  Let \Gamma be a geometrically finite tree lattice. We prove a
Khintchine-Sullivan type theorem for the Hausdorff measure of the points at
infinity of the tree that are well approximated by the parabolic fixed points
of G. Using Bruhat-Tits trees, an application is given for the Diophantine
approximation of formal Laurent series in the variable 1/X over the finite
field Fq by rational fractions in X over Fq, satisfying some congruence
properties
