In this study, the flotation of a copper gold ore in the presence of 10% bentonite in tap water and sea water was investigated. It was found that bentonite had a deleterious effect on copper and gold flotation in tap water, but this deleterious effect was mitigated in sea water. Rheology measurements, settling tests and Cryo-SEM analyses were conducted to understand the underpinning mechanism. It appears that the high viscosity produced by bentonite due to its swelling capacity and cross-linked network structures negatively affected flotation hydrodynamics and therefore copper and gold flotation in tap water. It is interesting that sea water reduced the swelling capacity of bentonite and modified the association modes of bentonite platelets in flotation pulp, resulting in the breakup of links between the structures with relatively large pores, which contributed to the improvement of copper and gold flotation. (c) 2015 Elsevier Ltd. All rights reserved.