Big Thunder Gold Mine (604 Blair, Hwy. 40, 605/666-4847 or 800/314-3917, www.bigthundermine.com [1], June–Aug. daily 8 a.m.–8 p.m., May and Sept.–Oct. daily 9 a.m.–6 p.m., closed in off-season, adult $8.75, child $5.75) is located just a few blocks east of U.S. 16A on Highway 40.
The facility has a small but nice gift shop and cafeteria on the grounds as well as the mine. The mine tour is somewhat short but the tour does include some interesting facts about the history of mining in Keystone [2] and there is a good collection of mining equipment inside the adjacent museum, which is included in the tour.
Gold panning ($5.75 with the tour, $7.75 without the tour) seems to be the highlight of the mine. Staring raptly at the bottom of their gold pans, children and adults alike demonstrate how easy it is to catch gold fever.
Links:
[1] http://www.bigthundermine.com
[2] http://www.moon.com/destinations/mt-rushmore-the-black-hills/mount-rushmore-and-the-central-hills/keystone