A POSSIBLE ROLE OF BOILING IN ORE DEPOSITION: A NUMERICAL APPROACH

Abstract

A possible role of boiling of the H2O-NaCl-CO2 (-H2S) fluid in ore deposition has been examined numerically by using the equations of state (EOS) of Duan et al. (1995, 1996), a modified EOS of Bowers and Helgeson (1983) and the water-rock interaction simulator MIX99 (Hoshino et al., 2000). The following three models are examined to evaluate an efficienc y of boiling on mineral precipitation: (1) hypothetical non-boiling process, (2) hypothetical boiling process with sulfur partitioned only in liquid phases and (3) boiling process in which partition ratios of H2S between liquid and vapor phases are assumed to be the same as those of CO2. The processes are simulated from 450°C and 900 bar to 310°C and 620 bar with an analytical step of 10°C/20 bar. Boiling occurs below 400°C in the latter two processes when the initial composition (in mole fraction) of the fluid is: XH2O = 0.84, XNaCl = 0.10 and XCO2 = 0.06. Ore deposition occurs abruptly at a boiling point when the part ition ratios of total sulfur (Xs vap/ Xs liq) are as high as those of total carbon during boiling. A decrease of concentration of sulfur in the liquid phase during boiling leads to an increase of pH of the solution, resulting in propelling mineral precipitation. It has been made clear that a possible role of boiling in ore formation mainly depends on the partition ratios of sulfur between the liquid and vapor phases, although they cannot be estimated accurately by the currently available EOS.