  We study the role played by a Chern-Simons contribution to the action in the
$QED_3$ formulation of a two-dimensional Heisenberg model of quantum spin
systems with a strictly fixed site occupation at finite temperature. We show
how this contribution affects the screening of the potential which acts between
spinons and contributes to the restoration of chiral symmetry in the spinon
sector. The constant which characterizes the Chern-Simons term can be related
to the critical temperature $T_c$ above which the dynamical mass goes to zero.
