Great photo.
I'd guess 1890s, but there really isn't anything to specifically date the photo.
Note that the "power house" is actually labeled as as a "compressor house", to generate compressed air for operating the drills.
The pile of pipe in the foreground looks a bit large for compressed air piping, perhaps it is stock piled ventilation pipe, as discussed in the Blue River branch thread.
Jim Courtney
Poulsbo, WA