The subject of "Mineable Widths" has been discussed on this board before. Once was March 09, and there are methods of mining narrow width veins. Here is one:

http://www.australiancontractmining.com.au/pdf/acm.pdf

"As a guide any large ore body that can be extracted economically using sub level, long hole, open stoping techniques can be mined successfully using Alimak mechanised stoping methods easily breaking over 4,000 tonnes per day with major reductions in over all mine waste development and with significantly less dilution reducing your costs dramatically. Even one metre wide narrow vein ore bodies with short strike lengths and low tones per vertical metre can be mined at over 1,000 tonnes per day using the Australian Contract Mining Alimak Stoping method."