The development of each project has been halted by community resistance centred on fears about contamination of water sources.
"We have already decided this. Conga and Tia Maria are unviable cases. I have been a watchman and as a social fighter, I defend the aquifer buffers," Castillo said in Cajamarca, according to local press.
"We must respect mining extraction has to take place where nature and mining extraction allow it. They will not go ahead and will never go ahead."
Castillo represents the radical left Partido Peru Libre party, which included in its manifesto the intention to nationalise major mining operations as part of a plan to create a socialist state, specifically identifying the Yanacocha, Antamina, Toromocho, Marcona, Cuajone, Quellaveco and Toquepala mines.
Castillo will face Keiko Fujimori in the June run-off in what is viewed as a left-versus-right battle in which neither candidate appeals to centrist voters.