Anhydrite precipitation

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).

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. 

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... 

Sr/Ca fractionation during anhydrite precipitation from brines. A solution of calcium sulfate contains 20mmol/kg Ca and 0.2mmol/kg Sr. Calculate the Sr concentration in the residual brine (res) when 50 percent Ca is removed. Use DCaSr = f)Sr/DCa = 0.35. 

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

Figure 15.12. Schematic diagram showing inorganic chemical processes occurring at warm- and hot-water vent sites. Deeply circulating seawater is heated to 350°-400°C and reacts with crustal basalts, leaching various species into solution. The hot water rises, reaching the sea floor directly in some places and mixing first with cold, down-welling seawater in others. On mixing, iron-copper-zinc sulfide minerals and anhydrite precipitate. (From Jannasch and Mottl, 1985.)...

As pointed out by Seal et al. (2000), many studies of ancient hydrothermal systems have utilized equilibrium sulfate-sulfide sulfur isotope fractionation models, but these should be applied with great caution. As shown in Figure 9, seafloor hydrothermal vent fluid 5" Sh2S values do not conform to simple equilibrium fractionation models. Shanks et al. (1981) first showed experimentally that sulfate in seawater-basalt systems is quantitatively reduced at temperatures above 250°C when ferrous minerals like the fayalitic olivine are present. When magnetite is the only ferrous iron-bearing mineral in the system, sulfate-reduction proceeds to sulfate-sulfide equilibrium, but natural basalts contain ferrous iron-bearing olivine, pyroxene, titanomagnetite, and iron-monosulfide solid-solution (mss) (approximately pyrrhotite). It is the anhydrite precipitation step... 

Still later the sulfate concentration in seawater reached 10% of the current value. Anhydrite precipitated in the intake regions of the hydrothermal systems. At that time, the sulfate concentration reaching the deep hot parts of the hydrothermal systems approached the present value. The flux of sulfate reduced per area of seafloor also approached its present value. The vent systems then ceased to be stabilizing buffers on seawater sulfate concentration. The subsequent increases in sulfate concentration resulted in more anhydrite sequestration but did not affect the amount of sulfate that got reduced. 

Calcium in the form of calcium carbonate contained in limestone is added in the neutralization processes after nickel laterite leaching. Calcium sulfate solids in the form of gypsum (and possibly hemihydrate and anhydrite) precipitate and have the potential to form scale on equipment and piping surfaces. 

Gypsum, CaS0i,.2H2 0, solubility in pure water and multicomponent solutions has been the topic of many studies. The reference list at the end of this chapter bears witness to this even though it does not list all the work reported. This body of work includes experimental measurements and attempts to predict gypsum and anhydrite precipitation. 

---