I don't really want to spend the time looking through the drill from eagles nest but I would

imagine that any ore is reltively close in density to the majority of the waste sulphide material (sulphide intrusion) around it, which is probably around double the density of the waste/country rock.

But all of this is besides the point as it is highly unlikely that either the weight of the core or the rods played much any role in the ability to drill. A driller only brings up the entire drill stem if s/he has to change a bit or something jams, and the same length of core is always brought up (I am not to familiar with surface operations but underground it is typically 10') on a wire line inside the hollow drill stem.

I would imagine ground conditions make is way to expensive to continue drilling, it would probably not be impossible it just doesn't make sense to spend the money and time drilling.