I've just thought that the ions available for dissolution would...

I've just thought that the ions available for dissolution would be severely limited by the nuclear decay process which would not be affected by the movement in water equilibrium point, removing any of the daughter nuclides through scaling, would not alter the rate at which this soluble product was produced. To the best of my knowledge you can't speed up or slow down a decay rate (well not without some high end experimental physics anyway).
The question then becomes one of what will be the increase in surface area of granite exposed to water in the field over time, which would be the only way to refresh supply, if there is no movement of water in or out of the field area and therefore the recirculating water inventory. But I am still waiting for an answer on trans boundary envelope water movement! The water chemistry in this project is definetly important. Its not just a question of thermal flows.