I thought that one of the nugget hunters might have jumped in here for you Bill. I'm not one of them, but I'll do my best to explain.
A mullock heap is the debris that is removed from a mine; in this case it is usually referring to what the Old times removed when they were digging into the ground and following a gold seem. The dirt was dumped into mounds all over the place. Being removed from gold producing ground, meant that there is some gold that is missed. It can also refer to the dirt that was processed by the miners in which they separated the gold from the rest of the 'wash' (dirt). The equipment they used relied on gravity to trap the gold, and also their eyes. In reality they missed half of the gold, which makes the mullock heaps a valuable place to look for it.
As far as reef heaps go, I would assume (dangerous thing to do when detecting) that would be very similar in nature to a mullock heap, but slightly different in location. Gold can be found different forms. A reef of gold, is simply referring to a main body of gold, at the source of where it was formed. I'm guessing here, but assume that it's just referring to the soil that has been processed at this location.
Hopefully, one of the more experienced guys can step in here and tidy up my explanation.
Mick Evans.
PS A dummy spit looks something like this.
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