Froth floatation process - why CuSO4 is activating floating character of ZnS?
I'm going to consult the literature later, but for me it seems to be as follows.

In the compounds discussed, there are following cations:


$\ce{Fe^2+}$. It is a transition metal with a moderate affinity to oxygen, a slightly larger affinity to sulfur and a high affinity to cyanide. 
$\ce{Zn^2+}$. It is a small cation with a high affinity to oxygen, a slightly larger affinity to sulfur and a moderate affinity to cyanide.
$\ce{Pb^2+}$. It is a large cation with a moderate affinity to oxygen and a slightly larger affinity to sulfur.


Xanthate bind to the surface via the sulfur atoms. So, in a basic cyanide solution the binding sites on the surfaces of pyrite and sphalerite are both completely filled with cyanide and hydroxide respectively (please, note that sulfur in xanthate is a lot less nucleophilic than in sulfide form). Copper has a high affinity to sulfur, a lot larger than zinc or iron. So it binds to xanthate and to the sulfur in either pyrite or sphalerite. However, the surface of pyrite is still occupied with cyanide, which binds really strongly there. And in an acidic environment, cyanide is transformed into $\ce{HCN}$ and thus the pyrite surface is open to binding with xanthate.