43 CFR §3832.33

1. (a)You may locate a mill site in the same manner as a lode or placer mining claim, except that—
   1. (1)It must be on land that is not mineral-in-character; and
   2. (2)You must use or occupy each two and a half acre portion of a mill site in order for that portion of the mill site to be valid.
2. (b)If the United States does not own the surface estate of a particular parcel of land, you may not locate a mill site on that land under the General Mining Law or the Stockraising Homestead Act (see part 3838 of this chapter).