  We consider a wide class of quantum spin systems obtained by adding a
transverse field to a classical Hamiltonian. We give explicit high-temperature
conditions which guarantee exponential decay of correlations. A
stochastic-geometric representation allows to reformulate the model as a
classical random field on a space of marked point processes on the circle
$[0,\beta)$, where $\beta$ is the inverse temperature. We then construct a
Markov process having this random field as invariant measure. By the mixing
properties of the process, the exponential decay of correlations follows by an
adaptation of a general argument.
