Slurries in mineral processing typically have average particles sizes of 20 to 100μm with maximum sizes of up to 500 to 1000μm. With solids volumetric concentrations varying from a few percent up to and over 40% the corresponding slurries are here termed complex because they cover an intermediate area between "non-settling" and "settling" types of mixtures. Depositional and flow resistance behaviours, some of which may be considered counter-intuitive, are discussed based on reported observations with heavy ore concentrates, tailings and sand slurries.