If by the fusion temperature you mean the melting point(?) then generally speaking the magnitude of the melting point of a substance with impurities will be lower than its 'pure' equivalent. In reality you actually get a 'range' of values for the melting point of a substance with impurities, it all depends in the type of impurity and quantity.
The theory behind this is that an impurity will disrupt the formation of the lattice structure of a solid, making it weaker, thus it is easier to break or weaken the bonds between the molecules in the solid. This means that less energy is required to break the lattice structure, hence a the melting point is lowered.
Hope this helps, if not let me know and I'll try again!