Alteration propylitic

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. 

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. 

It is rather difficult to determine the sequence of each type of alteration in a mine area. However, it is widely accepted that the hydrothermal alteration proceeds as follows propylitic alteration —> potassic alteration and intermediate argillic alteration advanced argillic alteration. The actual sequence alteration might be more complicated and superimposition of each type of alteration could be common. 

Usually propylitic alteration precedes the base metal and Au-Ag mineralizations. Potassic and intermediate argillic alterations are nearly contemporaneous with ore deposition. 

Generally, the chemical composition of rocks does not considerably change during the propylitic alteration. The components which are added to the rocks are only H2O, CO2 and S (e.g., Okabe and Bamba, 1976). 

I.4.2.5. Spatial and geochemical relationships between propylitic alteration and advanced argillic alteration a case study on the Seigoshi-Ugusu district, central Japan... 

The distribution of the Au-Ag vein-type deposits in this district is shown in Fig. 1.73. The propylitic alteration is intimately associoated with these deposits. 

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).

Figure 1.75. Zonal sequence of the propylitic alteration in. section A-B in Fig. 1.74 (Shikazono, 1985a).

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). 

Based on the hydrothermal alteration mineral assemblages and the fluid inclusion, the probable range of gas fugacities (/s2, /o2 /H2S) and temperature can be seen in Figs. 1.81 and 1.82 these estimated fugaeities are quite different from those of the propylitic alteration. 

Formation of albite which is characteristic mineral of propylitic alteration occurs by heating of rocks and descending fluids at recharge zone in the hydrothermal system (Giggenbach, 1984 Takeno, 1989). Thus, it is considered that the propylitic alteration takes place at recharge zone in the hydrothermal system, while potassic alteration at discharge zone. 

Shikazono (1985a) has studied hydrothermal alterations in the epithermal Au-Ag mine district in Izu Penin.sula, middle part of Honshu, and indicated that (1) the propylitic alteration occurs widely in the district (2) at the centre of the district and stratigraphically upper horizon, there exists advanced argillic alteration (3) epithermal Au-Ag vein-type deposits are distributed at marginal zone in the district (Fig. 1.125) ... 

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). 

Petrographic characterization of propylitic alteration associated with porphyry Cu-Mo deposits in the Collahuasi District, Northern Chile implications for mineral exploration... 

Keywords collahuasi District, porphyry Cu-Mo deposit, petrography, propylitic alteration, sub-greenschist metamorphism... 

In diorite, granodiorite and andesites, propylitic alteration is characterized by... 

Norman, D.K., Parry, W.T., Bowman, J.R. 1991. Petrology and Geochemistry of Propylitic Alteration at Southwest Tintic, Utah. Economic Geology, 86, 13-28. 

The major rock units in the East-Kahang are made up of andesite, volcanic breccia, dacite, quartz-diorite, diorite and locally mineralized hydrothermal breccia (Fig. 2). Eocene andesite and volcanic breccias showing propylitic alteration are the oldest units in the area. They have been intruded by dacite, mostly showing phyllic alteration associated with weak copper mineralization. Quartz-diorite and dioritic... 

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