Geothermal deposits

Shanks W. C. and Bischoff J. L. (1980) Geochemistry, srrlfur isotope composition, and accumulation rates of Red Sea geothermal deposits. Econ. Geol. 75, 445-459. 

The ocean is host to a variety and quantity of inorganic raw materials equal to or surpassiag the resources of these materials available on land. Inorganic raw materials are defined here as any mineral deposit found ia the marine environment. The mineral resources are classified generally as iadustrial minerals, mineral sands, phosphorites, metalliferous oxides, metalliferous sulfides, and dissolved minerals and iaclude geothermal resources, precious corals, and some algae. The resources are mosdy unconsoHdated, consoHdated, or fluid materials which are chemically enriched ia certain elements and are found ia or upon the seabeds of the continental shelves and ocean basias. These may be classified according to the environment and form ia which they occur (Table 1) and with few exceptions are similar to traditional mineral deposits on land. 

Chry sotile is a hydrated magnesium siHcate and its stoicliiometric chemical composition may be given as AIg2Si20 (0H)4 [12001 -29-5]. However, the geothermal processes wliich ield the chry sotile fiber formations usually involve the co-deposition of v arious other minerals. Tliese mineral contaminants comprise brucite [1317-43-7] (AIg(OH)2), magnetite [1309-38-2] (Fe O, calcite [13397-26-7] (CaCO ), dolomite [16389-88-1] (AIg,CaC02),... 

From the corrosion point of view, it is very important to control the deposition of scale. Removal of deposited scale by mechanical means is the first step. Standard, industrial water-treating techniques can be used to control scale deposition in general. In deep, hot wells or geothermal wells it is best to avoid untreated makeup water (i.e., geothermal brines). 

Figure 1.123. Schematic model for the formations of the Te-type and Se-type epithermal gold depositions in the fossil geothermal system. Reference Henley and Ellis (1983) (Shikazono et al., 1990).

Brown, K.L. (1986) Gold deposition from geothermal discharges in New Zealand. Econ. GeoL, 81, 979-986. Browne, P.R.L. and Ellis, A.J. (1970) The Ohaki-Broadlands hydrothermal area. New Zealand Mineralogy and related geochemistry. Am. J. Set, 269, 97-131. 

Casadevall, T. and Ohmoto, H. (1977) Sunnyside mine. Eureka mining district. Sun Juan County Geochemistry of gold and base-metal ore deposition in a volcanic environment. Econ. GeoL, 72, 1285-1320. Cathelineau, M. and Nieva, D. (1985) A chlorite geothermometer. The Los Azufres (Mexico) geothermal system. Contr. Mineral. Petrol., 91, 235-244. 

Hedenquist, J.W. and Henley, R.W. (1985) The importance of CO2 on freezing point mea.surements of fluid inclusions evidence from active geothermal systems and implications for epithermal ore deposition. Econ. Geol, 50, 1379-1406. 

Iwao, S. (1962) Silica and alunite deposits of the Ugusu mine a geochemical consideration on an extinct geothermal area in Japan. Japan. J. Geol. Geogr., 33, 131-144. 

Shikazono, N. (1978a) Possible cation buffering in chloride rich geothermal waters. Chem. Geol, 23, 234—259. Shikazono, N. (1978b) Selenium content of acanthite and chemical environments of Japanese vein-type deposits. Econ. Geol, 73, 524—533. 

Shikazono, N. (1989) Oxygen and carbon isotopic compositions of carbonates from the Neogene epithermal vein-type deposits of Japan Implication for evolution of terrestrial geothermal activity. Chem. Geol, 76, 239-247. 

Urabe, T., Scott, S.D. and Hattori, K. (1983) A comparison of footwall-rock alteration and geothermal systems beneath some Japanese and Canadian volcanogenic massive sulfide deposits. Econ. Geol. Mon., 5, 345-364. 

White, D.E, (1981) Active geothermal systems and hydrothermal ore deposits. Econ. Geol, 75th Anniv. Vol., 392-423. 

Chemical composition of geothermal water associated with base-metal deposits (White, 1967)... 

Comparison of active geothermal systems with epithermal vein-type deposits... 

Close similarities between epithermal vein-type deposits and active geothermal systems have been cited by various authors (e.g.. White, 1955, 1981 Henley and Ellis, 1983 Shikazono, 1985a,b Izawa and Aoki, 1991). 

In this section (2.2), geochemical, mineralogical and geological characteristics of epithermal vein-type deposits summarized in section 1.4 will be compared with subaerial active geothermal systems associated with base metal and Au-Ag mineralizations mentioned in sections 2.1.1 and 2.1.2. 

Fig. 2.23 shows the distributions of major geothermal systems and epithermal gold deposits of Japanese Islands. It is interesting to note that their distributions are similar and they are distributed close to the volcanic front. 

Fig. 2.23. The major geothermal system and epithermal gold deposits of Japan. Epithermal gold deposits are represented by gold mines or mining areas that have produced more than 10 metric tons of gold (Izawa and Aoki, 1991).

It has been pointed out by Giggenbach (1981) on the basis of thermochemical calculations that epidote occurs at higher temperatures of at least more than 240°C, and K-feldspar occurs at restricted temperatures, i.e. below ca. 250°C, in active geothermal systems. These theoretical results seem to be consistent with those observed in epithermal vein-type deposits in Japan. 

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