By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Pennington County lies just south of Lawrence County and includes part of what is known as the southern Black Hills. From available production records, which are very fragmentary and incomplete, it is estimated that Pennington County had a minimum production through 1959 of about 128,000 ounces of gold; most of it was from lode deposits and small amounts were from placers.
Gold was found in 1875 in the gravels of Spring Creek, Palmer Gulch, Castle Creek, and Rapid Creek by the Jenney expedition (Newton and Jenney, 1880, p. 238-272). Most of the placers were of low grade, and the discouraged prospectors turned northward to the more promising diggings in the Deadwood area and left the southern part of the Black Hills virtually deserted. The new arrivals found that most of the favorable ground around Deadwood had been claimed; accordingly, some of them, prospecting enroute, returned to the southern Black Hills (Hughes, 1924, p. 21-26). In 1876 the Columbia lode in the Keystone district was located; the nearby Bullion lode was found in 1877 (Connolly and O'Harra, 1929, p. 118-119). In the Hill City area, the Gold Metal deposit was explored as early as 1878 (Allsman, 1940, p. 72). Gold mining in the early years was apparently conducted in a desultory fashion, and production probably was small.
In May 1883, tin ore was discovered in what is now known as the Etta spodumene mine, and other discoveries of tin ore in the Harney Park area followed. The tin boom lasted until about 1894, after which gold prospecting was resumed and several significant discoveries were made (Connolly and O'Harra, 1929, p. 115-116). Among these were the Keystone and the Holy Terror lodes which were located in 1892 and in 1894 respectively. In 1898 the Keystone was sold to the Holy Terror Co.; the combined properties have been the largest producers in the southern Black Hills (Allsman, 1940, p. 91-94). After 1903 the most active period was in the early 1940's, when the Keystone mine was reopened briefly. Most of the mines in Pennington County were idle during 1906-27. In 1928 and 1929 some mines were revived in the Keystone district; in 1935 some lode mines and placers in the Hill City district were worked. Gold mining in Pennington County practically ceased from 1943 through 1959. In the county, only the Keystone and Hill City districts have produced more than 10,000 ounces of gold.
HILL CITY DISTRICT
The Hill City district is an area of widely scattered gold deposits in western Pennington County in the vicinity of Hill City, near the headwaters of Spring Creek and around Rochford to the northwest of Hill City. Although production figures are incomplete for the early years, it is estimated that the Hill City district had a minimum total output through 1959 of roughly 35,400 ounces of gold, mostly from lodes. The district was dormant from 1939 through 1959.
The Hill City district lies along the west side of the mass of Precambrian rocks that forms the core of the Black Hills. The Precambrian rocks consist of complexly folded and distorted schist and quartzite. A short distance to the southeast of the district these rocks are intruded by the Precambrian Harney Peak Granite (Darton and Paige, 1925, p. 3-5; Connolly and O'Harra, 1929, p. 129-134).
The lode-gold deposits occur in quartz fissure veins and lenses and mineralized shear zones. The deposits southwest and east of Hill City are chiefly quartz veins that cut the metamorphic rocks (Allsman, 1940, p. 69). The veins range in width from a few inches to 6 feet. Most of them are shallow, although a few have been mined to depths of 700 feet. The veins consist predominantly of quartz in which free gold is irregularly distributed. West of Silver City, a quartz vein in the schist contains masses and streaks of lead-antimony sulfide, arsenopyrite, pyrite, a little sphalerite, and free gold (Paige, in Darton and Paige, 1925, p. 28).
The mineralized shear zones are most common in a belt extending northwest from Hill City (Allsman, 1940, p. 69). These are zones of brecciated schist cemented by granular quartz and arsenopyrite, pyrite, sphalerite, and free gold. Most of these deposits are low grade.
In the Rochford area gold deposits are found in quartz veins and lenses that cut the cummingtonite schist (Connolly and O'Harra, 1929, p. 129-134). The ore minerals are arsenopyrite, pyrite, pyrrhotite, chalcopyrite, a little magnetite, and gold which is associated particularly with the arsenopyrite. The gangue minerals are cummingtonite, quartz, carbonates, biotite, garnet, and chlorite. The general character of the ore and the geologic relations are analogous to those in the Homestake ore body (Noble and Harder, 1948, p. 954-955).
The Keystone district is in western Pennington County and extends from about 2.5 miles northwest of the town of Keystone to 1.5 miles southeast. From available records, it is estimated that the Keystone district had a minimum output through 1959 of about 85,000 ounces of lode gold, of which about 76,000 ounces came from the Keystone-Holy Terror mine (Allsman, 1940, p. 91-94). No figures are available on the amount of placer production, which apparently was small.
The Keystone district is on the northeast side of the Harney Peak batholith near the eastern margin of the core of Precambrian rocks of the Black Hills (Darton and Paige, 1925). Precambrian rocks consisting of schist, quartzite, amphibolite, and many granite and pegmatite dikes, are the predominant rock units of the district. The rocks are tightly folded and are cut by faults and shear zones, some of which are mineralized (Connolly and O'Harra, 1929, p. 120-121).
The ore bodies are in quartz veins or lenticular replacement deposits which trend parallel to the foliation of the enclosing schist. The Holy Terror vein was mined for a maximum distance of 1,200 feet along the strike and to a depth of 1,200 feet, and it ranged from a few inches to 6 feet in width. The gangue consisted of white quartz and the ore mineral was coarse flaky gold (Allsman, 1940, p. 91). Other deposits contained a wider variety of minerals, including arsenopyrite, pyrrhotite, pyrite, chalcopyrite, sphalerite, and native gold as ore minerals and quartz, hornblende, biotite, ankerite, chlorite, graphite, and garnet in the gangue. In some deposits the gold was very fine grained. The ore deposits are believed to be Precambrian in age and genetically related to the Harney Peak Granite (Connolly and O'Harra, 1929, p. 123-129).