As Webby says below:
9. Mining: The upper part of a lode, or the roof of a horizontal underground passage.
I learned this term while touring the Vale Chasm demonstration mine at the Science North Dynamic Earth attraction in Sudbury, Ontario. In a mining context, back refers to to the roof of a cavern where mining takes place.
Why does the roof need a special term that's not, well, roof?
Hard rock mining typically happens oppositely to the way you might intuitively imagine. Mining often starts at the deepest part of the mineral vein, and then moves progressively upward. The (greatly simplified) reason for this is that mine construction starts with a large central vertical (or sloped) mine shaft, which is sunk near the lode. During the life of the mine, that central shaft carries men and equipment. It also takes broken ore (called muck) to the surface. It is most practical to build the muck conveyor system once, at the lowest work level. It would be costly and inefficient to keep relocating this equipment lower if work moved progressively downwards, or to haul muck up to it. Thus, the deposit is mined bottom-up. As work moves upwards, muck is dropped down shafts to the bottom level of the mine. Here, mucking machines scrape up and load the ore for transport to the surface. Thus, the back of the lode is part of the active work surface of the mine, along with the stope.
BQ'16 247