Seawater-rock interaction

Several workers have intended to estimate the chemical compositions of Kuroko ore fluids based on the chemical equilibrium model (Sato, 1973 Kajiwara, 1973 Ichikuni, 1975 Shikazono, 1976 Ohmoto et al., 1983) and computer simulation of the changes in mineralogy and chemical composition of hydrothermal solution during seawater-rock interaction. Although the calculated results (Tables 1.5 and 1.6) are different, they all show that the Kuroko ore fluids have the chemical features (1 )-(4) mentioned above. 

As will be discussed later, the experiments (Hajash, 1975 Mottl and Holland, 1978) and theoretical studies on seawater-rock interaction (Wolery, 1978 Reed, 1983) indicate that the Kuroko ore fluids characterized by (l)-(4) above are formed by seawater-crustal rock interaction at elevated temperatures. 

Figure 1.48. Change in the strontium content of anhydrite precipitated during the heating of normal seawater without any seawater-rock interaction (Shikazono et ah, 1983).

Origin of ore fluids is constrained by (1) chemical compositions of ore fluids estimated by thermochemical calculations (section 1.3.2) and by fluid inclusion analyses, (2) isotopic compositions of ore fluids estimated by the analyses of minerals and fluid inclusions (section 1.3.3), (3) seawater-rock interaction experiments, (4) computer calculations on the seawater-rock interaction, and (5) comparison of chemical features of Kuroko ore fluids with those of present-day hydrothermal solutions venting from seafloor (section 2.3). 

During the last two decades, many experimental studies on the seawater-rock interaction at elevated temperatures (100-400°C) have been conducted. Particularly, detailed seawater-basalt interaction experiments have been done. Several experimental studies on seawater-rhyolite interaction and seawater-sedimentary rock interaction are also available (Bischoff et al., 1981). Examples of chemical compositions of modified seawater experimentally interacted with various kinds of rocks are shown in Table 1.9. 

The above argument on the calculation of chemical composition of ore fluids, seawater-rock interaction experiments, and isotopic compositions of ore fluids clearly demonstrates that Kuroko ore fluids were generated by seawater-rock interaction at elevated temperatures. The chemistry of present-day hydrothermal solution venting from back-arc basins and midoceanic ridges (sections 2.3 and 2.4) also support this view. 

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. 

Wolery (1978) and Reed (1982, 1983) have indicated based on a computer calculation of the change in chemistry of aqueous solution and mineralogy during seawater-rock interactions that epidote is formed under the low water/rock ratio less than ca. 50 by mass. Humphris and Thompson (1978), Stakes and O Nell (1982) and Mottl (1983) have also suggested on the basis of their chemical and oxygen isotopic data of the altered ridge basalts that epidote is formed by seawater-basalt interaction at elevated temperatures (ca. 200-350°C) under the rock-dominated conditions. If epidote can be formed preferentially under such low water/rock ratio, the composition of epidote should be influenced by compositions of the original fresh rocks. 

Stable isotopic studies of 5 0 and 8D of hydrothermal solutions venting from back-arc basins show no evidence of contribution of magmatic fluids to the hydrothermal solutions at back-arc basins and midoceanic ridges. As noted already, the stable isotopic data (S S, S C, S 0, 8D) all indicate that hydrothermal solutions in submarine hydrothermal system in back-arc basins and midoceanic ridges were generated by seawater-rock interaction at hydrothermal conditions. 

Chemical and isotopic compositions of Kuroko ore fluids are reasonably explained by the seawater-rock interaction at elevated temperatures (200-350°C) in submarine hydrothermal system at back-arc basins. 

Seawater intrusion is usually limited to a distance of 1-2 km from the sea, but it varies. The depth at which seawater-induced salinization occurs increases with the distance of the well from the sea. The invading seawater may have direct hydraulic interconnection with the open sea, or it may originate from a stagnant compartment of seawater that was trapped during past hydrological conditions. The latter case will be recognizable because of the old age of the saline water and by deviations from the present seawater composition, reflecting seawater-rock interactions. 

Material supplied to the ocean originates from tlie atmosphere, rivers, glaciers and hydrothermal waters. The relative importance of these pathways depends upon the component considered and geographic location. River runolf commonly constitutes the most important source. Transported material may be either dissolved or particulate, but discharges are into surface waters and confined to coastal regions. Hydrothermal waters are released from vents on the seafioor. Such hydrothermal waters are formed when seawater circulates into the fissured rock matrix, and under conditions of elevated temperature and pressure, compositional changes in the aqueous phase occur due to seawater - rock interactions. This is an important source of some elements, such as Li, Rb and Mn. The atmosphere supplies particulate material globally to the surface of the ocean. In recent years, this has been the most prominent pathway to the World... 

Therefore, MgO content of altered rocks indicates the extent of seawater-rock interaction (degree of alteration). The contents of some elements (Fe, H2O) positively correlate with MgO content, but some (CaO, K2O) negatively do. 

