Hot spring-type deposits

Most of epithermal precious-metal vein-type deposits in Japan can be classed as adularia-sericite-type, and low sulfidation-type. Very few hot spring-type deposits (quartz-alunite-type, high sulfidation-type) are found in the Japanese Islands. A summary of various characteristic features of adularia-sericite type (low sulfidation-type) is given mainly in section 1.4. 

A few examples of hot spring-type deposits occur in the Japanese Islands. The characteristics of this type of deposits are described briefly in section 2.7. 

Figure 1.193. Locality map of the hot spring-type deposits in Kyushu.

Figure 1.196. /oj-pH ranges for hot-spring-type deposits and low sulfidation-type deposits. Temperature = 250°C, ES = 0.01 mol/kg H2O, ionic strength = 1. Ka kaolinite, Al alunite, SI liquid sulfur, Kf K-feldspar, Hm hematite, Mt magnetite, Py pyrite, Po pyrrhotite. Bn bomite, Cp chalcopyrite. 

The plot of K2O versus Si02 for volcanic rocks thought to be genetically related to Au mineralization indicates that high sulfidation deposits (hot spring-type deposits appear... 

The fluid inclusion studies of hot spring-type deposits (Takenouchi, 1981) show a wide range of homogenization temperatures in a given quartz crystal which suggests the boiling of ore fluids. These fluid inclusion studies demonstrate that the hot spring-type formed under shallow depth from the surface. 

These opaque minerals are common in Pb-Zn vein-type deposits in Japan Opaque minerals identified from silica sinter containing gold and mercury are krennerite, col-oradoite and metacinnabar (Osorezan, Japan Waiotapu, New Zealand). These minerals are not found in low sulfidation-type Au-Ag deposits in Japan but are reported from Kobechizawa, and Date in south Hokkaido which are massive and disseminated types and similar to hot spring type deposits. 

Recently, epithermal gold deposits were divided into several types based on gangue minerals, and physicochemical environment of ore deposition (pH, H2S concentration of ore fluids). They are hot spring-type (Silberman, 1982 Berger, 1983a Berger and Eimon,... 

Shikazono et al. (1990) divided epithermal precious-metal vein-type deposits into Te-bearing and Se-bearing deposits. As will be considered later, Te-bearing deposits are regarded as intermediate-type of adularia-sericite-type and hot spring-type. The distinction between these two types of deposits is discussed in section 1.4. 

Figure 1.89. Activity of 82(052 )-temperature diagram showing possible as and temperature ranges for epithermal Au disseminated-type (hot spring type), epithermal Au-Ag vein-type and epithermal base metal vein-type deposits in Japan (Shikazono 1986 Shikazono and Shimizu, 1988b).

Hot spring-type gold deposits (Nansatsu-type by Urashima et al. (1981,1987), high sulfidation-type by Hedenquist (1987), epithermal Au disseminated-type) are distributed in the Nansatsu district of southern Kyushu (Fig. 1.193). The deposits (Kasuga, Akeshi, Iwato) were formed at Pliocene age (5.5-3.7 m.y.) in the calc-alkaline volcanic rocks of nearly same age (Togashi and Shibata, 1984). The deposits, which are similar to Nansatsu-type deposits, occur in Southwest Hokkaido (Date, Hakurhu). 

The /oj-pH conditions for this type of deposits and other types of epithermal deposits and hydrothermal alteration are shown in Fig. 1.196 (Shikazono and Aoki, 1981). It is worth noting that the /oj-pH ranges for the epithermal Au deposits (hot spring-type, Te-type, and Se-typ>e) lie between the points A (high fo, and low pH) and B (low /oj... 

Silberman, M.L. (1982) Hot spring type large-tonnage, low-grade gold deposits. In Ericson, R.L. (ed.). Characteristics of Mineral Deposit Occurrences. U.S. Geol. Survey Open-File Report 82—795. 

The other important deposit types of the Neogene age are polymetallic (Cu, Pb, Zn, Sn, W, Au, Ag) (xenothermal or subvolcanic type deposits) vein-type deposits, hypo/mesothermal Au vein-type deposits, disseminated-type (hot spring type) Au deposits, and Hg-Sb disseminated and vein-type deposits. 

Several active geothermal systems in Japanese Islands are associated with precious- and base-metal mineralization. Base metal mineralization takes place from hot springs containing high chloride concentration probably due to the contribution of seawater. Precious-metal mineralization occurs in the Osorezan hot springs which are characterized by neutral pH, high H2S concentration, and low salinity. These chemical features are similar to those of epithermal precious metal vein-type deposits in Japan. 

Shallow probes have been used successfully at Lost River in Hardy County, West Virginia, Patrick Draw in Sweetwater County, Wyoming (Matthews et al., 1984 Richers et al., 1982), Arrowhead Hot Springs in San Bernardino County, California (Burtell, 1988) and on a large number of surveys conducted throughout the industry. Limited tests by Williams (1985) in the west Texas Permian Basin suggest that shallow probes are difficult to use in this area because of impermeable deposits of caliche and thick salt and anhydrite beds at a depth of about 300 m. An example of a halo-type anomaly reported by Williams (1985) is included in his thesis. 

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