Epidote

Figure 1.33. Frequency (number of analyses) histogram for Fc203 (wt%) of epidote from the Kuroko basalt. A epidote coexisting with albite, B epidote coexisting with chlorite, C epidote coexisting with pyrite, D epidote coexisting with hematite and calcite (Shikazono et al., 1995).

Negative Ce anomaly and positive Eu anomaly are observed for epidote-rich altered basalt near the orebody. 

Sverjensky (1984) calculated the dependency of Eu +/Eu + in hydrothermal solution on /oj (oxygen fugacity), pH and temperature. According to his calculations and assuming temperature, pH and /oj for epidote-stage alteration of basalt and Kuroko ores (Shikazono, 1976), divalent Eu is considered to be dominant in the rocks and hydrothermal solution. Thus, it is reasonable to consider that Eu in the rocks was removed to hydrothermal solution under the relatively reduced condition more easily than the other REE which are all tiivalent state in hydrothermal solution. Thus, it is hkely that Eu is enriched in epidote-rich altered volcanic rocks. Probably Eu was taken up by the rocks from Eu-enriched hydrothermal solution which was generated by seawater-volcanic rock interaction at relatively low water/rock ratio. 

Several factors such as Cl concentration, water/rock ratio and temperature are important in controlling the chemical composition of the hydrothermal solution interacted with the rocks. For example, water/rock ratio affects the alteration mineralogy (Mottl and Holland, 1978 Seyfried and Mottl, 1982 Shikazono, 1984). For example, at low water/rock ratio, epidote is stable, while chlorite at high water/rock ratio (Shikazono, 1984 Shikazono and Kawahata, 1987). 

Calcium silicates such as wairakite, epidote, prehnite, laumontite, and stilbite are common in the wall rocks of some Au-Ag deposits in the Izu peninsula. Epidote occurs as a gangue mineral coexisting with sulfides and quartz in some Cu deposits, but none of the other above-mentioned Ca and Mn silicates have been reported from these deposits. Laumontite is a common mineral in propylite, which is the host rock for Au-Ag deposits. Other zeolites such as mordenite and dachiardite are not generally common, but they are the main gangue minerals associated with Au-Ag minerals in the Ohnoyama and Awagano Au-Ag deposits. 

The predominant gangue minerals vary with different types of ore deposits quartz, chalcedonic quartz, adularia, calcite, smectite, interstratified mica/smectite, interstratified chlorite/smectite, sericite, zeolites and kaolinite in Au-Ag rich deposits chlorite, quartz, sericite, carbonates (calcite, rhodoehrosite, siderite), and rare magnetite in Pb-Zn rich deposits chlorite, serieite, siderite, hematite, magnetite and rare epidote in Cu-rich deposits (Sudo, 1954 Nagasawa et al., 1976 Shikazono, 1985b). 

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. 

Figure 1.74. Zonal sequence of the propylitic alteration in E-W section of the Seigoshi-Toi mine area (Yug = yugawaralite Heu = heulandite Stil = stilbite Opx = orthopyroxene Mont = montmorillonite Mor = mordenite Lm = laumontite Wr = wairakite Chi = chlorite pr = prehnite ep = epidote Py = pyrite Kf = K-feldspar Cpx = clinopyroxene) (Shikazono, 1985a).

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... 

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. 

Based on the analytical data of K-mica, epidote and K-feldspar and using thermochemical data on these minerals (Helgeson and Kirkham, 1974 Helgeson et al., 1978 Bird and Helgeson, 1981), the /coz range for the propylitic alteration was estimated (Fig. 1.78). 

The formation of epidote, K-feldspar, prehnite, wairakite and calcite in the geothermal area is considered to be due to the loss of CO2 gas and rapid precipitation from the solution supersaturated with respect to quartz (Browne, 1978). The widespread occurrence of these minerals in the Seigoshi district seems to be consistent with the above-mentioned consideration, namely that these minerals usually occur as veinlets rather than the replacements of original minerals and filling amygdule. In particular, many veinlets of epidote, prehnite and wairakite are found near the Au-Ag-quartz veins. 

Wolery (1978) and Reed (1982, 1983) have indicated based on a computer calculation of the change in chemistry of aqueous solution and mineralogy during seawater-rock interactions that epidote is formed under the low water/rock ratio less than ca. 50 by mass. Humphris and Thompson (1978), Stakes and O Nell (1982) and Mottl (1983) have also suggested on the basis of their chemical and oxygen isotopic data of the altered ridge basalts that epidote is formed by seawater-basalt interaction at elevated temperatures (ca. 200-350°C) under the rock-dominated conditions. If epidote can be formed preferentially under such low water/rock ratio, the composition of epidote should be influenced by compositions of the original fresh rocks. 

Shikazono (1984) summarized analytical data of the epidote from geothermal areas to consider the relationship between the composition of epidote and that of the original fresh rocks and to inspect the other factors controlling the compositional variations in epidote. The discussion on the epidote composition by Shikazono (1984) is described below. 

In Fig. 1.85 iron contents of epidote from two different geologic environments, island arc and oceanic ridge or ophiolite, are summarized. It can be seen in Fig. 1.85 that the iron content of epidote from ridge basalt and ophiolite is generally lower than... 

Figure 1.85. Relation between Fe20j content of epidote and that of original fresh rocks. S Seigoshi, Y Yugashima, N Noya, F Furotobe, O Ohtake, M Mid-Atlantic ridge, C Costa Rica rift. Mi Mitsuishi, Sh Shimokawa (Shikazono, 1984)...

The iron content of epidote coexisting with prehnite from the Seigoshi district is smaller than that without prehnite. It has been clarified that prehnite is stable at the low... 

CO2 fugacity and that the iron content of epidote in equilibrium with prehnite is lower than that in equilibrium with other minerals such as K-feldspar, K-mica and calcite under such low CO2 fugacity conditions (e.g., Cavaretta et al., 1982). Therefore, it seems clear that iron content of epidote is affected also by CO2 fugacity. 

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