Anhydrite hydrothermal

Negative Eu anomaly is observed for hydrothermally altered dacite underlying the Kuroko ores and anhydrite in the dacitic tuff breccia. 

Figure 1.46. REE patterns of the altered volcanogenic rocks and Kuroko ores. Data sources Shikazono (1999a). (A) Hydrothermally altered dacite and anhydrite underlying the Kuroko ores. (B) Barite, Kuroko ore and ferruginous chert. (C) Hydrothermally altered basalt overlying the Kuroko ores (Shikazono, 1999a).

The REE pattern for anhydrite is different from that of seawater, indicating that anhydrite did not precipitate due to the simple heating of seawater that was suggested by Sakai et al. (1970) and Sato (1973). This REE pattern could be explained in terms of the mixing of hydrothermal solution and cold seawater and low degree of seawater/hydrothermal solution mixing ratio (Shikazono et al., 1983). 

The precipitation of anhydrite from hydrothermal solutions has been studied extensively by various workers (e.g., Marshall et al., 1964a,b). The salinity of the inclu-... 

Figure 1.49. Change of the strontium content and Sr/ Sr ratio of Kuroko anhydrite during the deposition and dissolution due to the mixing of hot ascending solution and cold solution (normal seawater) (Shikazono et al., 1983). R mixing ratio (in weight) = S.W./(S.W.+H.S.) in which S.W. and H.S. are seawater and hydrothermal solution, respectively. Open triangle Fukazawa deposits. Solid triangle Hanawa deposits. Open square Wanibuchi deposits. Solid square Shakanai deposits. Concentration of Ca, Sr " " and SO of H.S. are assumed to be 1,(XX) ppm, 1 ppm, and 10 mol/kg H2O, respectively. Concentrations of Ca, Sr " and SO of S.W. are taken to be 412 ppm, 8 ppm, and 2,712 ppm. Temperatures of H.S. and S.W. are assumed to be 350°C and 5°C (Shikazono et al., 1983).

Amorphous silica and barite precipitate simultaneously from white smoker in midoceanic ridge hydrothermal system (Edmond et al., 1979). It is inferred that amorphous silica precipitates in the chimney at a later stage than sulfides and sulfates (anhydrite and barite) which constitute chimneys from which black smoker is emerging. 

Isotopic compositions of minerals and fluid inclusions can be used to estimate those of Kuroko ore fluids. Estimated isotopic compositions of Kuroko ore fluids are given in Table 1.10. All these data indicate that the isotopic compositions lie between seawater value and igneous value. For instance, Sr/ Sr of ore fluids responsible for barite and anhydrite precipitations is 0.7069-0.7087, and 0.7082-0.7087, respectively which are between present-day. seawater value (0.7091) and igneous value (0.704-0.705). From these data, Shikazono et al. (1983), Farrell and Holland (1983) and Kusakabe and Chiba (1983) thought that barite and anhydrite precipitated by the mixing of hydrothermal solution with low Sr/ Sr and seawater with high Sr/ Sr. 

The Okuaizu geothermal system is characterized by high temperatures (maximum 340°C), high salinity (about 2 wt% total dissolved solids (TDS)) and large amounts of non-condensable gases (1 wt% CO2 and 200 ppm H2S). The pH of the hydrothermal solution measured at 25°C is 6.44 (Table 2.6). However, the pH of the original fluid in the reservoir is computed to be 4.05. This pH as well as alkali and alkali earth element concentrations are plotted near the equilibrium curve of albite, K-mica, anhydrite and calcite (Fig. 2.19) (Seki, 1991). 

The H2S concentration of hydrothermal solution is plotted in Fig. 2.33. Based on these data, we can estimate the temperature of hydrothermal solution buffered by alteration mineral assemblages such as anhydrite-pyrite-calcite-magnetite and pyrite-pyrrhotite-magnetite for Okinawa fluids. 

For example, assuming anhydrite-magnetite-calcite-pyrite-pyrrhotite buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations of 21 °N EPR fluid indicate a temperature of 370-385°C. However, the estimated temperatures are higher than those of the measurement. This difference could be explained by adiabatic ascension and probably conductive heat loss during ascension of hydrothermal solution from deeper parts where chemical compositions of hydrothermal solutions are buffered by these assemblages. 

It is well known that anhydrite, pyrite, magnetite, and epidote are widespread in the country rocks in the Kuroko mine area. Therefore, it is likely that this assemblage controls the chemistry of the hydrothermal solution. 

The pH of hydrothermal solution of white smoker from which anhydrite is precipitating shows very low 2 for Lau Basin Vail Lili fluid. This low pH cannot be explained only by water-rock interaction process. One likely explanation is decreasing of pH due to precipitation of sulfides. The pH decreases by the following reaction,... 

Farrell and Holland (1983) cited ba,sed on Sr isotope study on anhydrite and barite in Kuroko deposits that the most appealing model for the formation of Kuroko strata-bound ores would seem to entail precipitation of the minerals from a hydrothermal solution within the discharge vent or in the interior of a hydrothermal plume formed immediately below above the vent exit in the overlying seawater (Eldridge et al., 1983). The study on the chimney ores from Kuroko deposits support this model which is discussed below. 

Sulfates (barite and anhydrite) precipitate due to the mixing of discharging hydrothermal solution with cold seawater above the seafloor at an early stage of hydrothermal activity. Ca and Ba in hydrothermal solution react with SO in cold seawater, leading to the precipitations of anhydrite and barite. It is observed that anhydrite precipitated earlier than barite. This may depend on the initial Ca and Ba concentrations of end member hydrothermal solutions, temperature and degree of mixing of hydrothermal solutions and... 

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. 

In locations where hydrothermal fluid exits directly into the bottom ocean with little subsurfece dilution, large chimneys are deposited by the rapid precipitation of anhydrite... 

The formation of these anhydrite walls prevents the hydrothermal fluids flowing through the chimney from mixing with seawater and provides a framework to enable precipitation of sulfide minerals. In some cases, the discharge of fluids is so rapid that the sulfide precipitates are emitted as clouds of black particles moving at a speed of... 

Anhydrite A mineral composed of calcium sulfate. It is a common hydrogenous mineral deposited in hydrothermal systems. 

To the category of ancient hydrothermal seafloor ore deposits belong volcanic associated massive sulfide deposits. They are characterized by massive Cu-Pb-Zn-Fe sulfide ores associated with submarine volcanic rocks. They appear to have been formed near the seafloor by submarine hot springs at temperatures of 150-350°C. Massive sulfide deposits have 5 S-values typically between zero and the 5-value of contemporaneous oceanic sulfate, whereas the sulfate has 5-values similar to or higher than contemporaneous sea water. According to Ohmoto et al. (1983) the ore-forming fluid is evolved sea water fixed as disseminated anhydrite and then reduced by ferrous iron and organic carbon in the rocks. 

Sleep N. H. (1991) Hydrothermal circulation, anhydrite precipitation, and thermal structure at ridge axes. J. Geophys. Res. 96, 2375-2387. 

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