Ann: Mt Thirsty Metallurgical Testwork Update, page-13

The heat comes from the exothermic reaction of the acid plant. 70C is actually good. There are other atmospheric pressure acid leach metallurgies that run at 95C - which means they lose a LOT of water through evaporation.

As for Mt Thirsty, the acid plant produces so much exothermic heat their plan it to use the excess heat to run steam steam turbines and generate power. The heat means reduced costs, not increased costs.