Argillic alteration

Among the epithermal vein-type deposits in Japan, four major types of hydrothermal alteration ean be diseriminated. They are (1) propylitie alteration, (2) potassic alteration, (3) intermediate argillic alteration, and (4) advaneed argillic alteration. The definitions of these types of alteration are mainly based on Meyer and Hemley (1967) and Rose and Burt (1979) who elassified the hydrothermal alteration in terms of alteration mineral assemblages. 

In contrast to the hardly investigated lateral zonation around Japanese epithermal vein-type deposits, a few examples of vertical zonation are known. Potassic alteration grades upwards into intermediate argillic alteration in the wall rocks for the Toyoha (Okabe and Bamba, 1976), Ohe (Tsukada and Uno, 1980), Chitose (Hasegawa et al., 1981) and Kushikino (Imai, 1986). 

Advanced arigillic alteration is found at the upper horizon than the sites of potassic and intermediate argillic alterations where the Au-Ag mineralization occurs (e.g., Seigoshi, Yatani, Kushikino, Hishikari). This type of alteration takes blanket-form in upper part and vein-form in lower part (Iwao, 1962 Shikazono, 1985a). The conspicuous zonation from upper to lower horizon is known at the Ugusu silica deposit, namely, silica zone, alunite zone, kaolinite zone and montmorillonite zone (Iwao, 1949, 1958, 1962). 

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. 

Considerable changes in the chemical composition of rocks occur during the advanced argillic alteration. For example, Si02 content of highly silicified rocks of the Ugusu silica mine reaches 99% (Iwao, 1962). This silicification is caused by the considerable leaching of elements from the rocks by acid hydrothermal solution except Si and addition of Si from hydrothermal solution. 

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... 

Figure 1.73. Distribution of epithermal Au-Ag vein-type deposits, propylitic and advanced argillic alterations and intrusive rocks of diorite prophyry (Shikazono, 1985a).

Figure 1.80. Zonal sequence of the advanced argillic alteration from the central to marginal zone in section of A -B in Fig. 1.79 and from upper horizon to lower horizon (Shikazono, 1985a)...

There are three possible mechanisms for generating the strongly acid solution which caused the advanced argillic alteration (1) alteration caused by the vapor-dominated system as inferred by White et al. (1971) (2) alteration caused by the oxidation of H2S near the surface and (3) alteration by volcanic gas and/or hot water condensed from a volcanic gas. Among them, (3) is the most attractive mechanism given the following evidence and considerations. 

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. 

There are two important chemical reactions which cause intermediate argillic and advanced argillic alterations... 

Oxidation of H2S (reaction (1-35)) occurs under the near-surface environment. Oxygen may be supplied from oxygenated groundwater. These oxidation reactions liberate H+ ion, leading to a decrease in pH. Under low pH conditions intermediate argillic alteration minerals (e.g., kaolinite, sericite) are stable. 

When temperatures of volcanic gases containing SO2 decrease, the reaction (1-35) proceeds to the right hand side. This reaction causes a considerable decrease in pH due to the formation of sulfuric acid. Advanced argillic alteration is formed by the interaction of volcanic gas with groundwater. 

The above interpretation of formation of epithermal Au-Ag vein-type deposits is supported by (1) thermochemical calculations on this type of mixing (Reed and Spycher, 1985), and (2) the geological occurrence of epithermal Au-Ag vein-type deposits and associated advanced argillic alteration. 

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) ... 

Therefore, it is thought that the mixing of acidic solution with hydrothermal solution occurred and andesite near the gold-quartz veins suffered superimposed potassic and advanced argillic alterations. 

For example, Shikazono (1985a) has shown that advanced argillic alteration (Ugusu silica deposit in west Izu Penninsula, central Honshu) occurs at the centre and... 

Such evolution of a hydrothermal system from acidic sulfate hydrothermal solution to neutral is common in the epithermal system associated with precious metal mineralization. For example, advanced argillic alteration and intense silicification occurred at earlier stage of hydrothermal system in the Seigoshi Au-Ag mine area. 

Gena et al. (2001) reported advanced argillic alteration of basaltic andesite from the Desmos caldera, Manus back-arc basin which was caused by interaction of hot acid hydrothermal fluid originated from a mixing of magmatic gas and seawater. It is noteworthy that the acid alteration is found in back-arc basins (Manus, Kuroko area) but not in midoceanic ridges. 

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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