I had a look at the 1880 US census and I would agree - it appears it was a census of employees at the mine. However, there are some wives and children included. Perhaps accommodation at the mine headquarters for special instances???

I checked out Matthew on Ancestry.com and discovered several "maybes", and if you don't already have a subscription I can do some further checking for you. I would need more info, ie name of wife, children, etc.

Pat