If I remember correctly they've mentioned  a few times that in this area these veins historically average around 1500 meters in depth. 

Considering the size of the Golden Mile I'd be absolutely stunned if it didn't turn out to have at least that....In my limited experience with geology, when something is large, it tends to be large in all dimentsions.

Now hopefully there is gold all the way to the bottom, so we can all retire when the results come in!