Epithermal

Mercury ore deposits occur in faulted and fractured rocks, such as limestone, calcareous shales, sandstones, serpentine, chert, andesite, basalt, and rhyolite. Deposits are mostiy epithermal in character, ie, minerals were deposited by rising warm solutions at comparatively shallow depths from 1—1000 m (6). 

For the neutron porosity measurement, fast neutrons are emitted from a 7.5-curie (Ci) americium-beryllium (Am-Be) source. The quantities of hydrogen in the formation, in the form of water or oil-filled porosity as well as crystallization water in the rock if any, primarily control the rate at which the neutrons slow down to epithermal and thermal energies. Neutrons are detected in near- and far-spacing detectors, located laterally above the source. Ratio processing is used for borehole compensation. 

The energy of the detected neutrons has an epithermal component because a high percentage of the incoming thermal neutron flux is absorbed as it passes through a 1 in. of drill collar steel. Furthermore, a wrap of cadmium under the detector banks shields them from the thermal neutron arriving from the inner mud channel. This mainly epithermal detection practically eliminates adverse effects caused by thermal neutron absorbers in the borehole or in the formation, such as boron. 

Neutron. The neutron porosity values recorded with most tools match closely the wireline thermal neutron logs in the lower porosity ranges (under 25 porosity units). In high porosity zones, the LWD neutron porosities lie between thermal and epithermal wireline values. 

For the same reason, this system will be self-scavenging. It thus seems unlikely that much can be learned about the hot or epithermal reactions in Ni(CO)4. 

Work on mercury alkyls has been done by Heitz and Adloff (31-33), who studied Hg(CH3)2, Hg(C2Hj)2 and HgPh2. They found no isotope effect between " Hg, Hg, and ° Hg, and no correlation with the respective conversion coefficients. They also noted that the retentions could not be satisfactorily explained by exchange of the respective ligands, and thus concluded that the molecules are reformed by an epithermal not by a thermal process. Parent yields were typically 74, 15, and 8% for the diphenyl-, dimethyl- and diethylmercury, respectively. 

The several polymeric metal carbonyls studied have led to some surprisingly high yields [e.g., Fe3(CO),2 and Ruj(CO)j2 in Table IV] but to no substantiated mechanisms. The 17% yield of Fe3(CO),2 in neutron-irradiated Fe(CO)j was interpreted as a reaction of Fe(CO)4 with the Fe(CO)5, but no further evidence is available. The study of Mn2(CO),o has been fruitful (44, 46). The insensitivity of the parent yield MnMn(CO),o to heat indicates that the molecule is formed by a reaction quite early in the sequence, perhaps epithermal. The discovery (46) of a species which reacts rapidly with I2 and exchanges with IMn(CO)5 led to the conclusion that the Mn(CO)5 radical is produced prominently (4.5%) by nuclear reactions in the solid decacarbonyl. The availability of this labeled Mn(CO)5 has made possible several interesting observations about the exchange properties of this radical in the solid (45) and in solution (42). 

Before mentioning the characteristics of Kuroko and epithermal vein-type deposits in Japan, it is worthwhile to briefly describe the metallogeny, geology, geophysics, and tectonic situations of the Japanese Islands. 

Large epithermal gold vein-type deposits occur at major arc-arc junctions (Figs. 1.5 and 1.6) specifically, Chishima (Kurile)-Northeast Honshu, Northeast Honshu-Izu-Bonin and Southwest Honshu-Ryukyu. This may result from hydrothermal activities and mineralizations caused by intense volcanism at the arc-arc junctions. 

Hydrothermal clay-silica deposits (kaolinite, halloysite, sericite, montmorillonite and silica) and zeolite deposits occur in Tertiary-Quaternary volcanic regions. These deposits are distributed in areas of epithermal gold mineralization. 

Gold-rich silica precipitates at the Osorezan volcano, which is located in the most northern part of Honshu, have features very similar to epithermal Te-bearing gold vein-type deposits of the Plio-Pleistocene. 

Main hydrothermal ore deposit types of Neogene age that formed in and around the Japanese Islands are Kuroko deposits and epithermal vein-type deposits. This classification is based on the form of the deposits. 

Y sub-type (yellow ore type), and B sub-type (black ore type), according to Cu, Pb and Zn ratios (Fig. 1.9). However, the variation in the ratio is not wide, compared with epithermal vein-type deposits. Therefore, characteristic differences in each sub-type of Kuroko deposits are not discussed here. 

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). 

