The estimate of 25 -30% cost for fuel was something that was told to me by a knowledgeable engineer. To me that seems quite reasonable as someone pointed out here that the type of trucks they use in mining are huge monster trucks that can haul up to 400t of rock. Clearly something this big will consume fuel like there's no tomorrow. In fact, I read somewhere that these trucks consume 3-4 gallons of diesel fuel for every mile. Compare that to the average dump truck that gets 6-10mpg.

In an average open pit mine you have dozens, if not hundreds for huge operations over 200K tons/day, of these trucks running constantly and you can see why fuel is one of the biggest expense in the operation.