Q: The rules say that the guardian spear can be both fired and
used in the Assault phase in a single turn, but as it’s fired
as a boltgun (Rapid Fire), the model surely cannot charge,
and could therefore only use it either as a ranged or as a
melee weapon?
A: In the event the Model gains the Relentless special
rule – or fires in Overwatch – it could both shoot and
fight with this weapon. Otherwise, yes, shooting would
prevent you from charging.