Numerical simulation of fluid flow and silica transport and deposition in boiling hydrothermal solutions: Application to epithermal gold deposits

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Abstract
A numerical fluid flow model quantifying silica precipitation in hydrothermal systems as a function of temperature, permeability, and mass flux has been employed to evaluate the role of boiling in precipitating quartz in the epithermal environment. Relative permeability relationships of liquid and vapor phases incorporated into the physical model have been modified to be temperature dependent and are consistent with current understanding of fluid flow in one‐component, two‐phase systems. The modifications extend the height of the boiling column and eliminate discontinuities in steam content on initiation of boiling. Results indicate that the degree to which boiling contributes to quartz precipitation is dependent on three related factors: the temperature of the fluid entering the base of the boiling system, the rate of fluid temperature decrease with respect to the distance of fluid travel, and the extent of fluid vaporization, particularly in regions of gradual temperature decline. Boiling contributes significantly to quartz precipitation in systems with high‐temperature fluids, and in deeper portions of systems in which extensive vaporization occurs. Temperature reduction is the dominant quartz precipitation mechanism in regions where temperature reduction is rapid, and in lower temperature systems. Owing to the smaller absolute difference in quartz solubility between the liquid and vapor phases at low temperatures as compared to higher temperatures, boiling is a less important precipitation mechanism in low temperature, nearsurface regions. Quartz precipitation is most intense in systems with short column heights, i.e., systems with high mass flux/permeability ratios, and low initial fluid temperatures. Vertical permeability variations within the flow channel produce steam jumps, or discontinuities, in mass fraction steam profiles, resulting in local zones of enhanced quartz precipitation or dissolution. Ore grade calculations indicate that high mass flux rates, low permeabilities, and low initial fluid temperatures promote precipitation of high gold grades in response to boiling. Under these conditions gold precipitates over a smaller interval of temperature decline and over a shorter ore horizon height.