Hydrothermal fluids

In this chapter, we develop geochemical models of two hydrothermal processes the formation of fluorite veins in the Albigeois ore district and the origin of black smokers, a name given to hydrothermal vents found along the ocean floor at midocean ridges. 

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Fig. 6. In a binary electricity generation plant, the hydrothermal water from the weU, A, is passed through a heat exchanger, B, where its thermal energy is transferred to a second, more volatile working fluid. The second fluid is vaporized and deflvered to a turbine, D. After exiting the turbine the spent working fluid is cooled and recondensed in another heat exchanger, E, using water or air as the coolant, F. It is then fed back to the primary heat exchanger to repeat the cycle. Waste hydrothermal fluid, C, can be reinjected into the producing field.

Direct Uses of Geopressured Fluids. Many of the uses typical of hydrothermal energy, such as greenhouse, fish farm, and space heating, have been proposed for geopressured resources, but none has been commercially developed (34). Hydrothermal fluids are widely used in enhanced oil recovery, however, to increase production from depleted oil fields. 

Gamo (1995) revealed based on the chemical and isotopic compositions of hydrothermal fluids from midocean ridges that the precipitation of minerals and interaction... 

Figure 1.105. Distributions of 8D and 5 0 in various waters, minerals, and hydrothermal fluids of epithermal...

Drummond, S.E. Jr. (1981) Boiling and mixing of hydrothermal fluids Chemical effects on mineral precipitation. Ph.D. Thesis, Pennsylvania State U. 

Gamo, T. (1995) Wide variation of chemical characteristic of submarine hydrothermal fluids due to secondary modification processes after high temperature water-rock interaction, a review. In Sakai, H. and Nozaki, Y. (eds.), Biogeochemical Processes and Ocean Flux in the Western Pacific, Terra Sci. Publ., pp. 425-451. 

Henley, R.W. (1984) Metals in hydrothermal fluids. Reviews in Econ. Geol, 1, 115-128. 

Michard, A. and Albarede, F. (1986) The REE content of some hydrothermal fluids. Chem. Geol, 55, 51-60. 

Mitra, A. (1994) Geochemical implications of rare earth element pattern in hydrothermal fluids from midocean ridges. Geochim. Cosmochim. Acta, 58, 5105-5113. 

Chemical compositions of hydrothermal fluids in the Western Pacific (Ishibashi and Urabe, 1995)... 

Submarine hydrothermal fluid end-member data observed at various plate boundary regions (end-member values are those obtained by extrapolation to an assumed value of zero magnesium, except for the values at site 2). BAB back-arc basin (Game, 1995)... 

Fig. 2.34. (A) Chondrite-normalized REE pattern of fresh basaltic andesite. (B) Chondrite-normalized REE pattern of altered basaltic andesite. (C) Ratios of REE content in the altered rock normalized to the fresh basaltic andesite (298-R-02). The dashed line is the ratio line of one. (D) Chondrite-normalized REE patterns of hydrothermal fluids from Vienna Wood, Pacmanus and Desmos, Tamagawa and Kusatsu-Yubatake (Gena et al., 2001).

Chemical compositions of major elements (alkali, alkali earth elements. Si) in back-arc and midoceanic ridge hydrothermal solutions are not so different (Table 2.15). This is thought to be due to the effect of water-rock interaction. For example, Berndt et al. (1989) have shown that mQ i+ of midoceanic ridge hydrothermal fluids is controlled by anorthite-epidote equilibrium (Fig. 2.37). Figure 2.37 shows that /Mca2+/m + of back-arc hydrothermal fluids is also controlled by this equilibrium. 

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. 

This reaction means that higher /hjS causes higher concentration of Au(HS)2 as well as higher fo. Higher fujS, /ss and /o, of back-arc hydrothermal fluids can explain higher Au, Hg, As and Sb who.se dominant dissolved species are probably thio complexes. 

Berndt, M.E., Seyfried, W.E. Jr. and Janeckey, D.R. (1989) Plagioclase and epidote buffering of cation ratios in midocean ridge hydrothermal fluids Experimental results in and near the supercritical region. Geochim. Cosmochim. Acta, 53, 2283-2300. 

