: Even melted down, gold would be a very poor chemical propellant. I can only surmise the gold acted as a medium or conduit for some kind of advanced gravity drive, perhaps of a type that projects a mini-gravity pull in the direction desired. Much in the same way the TARDIS console requires (or required) liquid mercury as a conduit for its advanced systems. While capable of lifting the vessel, its gold stores at takeoff were just barely insufficient to provide the field energy required to achieve orbital speed. The gold arrow (likely supplemented with a tracking device identical to what the Doctor used at the archery tournament) directly interacted with the energy field enveloping the vessel's outer hull on contact and provided the 'spark,' if you will, to achieve escape velocity.
Not sure if serious.