Pasco regional mining environmental engineer Juan Escalante said that an unknown quantity of toxic wastewater from the Atacocha mine escaped from a sedimentation well Wednesday into the Huallaga River.
The mine is owned by the Brazilian company Votorantim.
Peru's national water authority granted Atacocha a permit in 2011 to remove metals including mercury, cadmium, lead, iron and manganese from the mine's wastewater and release the treated water into the Huallaga.
Escalante said the area where the spill occurred is in the Andes Mountains at about 13,000 feet.
Peru is the world's No. 2 producer of zinc.