Shikazono et al., 1995). 

For example. Date et al. (1983) recognized the following alteration zones in the Fukazawa Kuroko mine area of Hokuroku district from the centre (near the orebody) to the margin (1) sericite-chlorite zone (zone 111 in Figs. 1.20-1.22) characterized by quartz + sericite Mg-rich chlorite (2) montmorillonite zone (zone 11 in Fig. 1.20) characterized by Mg-Ca-type montmorillonite + quartz kaolinite calcite sericite Fe-rich chlorite and (3) zeolite zone (zone 1 in Fig. 1.20) characterized by clinoptilolite + mordenite + Mg-Na-type montmorillonite cristobalite calcite or analcime + Mg-Na-type montmorillonite + quartz calcite sericite Fe-rich chlorite (Fig. 1.20). 

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. 

Date et al. (1983) found the existence of an Na20-depleted dacite mass with a lateral dimension of 1.5 x 3.0 km immediately below the ore horizon (Figs. 1.23 and 1.24) and the mass is useful indicator of exploration of Kuroko ore deposits. This Na20 depletion is considered to be due to the destruction of plagioclase attacked by potassium- 

Singer and Kouda (1988, 1992) confirmed based on the statistical analysis of the distribution of minerals and bulk compositions of altered rocks in the Hokuroku district 

---

Most of the developed hot-water fields are located by significant surface indications, particularly in the form of hot springs. Once a resource has been identified, a variety of techniques can be used to map the system and determine whether it is of a size sufficient to justify commercial development. Hidden hot-water resources are much more difficult to locate, but geologic indicators such as volcanic activity and evidence of hydrothermal alteration can be used. 

Talc of metasedimentary origin is formed by hydrothermal alteration of a dolomitic host rock by a silica-containing fluid. This type of deposit is typical of Montana and AustraUa. It is usually quite pure with talc content of 90 to 98% and often very white as well. Dolomite [17069-72-6], CaMg(C02)2, is the most common accessory mineral. The fourth type is of metamorphic origin, where a siUcaceous dolostone is first converted to tremolite [14567-73-8] or actinohte [13768-00-8] and then partially converted to talc. The Balmat, New York, and Death Valley, California, deposits are of this type. Tremolite, dolomite, and serpentine are common accessory minerals. This type of talc deposit has a variable talc content (30—80%), but is usually white and often commercially exploited because of the properties of its accessory minerals rather than the talc. 

Figure 1.7. Schematic distribution of Kuroko orebody and hydrothermally altered rocks (modified after T. Sato, 1974).

The Y, C and B sub-types roughly correspond to types 1, 2 and 3 as defined by Urabe (1974a), who classified Kuroko deposits based on hydrothermal alteration and ore mineral assemblages type 1, kaolinite-pyrophyllite-diaspore-type type 2, sericite-chlorite-type type 3, sericite—chlorite-carbonate-type. Hydrothermal alterations in the Kuroko mine area are described in section 1.3.2. 

Superimposed alterations are common in the Kuroko mine area (Inoue and Utada, 1991). For example, K-feldspar, kaolinite, alunite, pyrophyllite and diaspora alterations cut chlorite alteration, indicating that they formed later than chlorite alteration (Inoue and Utada, 1991). Inoue and Utada (1991) thought, based on detailed descriptions of the hydrothermal alterations in the Kamikita mine area. North Honshu, that hydrothermal alterations in this district started from 13 Ma and ended at 3-4 Ma. 

Pyrophyllite and diaspore alterations were reported from several Kuroko deposits, although they are not common (Urabe, 1974a). This type of hydrothermal alteration is thought to have occurred at a later stage than the hydrothermal alterations associated with Kuroko mineralization (sericite, chlorite, and zeolites) (Utada, personal communication, 1995). 

As well as felsic volcanic rocks, basalt occurs in the Kuroko mine area. It is also intensely and hydrothermally altered. Shikazono et al. (1995) studied the hydrothermal... 

Magnesite, dolomite and calcite occur in hydrothermally altered rocks near Kuroko orebody. The following equations are used to constrain /coa values of hydrothermal solutions (Shikazono et al., 1998) (Fig. 1.39). 

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). 

Rare earth elements (REE). Analytical results of REE contents of hydrothermally altered volcanic rocks in Kuroko mine area and Kuroko ores are summarized as follows (Shikazono, 1999a) (Fig. 1.46). 

Positive Eu anomaly is observed for barite, Kuroko ores, ferruginous chert (tet-susekiei), and hydrothermally altered basaltic and dacitic rocks overlying the Kuroko ores. 

Negative Eu anomaly is observed for hydrothermally altered dacite underlying the Kuroko ores and anhydrite in the dacitic tuff breccia. 

