K-feldspar

A specific example of this would be the weathering of K-feldspar and the formation of kao-linite (see Table 8-1 for mineral definitions), a layer-silicate clay ... 

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. 

Mg5Al2Si30io(OH)8 + KAlSi308 + 5CaC03 + 5C02 (Mg-chlorite) (K-feldspar) (calcite)... 

Principal gangue minerals in base-metal vein-type deposits are quartz, chlorite, Mn-carbonates, calcite, siderite and sericite (Shikazono, 1985b). Barite is sometimes found. K-feldspar, Mn-silicates, interstratified mixed layer clay minerals (chlorite/smectite, sericite/smectite) are absent. Vuggy, comb, cockade, banding and brecciated textures are commonly observed in these veins. 

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... 

K-mica + 4 calcite + 6 quartz = 2clinozoisite - - 4 K-feldspar - - 4C02 + 2H2O (1-29)... 

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. 

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. 

Few data on the chemical compositions of feldspars (albite, K-feldspar) are available. Fuji (1976) indicated that K-feldspar and albite in the propylite of west Izu Peninsula, middle Honshu are of nearly end member composition. Nagayama (1992) showed that K-feldspars in the Hishikari Au-Ag vein and in the host andesitic rock have different composition Na/K ratio of K-feldspars from the vein is lower than that from the host rocks. 

If OTci- and pH are assumed to be 1-5 molal and lower than that for K-feldspar-K-mica-quartz equilibrium, respectively, EAu/EAg is estimated to be considerably lower than 0.1. Therefore, EAu/EAg of ore fluids for epithermal base-metal vein-type deposits is thought to be considerably lower than 0.1. 

Oxygen isotopic fractionation factors used for the calculation were taken from Taylor (1997). Initial 8 0 value of hydrothermal solution (0%o) was estimated from 8 0 values of K-feldspar and quartz in the veins and homogenization temperatures (Shikazono and Nagayama, 1993), and that of groundwater (—7%c) was estimated from meteoric water value of the south Kyushu district (—7%c) (Matsubaya et al., 1975). 

These results are consistent with XRD (X-ray diffraction) results. The amounts of K-feldspar, K-mica and chlorite are higher in the altered rocks closer to the veins and Ca-zeolites and smectite decrease in amounts towards periphery of the alteration zones. 

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.

The ages of Neogene mineralization and hydrothermal alteration in and around the Northeast Honshu and Hokkaido have been determined by K-Ar data on K-minerals (K-feldspar, sericite). These data are summarized in Fig. 1.147 and Table 1.26. 

Common gangue minerals are kaolinite and sericite, but K-feldspar is not found, suggesting low pH of ore fluids. 

Figure 1.189. The relationship between tAuCl / Au(HS)" temperature. Hatched and dotted areas represent the probable geochemical environment for typical Japanese gold-silver vein and auriferous vein deposits, respectively. A, mci- = 10, mK+ =2, qh2S = 10, K-feldspar/K-mica/quartz equilibrium B, mQ- = 1. niK+ =0.2, H2S = 10 - , K-feldspar/K-mica/quartz equilibrium C, mci- — 1, Wk+ =0.2, qh2S = 10, K-feldspar/K-mica/quartz equilibrium D, mci- =0.2, mK+ =0.04, oh2S = 10 , K-feldspar/K-mica/quartz equilibrium E, mci- =0.2, m <+ =0.04, uh s = 10 K-feldspar/K-mica/quartz equilibrium F, mci- =0.2, = 0.04, UHiS = 10 , K-feldspar/K-mica/quartz equilibrium. Thermochemical data for the calculations were taken from Helgeson (1969), Seward (1973), Drummond (1981), and Henley et al. (1984). (Shikazono and Shimizu, 1987).

Figure 1.196. /oj-pH ranges for hot-spring-type deposits and low sulfidation-type deposits. Temperature = 250°C, ES = 0.01 mol/kg H2O, ionic strength = 1. Ka kaolinite, Al alunite, SI liquid sulfur, Kf K-feldspar, Hm hematite, Mt magnetite, Py pyrite, Po pyrrhotite. Bn bomite, Cp chalcopyrite. 

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. 

Hence, it is assumed that the minerals in equilibrium with geothermal waters are albite, K-feldspar, muscovite, quartz, calcite, anhydrite, chlorite and wairakite. 

Albite and K-feldspar are commonly observed to coexist. If the following reaction is in equilibrium,... 

Fig. 2.3. Relation between the K+ and CI concentration of geothermal waters and inclusion fluids. The solid line defines the equilibrium condition between the solution and the assemblage albite-K-feldspar at 250°C. For symbols used, see caption to Fig. 2.2. (Shikazono, 1978a).

If muscovite, K-feldspar and quartz are saturated with geothermal waters, the following reaction can be written ... 

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