Water meteoric

Noble gases in rainwater are in solubility equilibrium with air and, although we know of no investigations addressed to the question, there is no reason to believe any differently for river and lake water. Most work on meteoric water has thus been on groundwater, which is removed from contact with air, especially geothermal water (see review by Mazor, 1975). Matters of interest are the temperature at which the water was last equilibrated with air, whether it has behaved as a closed system since air equilibration, and indeed whether or not it is actually meteoric. 

Concerning the latter question, the alternative to meteoric water is juvenile water. Distinction between these two alternatives for the origin of geothermal waters has 

No data for noble gas solubility in ice are available, but solubilities can be expected to be much lower than for liquid water. The only noble gas observations are these of Matsuo and Miyake (1966), who analyzed Ar along with N2, 02, and C02 in natural ices. They found the major gases and Ar present in roughly atmospheric proportion, evidently contained principally in occluded gas bubbles. The bubbles were present in 

Sorption of the heavier noble gases on ice might be interesting (cf. Section 7.5), but there are no relevant data. 

In many examples of hot springs, fumaroles, and the like, associated with tectonic activity, the water involved is meteoric but the noble gases are, in part, juvenile. Observations are described in Chapter 6. 

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Fig. 18-24 Observed correlation (the Meteoric Water Line) of the two most important isotopic ratios in precipitation (gray diamonds Jouzel et al., 1987 and Dahe et al., 1994), and predictions of simple isotopic models. A, prediction with constant a B, prediction with temperature-dependent a.

Craig, H. (1961). Isotopic variations in meteoric waters. Science 133,1702-1703. 

These different sites of hydrothermal and ore-forming activity may have resulted from the mode of subduction of the Pacific Plate. Mariana-type subduction (characterized by a steep angle of subduction and back-arc basin formation Uyeda and Kanamori, 1979) during middle Miocene caused WNW-ESE extension, submarine hydrothermal activity, thick accumulation of bimodal (basaltic and dacitic) volcanic activity (Green tuff) and Kuroko-type formation (Shikazono and Shimizu, 1993). Plio-Pleistocene Chilean-type subduction (shallow-dipping subduction zone, E-W compression Uyeda and Kanamori, 1979) and oblique subduction of the Pacific Plate beneath the North American Plate led to uplift and expansion of land area, subaerial hydrothermal activity accompanied by meteoric water circulation, subaerial andesitic volcanic activity and formation of vein-type deposits. 

Horikoshi and Shikazono (1978) indicated that 8D of ore fluids for B sub-type which is located at centre of Hokuroku basin is higher, suggesting large contribution of seawater, while 8D of ore fluids of Y sub-type located at the margin of Hokuroku basin is lower, suggeting meteoric water contribution. 

The most important conclusion derived from the isotopic studies mentioned above is that isotopic characteristics of Kuroko ore fluids were caused dominantly by seawater-volcanic rock interaction at elevated temperature and by the mixing of seawater with small portions of igneous water or the hydrothermal solution whose chemical and isotopic compositions are controlled by water-rock interaction under the rock-dominated condition and also small proportion of mixing of meteoric water. 

However, it cannot be decided at present which processes (degree of seawater-rock interaction or mixing ratio of seawater, igneous water and meteoric water) are important for the generation of Kuroko ore fluids solely from the isotopic studies. But experimental and theoretical considerations on seawater-volcanic rocks interaction and origin of hydrothermal solution at midoceanic ridges suggest that Kuroko ore fluids can be produced dominantly by seawater-volcanic rock interaction. 

SD and S O. 8D and of the ore fluids responsible for epithermal Au-Ag and base-metal vein-type deposits in Japan have been estimated from analyses of fluid inclusions (Hattori and Sakai, 1979) and minerals (Watanabe et al., 1976). These data are shown in Fig. 1.103. 8D values of ore fluids for epithermal Au-Ag vein-type deposits are similar to those of present-day meteoric water values. 8D values of epithermal ore fluids for base-metal vein-type deposits are slightly higher than those of epithermal Au-Ag vein-type deposits. This may be due to the boiling of epithermal base-metal ore fluids and involvement of seawater. 

Figure 1.103. 8D and 8 0 of ore fluids responsible for epithermal Au-Ag vein-type deposits in Japan (Hattori and Sakai, 1979 Imai et al., 1998). S.W. seawater, M.W. line meteoric water line, KK Kushikino, SG Seigoshi, YUG Yugashima, TK Takatama, FUK Fuke, YN Yatani, KN Kanisawa (Yatani), HK Hishikari. 

SD and 8 0 values for epithermal deposits from other countries are summarized in Fig. 1.105 (Field and Fifarek, 1985). The oxygen shift away from the meteoric water line is always observed, but 8D is similar to meteoric water value, suggesting meteoric water source of epithermal ore fluids. Magmatic contribution to ore fluids has not been found except in some ore fluids responsible for the deposits in the other countries Tui... 

These detailed studies on individual mine district suggest that carbon in carbonates was derived from the country rocks underlying the ore deposits and oxygen in ore fluids is controlled by origin of ore fluids (mostly meteoric water) and boiling of ore fluids. 

Mixing of high temperature hydrothermal solution with high salinity and low temperature solution with low salinity of meteoric water origin seems the most likely mechanism for the base-metal vein-type deposition. 

D and 8 0 data on fluid inclusions and minerals at main stage of epithermal Au-Ag mineralization clearly indicate that the dominant source of ore fluids is meteoric water. Meteoric water penetrates downwards and is heated by the country rocks and/or intrusive rocks. The heated water interacts with country rocks and/or intrusive rocks and extracts sulfur, Au, Ag and other soft cations (e.g., Hg, Tl) from these rocks. If hydrothermal solution boils, it becomes neutral or slightly alkaline, leading to the selective leaching of soft cations such as Au, Ag, Hg and Tl from country rocks. However, a contribution of sulfur gas and other components from magma cannot be ruled out. 

D and 5 0 data on fluid inclusions and minerals, 8 C of carbonates, salinity of inclusion fluids together with the kind of host rocks indicate that the interaction of meteoric water and evolved seawater with volcanic and sedimentary rocks are important causes for the formation of ore fluids responsible for the base-metal vein-type deposits. High salinity-hydrothermal solution tends to leach hard cations (base metals, Fe, Mn) from the country rocks. Boiling may be also the cause of high salinity of base-metal ore fluids. However, this alone cannot cause very high salinity. Probably the other processes such as ion filtration by clay minerals and dissolution of halite have to be considered, but no detailed studies on these processes have been carried out. 

Oxygen isotopic fractionation factors used for the calculation were taken from Taylor (1997). Initial 8 0 value of hydrothermal solution (0%o) was estimated from 8 0 values of K-feldspar and quartz in the veins and homogenization temperatures (Shikazono and Nagayama, 1993), and that of groundwater (—7%c) was estimated from meteoric water value of the south Kyushu district (—7%c) (Matsubaya et al., 1975). 

D, 8 0 and Cl concentration data suggest the mixing of meteoric water, connate seawater and magmatic gas (Seki, 1991) (Fig. 2.20). Br/Cl and B/Cl ratios are different from those of seawater (Fig. 2.21). This difference and N2-H2-Ar gas composition indicate a contribution of magmatic gas (Seki, 1991, 1996). 

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). 

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