Wherever the removed overburden is, it would make a much bigger pile than that; look at its size relative to that of the pit.
I think you are right and I am wrong. I cannot find anything that clearly maps it out. I played around on the 3D in Google Earth and that likely was a “mountain” that had been “edged” but I could be wrong.
What is a nice view without answering the question is this:
http://www.3d-exposure.com/panos1/copper-mine-1.html