Mineral precipitation using REACT module

[SebFuchs]

Dear GWB - Team,

I use the REACT module to study the precipitation of minerals when a hot hydrothermal fluid is mixed with cold seawater (simple titration model, only dump command is on).

I can read from the plot (Fig. 1) that sphalerite and gold start to precipitate somewhat above 100°C. With further cooling the mineral concentrations slightly decrease again, which means to me that the minerals (re-)dissolve back into the fluid again (please correct me if I am wrong). The concentrations of these metals in the fluid, however, remain low (Fig. 2) and the Saturation Index (Fig. 3) remain high showing that there is no re-dissolution of the minerals into the fluid. This does not make sense to me.

Is there a fundamental problem in the calculation / model or am I reading Figure 1 in a wrong way?

 

Best greets,

Sebastian

[Brian Farrell]

Hi Sebastian,

Looking only at Figure 1, the decrease in concentration could be explained by dissolution. Another possibility is dilution, due to the addition of seawater. I suggest you compare your plot of minerals in mmol/kg solution units with a second using absolute mmol units. Plot as well the mass of solution as a function of reaction progress or temperature. You might find that as the solution mass increases, the mineral is being diluted, but doesn’t necessarily dissolve. That’s the case for a few of the sulfide minerals in the black smokers example in Craig Bethke’s Geochemical and Biogeochemical Reaction Modeling text. The concentration of dissolved components will also reflect both reaction and dilution.

Are these kinetic minerals? The mineral saturation (Q/K) should be at 1 wherever equilibrium minerals exist. If they’re kinetic minerals, however, they won’t necessarily remain in equilibrium with the fluid, even if present. You can actually plot the dissolution rate (positive rate indicates dissolution, negative rate indicates precipitation) for kinetic minerals. Just look in the Reactant properties variable type.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC