In event-chain Monte Carlo simulations we model colloidal particles in two dimensions that interact according to an isotropic short-ranged pair potential which supports the two typical length scales present in decagonal quasicrystals. We investigate the assembled structures as we vary the density and temperature. Our special interest is related to the transition from quasicrystal to liquid. We find a one-step first-order melting transition without a pentahedratic phase as predicted within the KTHNY melting theory for quasicrystals. However, we discover that the slow relaxation of phasonic flips, i.e. rearrangements of the particles due to additional degrees of freedom in quasicrystals, changes the positional correlation functions such that structures with long-range orientational correlations but exponentially decaying positional correlations are observed.