The present study investigates the effect of aeration and diethylenetriamine (DETA) on the selective depression of pyrite in a porphyry copper-gold ore, after regrinding (at grind sizes, d(80) = 38 and 8 mu m) with respect to Au recovery and grade using oxygen demand tests, flotation, QEMSCAN, X-ray spectroscopy (XPS) and EDTA extraction analysis. It was found that pyrite depression increases after aeration and with decreasing grind size. This was observed to be due to the markedly higher oxygen consumption rate of pyrite at the 8 mu m (kla = 0.10 min(-1)) than at the 38 mu m grind size (kla = 0.02 min(-1)). The addition of DETA improved pyrite depression (9% with aeration only versus 39% with aeration + DETA) at the 38 jam grind size. Gold and copper flotation recovery followed pyrite recovery for the two grind sizes using XD5002 in the presence of air and DETA. The surface analysis (XPS and EDTA extraction) revealed that the significant pyrite depression at the 8 mu m grind size was due to increased amount of surface iron oxides, oxy-hydroxides (Fe-O/OH), sulphate species and increased liberation of mineral phases (QEMSCAN analysis), whilst the poorer pyrite depression at the 38 mu m grind size was due to insufficient liberation of mineral phases and the persistence of activating Cu on the pyrite surface. The addition of DETA increased pyrite depression at the coarser grind size due to a significant reduction in Cu(I)S and increased Cu(II)-O species, correlating with the flotation results of pyrite under this test condition. Two-stage copper and pyrite flotation, followed by Au cleaning after regrinding to 38 mu m grind size, under high pH or aerated condition is proposed as the recommended route to optimise Au flotation. (C) 2014 Elsevier Ltd. All rights reserved.