the gap zone

Can anyone help those of us who are in not so well informed?

Just what is the Gap Zone and how did it come about? Why did WMC miss out taking this supposed section of rich ore? And why did they go to the trouble (and cost) of going deeper? Why not take out the ore on the way? Is it in 'bad ground'?

[Before someone exhorts me to DYOR, I rather think the story of the Gap Zone is largely lost (or at least concealed in the mists of time) and some of the well-informed old timers on this thread could very easily satisfy my curiosity.]