Cowan, J.E. and Cann, J. (1988) Supercritical two-phase separation of hydrothermal fluids in the Troodos ophiolite. Nature, 333, 259-261. 

Grimaud, D., Ishibashi, J., Lagabrielle, Y, Auzende, J.M. and Urabe, T. (1991) Chemistry of hydrothermal fluids from the 1°S active site on the North Fiji Basin Ridge (SW Pacific). Chem, Geol, 93, 209-218. 

Tsunogai, U., Ishibashi, J., Wakita, H., Gamo, T., Watanabe, K., Kajimura, T., Kanagawa, S. and Sakai, H. (1994) Peculiar features of Suiyo seamount hydrothermal fluids, Izu-Bonin arc Differences from subaerial volcanism. Earth Planet. Sci. Lett., 126, 289-301. 

Berndt et al. (1989) have indicated that aQ +/a + and aNa+/r H+ of midoceanic ridge hydrothermal fluids is controlled by clinozoisite, Ca-feldspar, and Na-feldspar. In addition to these assemblages, calcite is in equilibrium with fluids. Therefore, we can derive the /CO2 temperature relationship from the following equilibrium relations. 

Barne.s, H.L. and Czamanske, G.K. (1967) Solubilities and transport of ore minerals. In Barnes, H.L. (ed.). Geochemistry of Hydrothermal Ore Deposits. New York Holt, Rinehart and Win.ston, pp. 334-381. Berndt, M.E., Seyfried, W.E. Jr. and Janeckey, D.R. (1989) Plagiocla.se and epidote buffering of cation ratios in midocean ridge hydrothermal fluids Experimental results in and near the supercritical region. Geochim. Cosmochim. Acta, 53, 2283-2300. 

Salts with the [Au(CO)2]+ cation have been proposed to be important intermediates in the transport of gold in hydrothermal fluids. Gold deposits may accumulate through formation and decomposition of (carbonyl)gold salts under geochemically relevant conditions, similar to the mechanism advanced with salts based on the [Au(SH)2]- anion.303... 

Pigna M, Colombo C, Violante A (2003) Competitive sorption of arsenate and phosphate on synthetic hematites (in Italian). Proceedings XXI Congress of Societa Italiana Chimica Agraria SICA (Ancona), pp 70-76 Quirk JP (1955) Significance of surface area calculated from water vapour sorption isotherms by use of the B. E. T. equation. Soil Sci 80 423-430 Rancourt DG, Fortin D, Pichler T, Lamarche G (2001) Mineralogical characterization of a natural As-rich hydrous ferric oxide coprecipitate formed by mining hydrothermal fluids and seawater. Am Mineral 86 834-851 Raven K, Jain A, Loeppert, RH (1998) Arsenite and arsenate adsorption on ferrihydrite kinetics, equilibrium, and adsorption envelopes. Environ Sci Technol 32 344-349... 

In light of the small solubilities of many minerals, the extent of reaction predicted by this type of calculation may be smaller than expected. Considerable amounts of diagenetic cements are commonly observed, for example, in sedimentary rocks, and crystalline rocks can be highly altered by weathering or hydrothermal fluids. A titration model may predict that the proper cements or alteration products form, but explaining the quantities of these minerals observed in nature will probably require that the rock react repeatedly as its pore fluid is replaced. Local equilibrium models of this nature are described later in this section. 

Bowers and Taylor (1985) were the first to incorporate isotope fractionation into a reaction model. They used a modified version of EQ3/EQ6 (Wolery, 1979) to study the convection of hydrothermal fluids through the oceanic crust, along midocean ridges. Their calculation method is based on evaluating mass balance equations, as described in this chapter. 

The model calculated in this manner predicts that two minerals, alunite [KA13(0H)6(S04)2] and anhydrite (CaSC>4), are supersaturated in the fluid at 175 °C, although neither mineral is observed in the district. This result is not surprising, given that the fluid s salinity exceeds the correlation limit for the activity coefficient model (Chapter 8). The observed composition in this case (Table 22.1), furthermore, actually represents the average of fluids from many inclusions and hence a mixture of hydrothermal fluids present over a range of time. As noted in Chapter 6, mixtures of fluids tend to be supersaturated, even if the individual fluids are not. 

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