I am afraid that my geology studies were limited to first year...

I am afraid that my geology studies were limited to first year university as I went on to major in physics. However, as I understand it, most mineral deposits result from ancient volcanic activity and so I would expect the extent would often be quite large although the grade could only be tested by drilling. Grade would, I expect, depend on the make up and variety contained in the original heated rock intrusions. Whatever, I would expect the deposit to be much more than 15 metres deep unless we are looking at just what is left after millions of years eroding away.

As I said my geology is poor so don't rely on it much.