Figure 1.6. Di.stribution and temporal and spatial relationship of late Cenozoic gold deposits in the Japanese Islands. 1 Quartz vein-type gold deposits with little to no base metals. 2 Gold-silver deposits with abundant base metals. 3 Distribution boundary of gold deposits formed during the Miocene. 4 Location of Plio-Pleistocene gold deposits at the actual island arc junctions. 5 Location of Plio-Pleistocene gold deposits in front of the actual island arc junctions. Numbers in the figure are K-Ar ages of epithermal Au-Ag veins (Kubota, 1994).

Several sub-classification.s of epithermal preciou.s-metal deposits have been proposed mineralogy, host-rock composition and elemental association (Lindgren, 1928), gold-silver ratios of metal weights (Ferguson, 1929 Nolan, 1933), mineral paragenesis (Nishiwaki et al., 1971), and production ratios of metals (Heald-Wetlaufer et al., 1983). 

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,... 

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. 

Each deposit type is distributed in a different metallogenic province (Fig. 1.3) (Tat-sumi, 1970). Epithermal vein-type deposits occur in Miocene-Pliocene volcanic terrain. 

Compositional zoning in electrum grain is common (Shimazaki, 1974 Imai et al., 1981). The Ag content of rim of electrum grain is higher than that of core. Although Ag content varies widely, it is generally lower than that of epithermal vein-type deposits. 

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-... 

Epithermal base-metal vein-type deposits are distributed in the Green tuff region (Southwest Hokkaido, Northeast Honshu) (Fig. 1.62). The distribution area of this type of deposits is nearly same as that of Kuroko deposits. For example, large deposits (Osarizawa Cu-(Au) Ani Cu-Au Hosokura Pb-Zn deposits) occur in Northeast Honshu, but are more widely distributed in the Green tuff region than Kuroko deposits. 

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... 

Figure 1.62. Location of epithermal-type deposits in Japan (Shikazono and Shimizu, 1988a). 1 Green tuff and subaerial volcanic region of Tertiary/Quaternary ages, 2 Main Paleozoic/Mesozoic sedimentary terranes, 3 Main metamorphic terranes. TTL Tanakura tectonic line, ISTL Itoigawa-Shizuoka tectonic line, MTL Median tectonic line. Open circle epithermal Au-Ag vein-type deposits, solid circle epithermal base metal vein-type deposits, open triangle epithermal Au disseminated-type deposits.

The K-Ar age data are summarized in Figs. 1.64 and 1.65. It is obvious in these figures that (1) ages of formation of epithermal vein-type deposits vary widely from 15 to 1 Ma, but are mostly 6-1 Ma, (2) epithermal vein-type deposits have been formed... 

Epithermal base-metal vein-type deposits are characterized by the abundant occurrence of sulfides (chalcopyrite, pyrite, sphalerite, galena), and a scarcity of Au-... 

A large number of analytical data on chemical composition of sphalerite are available (Shikazono, 1974a Watanabe and Soeda, 1981). The FeS content of sphalerite from epithermal base-metal vein-type deposits varies widely mostly from 1 to 20 mol% (Fig. 1.68). 

Figure 1.68. Iron content of sphalerite from Kuroko, epithermal Au-Ag vein-type and epithermal base metal vein-type deposits (Shikazono, 1977a).

The Ag content of electrum from epithermal Au-Ag vein-type deposits is mostly in a range of 40-70 atomic% (Fig. 1.69). 

Very few data on the chemical composition of electrum from epithermal base-metal vein-type deposits are available. However, it is evident that the Ag content varies widely (Fig. 1.71). The Ag content of electrum from the Osarizawa and Okuyama Cu deposits is low N g (Ag atomic%) = 8.6-17.7), while the Ag content of electrum from Pb-Zn-Mn deposits (Toyoha, Oe, Inakuraishi, and Imai-Ishizaki) is high NAg = 60-80). Motomura (1986) reported that the Ag content of electrum from these deposits is higher than that from epithermal Au-Ag vein-type deposits. The geochemical implication of the Ag content of electrum is discussed in section 1.4.4. 

Chemical compositions of tetrahedrite-tennantite from epithermal base-metal vein-type deposits are characterized by (1) wide compositional variations, and (2) higher Zn and Sb contents and Ag and lower Fe, As, and Cu contents, compared with Kuroko deposits (Shikazono and Kouda, 1979). 

The differences in Zn/Fe ratio of tetrahedrite-tennantite in epithermal vein-type and Kuroko deposits and that of sphalerite in these deposits can be interpreted in terms of the following exchange reaction ... 

Figure 1.71. Frequency histogram for the Ag content of electrum from epithermal base metal vein-type deposits in Japan (Shikazono and Shimizu, 1988a).

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