  In the classical pyrochlore lattice Heisenberg antiferromagnet, there is a
macroscopic continuous ground state degeneracy. We study semiclassical limit of
large spin length $S$, keeping only the lowest order (in 1/S) correction to the
classical Hamiltonian. We perform a detailed analysis of the spin-wave modes,
and using a real-space loop expansion, we produce an effective Hamiltonian, in
which the degrees of freedom are Ising variables representing fluxes through
loops in the lattice. We find a family of degenerate collinear ground states,
related by gauge-like $Z_2$ transformations and provide bounds for the order of
the degeneracy. We further show that the theory can readily be applied to
determine the ground states of the Heisenberg Hamiltonian on related lattices,
and to field-induced collinear magnetization plateau states.