Figure 1.46. REE patterns of the altered volcanogenic rocks and Kuroko ores. Data sources Shikazono (1999a). (A) Hydrothermally altered dacite and anhydrite underlying the Kuroko ores. (B) Barite, Kuroko ore and ferruginous chert. (C) Hydrothermally altered basalt overlying the Kuroko ores (Shikazono, 1999a).

Heavy Rare Earth Element). Therefore, it is considered that negative Ce and positive Eu anomalies in hydrothermally altered volcanic rocks, Kuroko ores, and ferruginous chert and LREE enrichment in the Kuroko ores have been caused by hydrothermal alteration and precipitations of minerals from hydrothermal solution responsible for sulfides-sulfate (barite) mineralization. 

A negative correlation between Mg content and Ca content of hydrothermally altered basalt and dacite from the Kuroko mine area exists. This correlation indicates that Ca in the rocks is removed to fluid by the exchange of Mg in seawater. Eu may behave in the manner similar to Ca during seawater-volcanic rock interaction because of the similarity of their ionic radii. 

Positive Eu anomaly is observed for hydrothermal solution issuing from the hydrothermal vent on the seawater at East Pacific Rise (Bence, 1983 Michard et al., 1983 Michard and AlbarMe, 1986). Guichard et al. (1979) have shown that the continental hydrothermal barites have a positive Eu anomaly, indicating a relatively reduced environment. Graf (1977) has shown that massive sulfide deposits and associated rocks from the Bathurst-Newcastle district. New Brunswick have positive Eu anomalies. These data are compatible with positive Eu anomaly of altered basaltic rocks, ferruginous chert and Kuroko ores in Kuroko mine area having positive Eu anomaly and strongly support that Eu is present as divalent state in hydrothermal solution responsible for the hydrothermal alteration and Kuroko mineralization. 

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). 

The age of formation of epithermal vein-type deposits can be estimated from K-Ar ages of K-bearing minerals (adularia, sericite) in veins and in hydrothermal alteration zones nearby the veins. A large number of K-Ar age data have been accumulated since the work by Yamaoka and Ueda (1974) who reported K-Ar age data on adularia from Seigoshi Au-Ag (3.7 Ma) and Takadama Au-Ag deposits (8.4 Ma). Before their publication on the K-Ar ages of these deposits it was generally accepted that epithermal... 

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. 

It is rather difficult to determine the sequence of each type of alteration in a mine area. However, it is widely accepted that the hydrothermal alteration proceeds as follows propylitic alteration —> potassic alteration and intermediate argillic alteration advanced argillic alteration. The actual sequence alteration might be more complicated and superimposition of each type of alteration could be common. 

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. 

Hydrothermal alteration is reflected by the changes in many variables (temperature, water/rock ratio, extent of water-rock interaction (reaction progress), reaction rate, flow rate of fluids etc.) (Fujimoto, 1987). Theoretical and experimental works on hydrothermal alteration were reviewed by Meyer and Hemley (1967), and Rose and Burt (1979). 

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. 

The coupled precipitation kinetics-fluid flow model was applied to the distribution of Si02 content and K2O content of the hydrothermally altered andesite in the Hishikari Au-Ag mine area, south Kyushu, Japan by Shikazono et al. (2002). This will be described in section 1.4.6. 

Shikazono (1985a) has studied hydrothermal alterations in the epithermal Au-Ag mine district in Izu Penin.sula, middle part of Honshu, and indicated that (1) the propylitic alteration occurs widely in the district (2) at the centre of the district and stratigraphically upper horizon, there exists advanced argillic alteration (3) epithermal Au-Ag vein-type deposits are distributed at marginal zone in the district (Fig. 1.125) ... 

The Special Issue of Resource Geology on the Hishikari deposits (Shikazono et al., 1993) includes various aspects of the Hishikari deposits (oxygen isotopes of gangue minerals, hydrothermal alteration, precipitation sequence, fluid inclusions, vertical electric profiling and electric sounding surveys, structural geological analysis, opaque minerals. 

Supergroup rocks in the Hishikari district suffered hydrothermal alteration. Chlorite, quartz and sericite occur abundantly near the veins. The other constituents are pyrite, albite, calcite and organic matter. 

They calculated the change in 8 0 values of hydrothermally altered volcanic rocks as a function of water to rock ratio by weight and temperature, assuming that oxygen isotopic equilibrium is attained in a closed system, and demonstrated that the increase in 8 0 values of altered andesitic rocks from the veins towards peripheral zones can be interpreted as a decrease in temperature from the vein system (Fig. 1.135). In their calculations, the effect of mixing of hydrothermal solution with groundwater was not considered. 

Figure 1.135. Change in S 0 values of hydrothermally altered rocks as a function of water to rock ratio with several different temperatures. The initial values of rock and water were taken as +8.5%c and —5.0%o, respectively (Naito et al., 1993).

---