Alteration zonings

Note also that the relative sizes cf the altered zones are not to scale (e.g., choosing a higher value for the level of concern does not always result in a smaller zone than the use of greater wind speed and less atmospheric stability). 

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. 

Ohmoto et al. (1983) found that values alteration zones and major and base metal contents of footwall dacite along the section in the Fukazawa area correlate with each other (Fig. 1.30). 

Figure 1.27. Areal distribution of the whole-rock 5 0 values of footwall volcanic rocks in the Fukazawa area. The boundaries for the alteration zones are modified from Date et al. (1983) (Green et al., 1983).

Figure 1.28. Whole-rock 0 values of Miocene volcanic and sedimentary rocks from the Hokuroku district, grouped by alteration zones. Each square represents one sample. Mont. = montmorillonite, Ser. = sericite, Chi. = chlorite, av. = average (Green et ah, 1983).

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... 

The area of the potassic alteration is not wide, compared with the propylitically altered area. The width of potassic alteration zone away from the vein is generally within several tens of meters (ca. 50 m) (Shikazono and Aoki, 1981 Imai, 1986). The potassic alteration is usually found in the intermediate vicinity of the vein in the epithermal deposits in Japan. Thus it is evident that this type of alteration occurs genetically related to the ore deposition. 

The range of temperatures for each alteration zone can be estimated from the following chemical reactions and thermochemical data available for these reactions... 

Figure 1.79. Geology and alteration zoning in the Ugusu silica mine (modified from Iwao, 1949, 1962). Toi F = Toi Formation Koshimoda = Koshimoda andesite Hakko = Hakko orebody Shibayama = Shibayama orebody. Numbers indicate metres above sea level. Alteration zoning in the section of A -B is shown in Fig. 1.80 (Shikazono, 1985a)...

The above interpretation on the alteration zoning is mainly based on thermodynamics. However, it is necessary to consider the influence of kinetics and fluids flow on... 

Izawa et al. (1990) recognized the following alteration zones from the vein towards margin of the Hishikari Au-Ag mine area, chlorite-sericite zone (zone IV), interstratified clay mineral zone (zone III), quartz-smectite zone (zone II) and cristobalite-smectite zone (zone I) and least altered zone (L.A. (least altered) zone) (Fig. 1.131). 

Figure 1.134. Histograms of values of the volcanic rocks for five distinct alteration zones (Naito et al., 1993).

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 1.142 shows the dependence of solubility of Si02 minerals (quartz, cristobalite) on temperature. As described already, cristobalite occurs in peripheral and shallower part of hydrothermal alteration zone. Quartz is present in zones occurring in deeper and closer to the gold-quartz veins. Such zoning from quartz to cristobalite is also common in main active geothermal systems (Hayashi, 1973 Takeno et al., 2000). 

As shown in Fig. 1.143, the system is divided into four reservoirs. Each reservoir corresponding to alteration zone IV, III, II and I is assumed to be homogeneous with respect to temperature and concentrations of dissolved silica in aqueous solution. 

Temperature of each reservoir was estimated from the assemblage of hydrothermal alteration minerals and temperature of alteration zone in active geothermal system (e.g., Hayashi, 1973 Takeno et al., 2000). 

Volume of each reservoir was calculated from the volume of each alteration zone and assuming porosity of alteration zone to be in a range of 3-1%. 

Figure 1.144 shows the results of calculation based on multireservoirs (40 reservoirs) model in which each reservoir corresponding to each alteration zone is divided into... 

The deposits are characterized by conspicuous alteration zoning from the centre (orebody) to margin (Tokunaga, 1955 Doi, 1972 Urashima et al., 1981, 1987). They are siliceous zone, alunite zone, kaolinite zone, sericite zone and montmorillonite zone. 

Fujimoto, K. (1987) Factors to control the width of a partial altered zone. Mining Geology, 37, 45-54 (in Japanese). 

Utada, M., Minato, H., Ishikawa, T. and Yoshizaki, Y. (1974) The alteration zones surrounding Kuroko-type deposits in Nishi-Aizu district, Fukushima 1306cture with emphasis on analcime zone as an indicator in exploration for ore deposits. Mining Geology Special Issue, 6, 291-302. 

Utada, M., Tokoyo, T. and Aoki, H. (1981) The distribution of alteration zones in the central area of the Hokuroku district, northern Japan. Mining Geology, 31, 13-25 (in Japanese with English abst.). 

Janecky, D.R. and Shanks, W.C. Ill (1988) Computational modeling of chemical and sulfur isotopic reaction processes in seafloor hydrothermal systems, chimney, massive sulfides, and subjacent alteration zones. Can. Mineral, 26, 805—826. 

Argentite, natural occurrence of, 22 668 Argentium Sterling, 12 562 Argentothiosulfate complexes, 19 215 Argillaceous limestone, 15 26 Argillic alteration zones, gallium in,... 

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