...I doubt that radioactive elements beneath the surface of Charon would be sufficient to energize its geysers...alone.

I'm convinced tidal flexing would have a major role, even though
I've admitted from the start that that far from the sun the heat energy
generated by such flexing would be puny compared to what happens on the jovian moons Io and Europa....
Feeble as the tidal flexing energy would be out at Plutonian distances,
it might be enough to repeatedly fracture Charon's crust, releasing trapped inert gases, causing them to spew out mingled ice crystals onto Charon's surface as well.
I hope this spacecraft bound for Pluto will reveal evidence one-way
or the other.

cheers.

Jacob