  We investigate the structure of a binary mixture of particles interacting via
purely repulsive (point) Yukawa pair potentials with a common inverse screening
length $\lambda$. Using the hyper-netted chain closure to the Ornstein-Zernike
equations, we find that for a system with `ideal' (Berthelot mixing rule) pair
potential parameters for the interaction between unlike species, the asymptotic
decay of the total correlation functions crosses over from monotonic to damped
oscillatory on increasing the fluid total density at fixed composition. This
gives rise to a Kirkwood line in the phase diagram. We also consider a
`non-ideal' system, in which the Berthelot mixing rule is multiplied by a
factor $(1+\delta)$. For any $\delta>0$ the system exhibits fluid-fluid phase
separation and remarkably the ultimate decay of the correlation functions is
now monotonic for all (mixture) state points. Only in the limit of vanishing
concentration of either species does one find oscillatory decay extending to $r
= \infty$. In the non-ideal case the simple random phase approximation provides
a good description of the phase separation and the accompanying Lifshitz line.
