A large number of lands are still open to mineral location, but you should know where to find them.
A mining claim gives the holder the right to mine on federal land, while a patent gives him outright ownership of mineral-rich land that belongs to the federal government.
Federal lands can be determined from maps, however there are no maps representing mining claims.
A mining claim can be located on these lands which are open to mineral location. If you have a specific piece of land in mind and no active claims are shown on the county’s records, the land may then be open for staking.
Here is a pretty good Ebook put out by the Bureau of Land Management that will explain things better: BLM Mining Info.