pH Calculations for Hydrothermal Fluids
In the last chapter the relationship between alteration mineralogy and fluid chemistry was discussed using a greatly oversimplified scheme for estimating the pHt of deep system fluids. We use this as a starting point to consider some further implications of the fluid- mineral equilibria we have considered and then develop some calculation procedures to improve our calculation of pHt for high temperature fluids from analytical data.
Drill core from hydrothermal systems contains a number of secondary minerals such as Kmica, K-feldspar, and epidote that result from the alteration of primary minerals. In many hydrothermal studies it is assumed that fluid composition and pH were controlled--or buffered—by simple alteration phase equilibria. We can examine this proposition as follows. If the pH controlling equilibria are