It's a bit early in the morning, but if I recall correctly, radioactive decay within the granite provided the heat to drive the circulating fluids. The Permian Zechstein sea was highly saline, so the circulating fluids were as well, these concentrated the minerals, precipitating them in the veins and flatts.
The heat is still there, with boreholes on the Slit vein at Eastgate and the 90 Fathom dyke in Newcastle demonstrating high heat flows and the potential for geothermal energy.