The interaction of two clay minerals, kaolinite and bentonite with gypsum and its effects on the flotation of a copper-gold ore was investigated in this study. It was found that bentonite increased the viscosity more than kaolinite when mixed with the copper-gold ore at low shear rates. The detrimental effect of these clay minerals on flotation was attributed to the entrainment of clay particles when kaolinite was added to the ore and to a decrease in true flotation by bentonite. Bentonite formed a sponge-like structure with predominant edge-edge (E-E) interactions which might affect hydrodynamics in the flotation cell and have a detrimental effect on flotation recovery. Kaolinite did not form a particular network structure and its aggregates mostly consisted of face-face (F-F) type associations which did not affect flotation hydrodynamics. The addition of gypsum to the ore-bentonite mixture inhibited the formation of interconnected network structures. This led to lower viscosity values with flotation behaviour similar to that of mixtures with kaolinite. In this case, there was an improvement in recovery, but the grade decreased due to entrainment. The addition of gypsum to the ore-kaolinite mixture created aggregates with long strings further enhancing particle entrainment with more mass transported to the froth. (c) 2015 Elsevier Ltd. All rights reserved.