The mining of vertical or sub-vertical ore "pipes" is very...

The mining of vertical or sub-vertical ore "pipes" is very common. There may be a small open cut and this devolves into underground mining.

This style of mining or "stoping" is efficient as there is limited horizontal development - there is a drive above and below the section to be mined - long hole drilling and there the section is blasted to fall into the lower drive where it can be mucked out and delivered to the surface. It does depend on the strength of the surrounding rock but I understand that this rock was bending the drilling rods.

A link gives more details:

http://www.ct.ufrgs.br/laprom/Underground%20Mining%20Methods.pdf