Epithermal precious-metal deposits

Berger, B.R. and Eimon, R.I. (1983) Conceptual models of epithermal precious metal deposits. In Shanks, W.C. Ill (ed.), Cameron Volume on Unconventional Mineral Deposits. New York Society of Mining Engineers of the American Institute of Mining, Metallurgical and Petroleum Engineers Inc., pp. 191-205. 

Bonham, Jr., H.R (1984) Three major types of epithermal precious-metal deposits (abst.). GeoL Soc. Am. Abst. with Programs, 16, 449. 

Bonham, Jr., H.E (1986) Models for volcanic-hosted epithermal precious metal deposits a review. Intemat. 

Hayba, D.O., Bethke, P.M., Heald, P. and Foley, W.K. (1985) Geologic, mineralogic, and geochemical characteristics of volcanic-hosted epithermal precious metal deposits. Reviews in Economic Geol, 2, 129-168. 

Nolan, T.B. (1933) Epithermal precious-metal deposits. In Ore Deposits of the Western States (Lindgren Volume). New York American Institute of Mining and Metallurgical Engineers, pp. 623-640. 

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. 

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. 

Based on the study of precious-metal vein-type deposits in western U.S.A., White (1981) classified the epithermal precious-metal vein-type deposits into an Au-dominated and an Ag-dominated groups. White (1981) considered that the Ag-dominated group... 

The ore fluids responsible for epithermal base-metal vein-type deposits were generated predominantly by meteoric water-rock interaction at elevated temperatures (200-350°C). Fossil seawater in marine sediments was also involved in the ore fluids responsible for this type of deposits. Epithermal precious metal ore fluids were generated by meteoric water-rock interaction at 150-250°C. Small amounts of seawater sulfate were involved in the ore fluids responsible for epithermal precious metal vein-type deposits occurring in Green tuff region (submarine volcanic and sedimentary rocks). 

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. 

Major epithermal vein-type deposits in Japan are base-metal type and precious-metal type which are classified based on the ratios of base metals and Au and Ag which have been produced during the past (Table 1.2). 

Epithermal vein-type deposits can be divided into four types based on total metal produced and metal ratio base-metal type, precious-metal (Au, Ag) type, Sb-type and Hg-... 

Pb. Their results clearly demonstrate that basement rocks affect vein components in epithermal precious and base-metal quartz-adularia-type deposits. 

In and nearby the Japanese Islands, which are situated close to plate boundaries (Pacific, Philippine, North American and Asian plates), many hydrothermal ore deposits were formed during the Miocene-present eras. Major deposit types are Kuroko and epithermal vein-types. Epithermal vein-type deposits are classified into precious (Au, Ag)-types and base-metal (Cu, Pb, Zn, Fe, Mn, Ag)-types. 

The K-Ar age and foraminiferal age of epithermal vein-type and Kuroko deposits clearly demonstrate that epithermal precious and base metal vein-type deposits formed... 

Fe-minerals and sulfides are common in hydrothermal ore deposits. If we know these mineral species, chemical compositions of minerals and temperature, fo2-pH and Eh-pH conditions can be restricted. As an example of the estimates of foj and pH, vein-type deposits in Japan are taken into account. The vein-type deposits considered here include precious metal (Au, Ag) and base metal (Cu, Pb, Zn, Fe, Mn) deposits and are classed as epithermal-type (Shikazono 2003). The oxidation-reductimi conditirm can be estimated based on mineral assemblages and chemical compositions of minerals. Sphalerite is the most conunon sulfide mineral in these deposits. It contains iron and is zinc-iron solid solutimi (Zni xFexS). The relationship among FeS content of sphalerite, foj, pH and temperature is obtained from the reaction. 

Spycher, N. and Reed, M. (1989) Evolution of a Broadlands-type epithermal ore fluids along alternative P-T paths Implications for the transport and deposition ofba.se precious and volatile metals. Econ. Geol, 84, 328-359. 

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