K+-mica

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). 

If alunite, K-mica and kaolinite (which are common minerals in the advanced argillic alteration) are in equilibrium, the concentration of H2SO4 can be estimated based on the experimental work by Hemley et al. (1969) the concentration of H2SO4 at 200°C and 300°C is 0.002 and 0.012 M, respectively. This may suggest that it is difficult to form such a high concentration of sulfate ion only by oxidation of H2S. 

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. 

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. 

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. 

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).

Fig. 2.11. The temperature dependence of cation/proton activity ratios of geothermal well discharges in Japan. The lines in the figure are recalculated temperature dependences of cation/proton ratios in Icelandic geothermal waters. The dashed curve in B represents the reaction 1.5 K-feldspar + H+ = 0.5 K-mica + 3 quartz (or chalcedony) + K+ (Chiba, 1991). Open circle Takigami, open triangle Kakkonda, open square Okuaizu, solid circle Kirishima, solid triangle Sumikawa, solid square Nigoiikawa.

Fig. 2.13. (A) Temperature dependence of pH in Japanese thermal waters. Lines indicate the temperature dependence of pH when pH is buffered by the K-feldspar-K-mica-quartz (or chalcedony at less than 200°C) assemblage at a Na + K concentration of 0.1 and 0.01 mol/kg H2O. Symbols are as in Fig. 2.11. (B) Temperature dependence of pH of Icelandic thermal waters. Large circles indicate well discharges. Small dots represent hot spring waters (Chiba, 1991).

The Okuaizu geothermal system is characterized by high temperatures (maximum 340°C), high salinity (about 2 wt% total dissolved solids (TDS)) and large amounts of non-condensable gases (1 wt% CO2 and 200 ppm H2S). The pH of the hydrothermal solution measured at 25°C is 6.44 (Table 2.6). However, the pH of the original fluid in the reservoir is computed to be 4.05. This pH as well as alkali and alkali earth element concentrations are plotted near the equilibrium curve of albite, K-mica, anhydrite and calcite (Fig. 2.19) (Seki, 1991). 

Fig. 2.19. Reservoir temperature versus saturation indices (logQ/K) for calcite, anhydrite, K-feldspar and K-mica based on the estimated composition of reservoir fluid (Seki, 1991). Estimation based on gas results of Seki (1990), with saturation calculations carried out by PECS (Takeno, 1988). Gas concentrations were assumed to be 1 wt% of CO2 and 250 mg/kg for H2S for all wells (Seki, 1991).

Fig. 2.26. Range of carbon dioxide fugacity (fco ) and temperature for the propylitic alteration (epidote zone) in the Seigoshi area and same active geothermal systems. Seigoshi = propylitic alteration of the Seigoshi district. The curves A-B and A -B are equilibria for epidote (Xpis = 0.30) - K-mica (oK-mica = 0-9) -K-feldspar (aK-feidspar = 0.95) - calcite assemblages at saturated water vapor pressure condition (Shikazono, 1985a).

The equilibrium relations of epidote-K-mica-K-feldspar-pyrite-chlorite, hematite S jq = pyrite -I- H2S, anhydrite-magnetite-pyrite-clinozoisite and pyrite-hematite-magnetite assemblage are shown in Fig. 2.27. Based on the equilibrium curves and analytical data on epidote and chlorite, /hjS of the epithermal Au-Ag vein ore fluids for some propylitic alterations is also estimated (Shikazono, 1985a). 

The main alteration minerals surrounding Kuroko ore body are K-mica, K-feldspar, kaolinite, albite, chlorite, quartz, gypsum, anhydrite, and carbonates (dolomite, calcite, magnesite-siderite solid solution), hematite, pyrite and magnetite. Epidote is rarely found in the altered basalt (Shikazono et al., 1995). It contains higher amounts of ferrous iron (Fe203 content) than that from midoceanic ridges (Shikazono, 1984). 

It is likely that the minerals controlling /CO2 of hydrothermal solution at back-arc basins are dolomite, siderite, calcite, hematite, magnetite, graphite, K-mica and kaolinite. Most of these minerals are not found in altered ridge basalt. 

Shikazono (1978) theoretically derived that the concentrations of alkali and alkali earth elements in chloride-rich hydrothermal solution are nearly in equilibrium with hydrothermal alteration minerals such as albite, K-feldspar, K-mica, quartz, calcite, wairakite, and Mg-chlorite. If we use 500 mmol/kg H2O as the average Cl concentration of hydrothermal solution from the back-arc basin, which is in equilibrium with... 

The kinetics of reesterification model reaction of methylbenzoate by heptanole-1 in mica presence was studied at 443 K. Mica catalytic activity was determined on the observed rate constant of the first order k at the twentieth multiple of heptanole-1 excess and mica contents 30 mass.% in calculation on the methylbenzoate [2],... 

Let us now imagine a process of hydrothermal alteration of arkose sandstones composed of Mg-chlorite, K-feldspar, K-mica, and quartz. Because precipitating Si02 during alteration is amorphous, we will assume the presence of amorphous silica instead of quartz, and we will consider MgO as the generic oxide / and K2O as the generic oxide j. 

We can understand why this is true if we consider the equilibrium between kaolinite and K-mica ... 

From HemleyJs work on the potassium system (11) one may infer that kaolinite, quartz, and K-mica ( illite) may be stable together, and the equilibrium constant [K+]/[H+] may be extrapolated, (from 200°C.) to 106 at 25°C.—e.g., Hollands (15) value of 10,50 O5. Hem-ley s work on the sodium system (12) in the same way indicates that quartz, Na-montmorillonite, and kaolinite can form a stable assemblage, and a somewhat risky extrapolation of the equilibrium ratio [Na+]/[H+] from 300° to 25°C. gives 107° (15). These ratios are not far from the corresponding ratios in sea water. One could not expect them to be exactly the same since the hydromica and montmorillonite phases in sea water are solid solutions, containing more components than the phases in Hemley s experiments. His experiments surely do not contradict the idea that the previously mentioned phases could exist together at equilibrium. 

Figure 4. Silicate stability. KF, KM, G, K, and Q are K+-feld-spar, K+-mica, gibbsite, kaolinite, and amorphous silica, respectively. M and AB are montmorillonite and albite. W, S, FW, and SW represent areas of winter lake data, summer lake data, extracted fresh water sediments, and extracted sea water sediments, respectively...

With an uncertainty of 0.06, the use of the gaseous phase C02 and the mineral phases calcite, dolomite, and halite (from the Cretaceous limestone), quartz, K-mica, albite, anorthite, and Ca-montmorillonite (from the Quaternary aquifer) as well as Fe(OH)27Clo3 pyrite, pyrolusite (from the crystalline basement) and assuming additionally that halite, K-mica, albite, and anorthite can only be dissolved while Ca-montmorillonite can only precipitate, the following model was found (Table 46). 

The mineral phases calcite, dolomite, halite, and gypsum for the Cretaceous limestone, as well as albite, quartz, anorthite, K-mica for the sandstone, and the gaseous phase C02 have to be defined. Furthermore it is assumed that dolomite, gypsum, and halite only dissolve, while calcite precipitates and C02 degasses. Under those conditions and with an uncertainty of 4%, two models are obtained (Table 47). 

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