Sverjensky (1984) calculated the dependency of Eu +/Eu + in hydrothermal solution on /oj (oxygen fugacity), pH and temperature. According to his calculations and assuming temperature, pH and /oj for epidote-stage alteration of basalt and Kuroko ores (Shikazono, 1976), divalent Eu is considered to be dominant in the rocks and hydrothermal solution. Thus, it is reasonable to consider that Eu in the rocks was removed to hydrothermal solution under the relatively reduced condition more easily than the other REE which are all tiivalent state in hydrothermal solution. Thus, it is hkely that Eu is enriched in epidote-rich altered volcanic rocks. Probably Eu was taken up by the rocks from Eu-enriched hydrothermal solution which was generated by seawater-volcanic rock interaction at relatively low water/rock ratio. 

A negative correlation between Mg content and Ca content of hydrothermally altered basalt and dacite from the Kuroko mine area exists. This correlation indicates that Ca in the rocks is removed to fluid by the exchange of Mg in seawater. Eu may behave in the manner similar to Ca during seawater-volcanic rock interaction because of the similarity of their ionic radii. 

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. 

These differences are considered to be attributed to the dilferences in compositions of rocks and alteration minerals interacted with circulating seawater or modified seawater at elevated temperatures. For example, high K and Li concentrations in the hydrothermal solution in the Mid-Okinawa Trough baek-arc basin (Jade site) are due to the interaction of hydrothermal solution with acidic volcanic rocks (Sakai et al., 1990). It is evident that the chemical compositions of hydrothermal solution are largely alfected by water-rock interaction at elevated temperatures. 

Kawahata and Shikazono (1988) summarized S S of sulfides from midoceanic ridge deposits and hydrothermally altered rocks (Fig. 2.42). They calculated the variations in 5 " S of H2S and sulfur content of hydrothermally altered basalt as a function of water/rock ratio (in wt. ratio) due to seawater-basalt interaction at hydrothermal condition (Fig. 2.43) and showed that these variations can be explained by water/rock ratio. The geologic environments such as country and host rocks may affect S S variation of sulfides. For example, it is cited that a significant component of the sulfide sulfur could... 

These differences are caused by the influences of /02, /s2> pH, temperature of ore fluids and chemical compositions of rocks interacted with seawater in hydrothermal system. 

We saw in section 2.3.2 that present-day hot spring venting and sulfide-sulfate depositions have been discovered in back-arc basins in the Western Pacific. These intense hydrothermal activities indicate that seawater-volcanic rock interactions are taking place at these environments. 

The Mg content of hydrothermally altered volcanic rocks is reflected by the extent of seawater-volcanic rock interaction at elevated temperatures, because it has been experimentally and thermodynamically determined that nearly all of the Mg in seawater transfer to volcanic rocks, owing to the reaction of the cycled seawater with volcanic rocks at elevated temperatures (Bischoff and Dickson, 1975 Mottl and Holland, 1978 Wolery, 1979 Hajash and Chandler, 1981 Reed, 1983 Seyfried, 1987). It has been shown that the CaO content of hydrothermally altered midoceanic ridge basalt is inversely correlated with the MgO content with a slope of approximately — 1 on a molar basis (Mottl, 1983). This indicates that Ca of basalt is removed to seawater and Mg is taken up from seawater by the formation of chlorite and smectite during the seawater-basalt interaction. This type of reaction is simply written as ... 

Kaiho and Saito (1994) estimated 20 x 10 km /m.y. and 2x 10 km /m.y. for present-day midoceanic ridge crustal production rate and back-arc basin crustal production rate, respectively. If their estimates are correct. Mg removal to midoceanic ridge basalt during early-middle Miocene age is estimated to be 2.6 1 x 10 g/year. Although estimates of annual Mg removal by interaction of circulating seawater with midoceanic ridge basalt are uncertain, it seems likely that Mg removal by seawater-volcanic rock interaction at back-arc basins corresponds to that of Mg removal at midoceanic ridge axis. 

Using 2.5 X lO g/m.y. as oceanic production rate and 5-20 as seawater/rock ratio and assuming that 30% of oceanic crust interacts with circulating seawater, and the crustal production rate is (0.8-1.1) x 10 kg/m.y., then the rate of seawater cycling through back-arc basin is estimated to be (4-22) x lO kg/m.y. Using this value and the CO2 concentration of hydrothermal solution ((0.05-0.3) mol/kg -H20) (Table 3.2), hydrothermal CO2 flux into the ocean is estimated to be (0.2-6) x lO kg/m.y. 

Shikazono, N. (1994) Hydrothermal alteration of green tuff belt, Japan Implications for the influence of seawater/volcanic rock interaction on the seawater chemistry at a back arc basin. The Island Arc, 3, 59-65. 

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). 

Oxygen isotopes. 5 0 in ocean-crust studies is typically dehned as the per mil deviation in 180/1 0 ratio of a rock relative to a standard mean ocean water (S Osmow) and it is widely used to understand ocean-crust alteration processes. Fresh MORB has an S Osmow value of - -5.7%o, and water-rock interaction with seawater (S OsMow = at low temperatures increases the value, while high-temperature alteration decreases it. Muehlenbachs and Clayton (1972) drew attention to this relationship and suggested that hydrothermal alteration of the crust may buffer the oxygen isotopic composition of seawater. Oxygen is the major component in the oceanic crust, and therefore, changes in 5 0 are a rather profound indicator of hydrothermal alteration. 

---