Feldspar alteration

The mixing calculation is interesting in that it demonstrates a common ion effect by which dolomite precipitation drives feldspar alteration. In the model, dolomite forms because the saline water is rich in Ca++ and Mg++, whereas the fresh water... 

Hellmann R., Penisson J.-M., Hervig R. L., Thomassin J.-H., and Abrioux M.-F. (2003) An EFTEM/HRTEM high-resolution study of the near surface of labradorite feldspar altered at acid pH evidence for interfacial dissolution-reprecipitation. Phys. Chem. Min. 30, 192-197. 

K Feldspars Altered Plagio -clases Quartz Black minerals (Biotite mainly)... 

In igneous rocks, the general tendency is an increase of radiation from ultra-basic to acid rocks. This is attributed to the higher uranium, thorium and potassium content of mica and alkali feldspars. Alteration can change the radioactivity. 

The experimental study of feldspar alteration is fully summarized in Deer, et al [1963], vol. IV, pp. 54-55, 127-129. The recent thesis by Pedro [1964] contains an extensive review and bibliography. 

Carbonate reservoirs are usually affeoted to varying degree by diagenesis. However the process of dissolution and replacement is not limited to carbonates. Feldspar for instance is another family of minerals prone to early alterations. 

Kaolin most commonly originates by the alteration of feldspar or other aluminum siHcates via an intermediate solution phase (97,98) usuaHy by surface weathering (26,99) or by rising warm (hydrothermal) waters. A mica, or hydrated alumina soHd may form as an intermediate phase during the alteration from parent material to kaolin minerals. 

Bentonite is a rock rich in montmorillonite that has usually resulted from the alteration of volcanic dust (ash) of the intermediate (latitic) siliceous types. In general, reUcts of partially unaltered feldspar, quartz, or volcanic glass shards offer evidence of the parent rock. Most adsorbent clays, bleaching clays, and many clay catalysts are smectites, although some are palygorskite [1337-76 ]. 

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. 

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... 

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). 

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. 

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. 

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). 

---