Thanks for you answer.
I got mixed up with John Biggar's information saying that the Nevado Cachi is also called Libertador. As you may have seen on Google Earth there are actually two different summits : Cachi (6334m with coordinates 24°55'51.24"S / 66°23'23.57"W
) and Libertador (5983m).
I guess you spotted the same...
Anyway if you find any good description on the ascent, please let me know