Of the recorded production of 34,694,552 troy ounces of gold mined in South Dakota through 1971, about 90 percent has come from Precambrian ore bodies in the Homestake mine at Lead in the northern Black Hills. The site of the original outcrop of the deposit has outliers of the Deadwood Formation (Cambrian) in its immediate vicinity, and the edge of the main body of Paleozoic cover is as little as 2 miles (3.2 km) away. Had erosion never removed the Paleozoic blanket from a small but critical area, the Homestake deposit might still be undiscovered.
With the exception of the Homestake mine and the Bald Mountain mine, which is in the Deadwood Formation 3 miles (almost 5 km) west of Lead, all the gold mines in the Black Hills have been closed for more than 50 years. Many have not been worked since the period of intense gold activity near the turn of the century. Few workings are now accessible, and opportunities for detailed geologic mapping and comprehensive modern geologic investigations have not existed for more than a generation.
Renewed gold activity in the Black Hills seems inevitable. Old mines will be reopened, though most of them are small and have had little or no success. Only one mine, the Homestake, has been large by modem standards; two other groups of properties, the Bald Mountain and the Golden Reward have produced nearly 1 million ounces of gold apiece; only four other localities have exceeded the 100,000 ounce level.
Obviously, an effective enlargement of Black Hills gold production can be achieved only by finding deposits at least as large as those of the Bald Mountain or the Golden Reward operations and preferably of a size approaching that of the Homestake deposit. Perhaps the Homestake is the only major gold deposit in the Black Hills, but existing evidence, indicates that areas of considerable potential have been only slightly investigated or totally neglected.
Many thousands of prospect pits dot the Black Hills. Most gold pits are in locations that indicate the prospector knew well enough which rocks and perhaps even which structures are most favorable; some pits are in unlikely or even absurd places) such as the quartz cores of pegmatites and ordinary schist with quartz veinlets, whereas others were merely dug in handy locations to meet assessment requirements.
Of course major deposits may have been overlooked because mineralization stopped short of the present surface or because the gold is very fine grained or occurs in such odd minerals that it was not readily detected by the panning and assaying techniques generally used in the past.
There is no doubt that possibilities can, and undoubtedly soon will, be tested by diamond drilling at old mines or at places determined to be favorable. Such work should at the very least bring to light the existence of previously undetected, but probably small, ore shoots.