By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Most of the gold-producing districts in Elmore County are clustered in its northeastern part, in an area underlain by granitic rocks of the Idaho batho-lith. Shear zones that trend nearly east contain lode deposits of gold and silver, which have yielded most of the gold produced in Elmore County, although placers were important in the Featherville and Rocky Bar districts, especially before 1900.
Gold lodes were discovered in 1862 in the Rocky Bar district and soon afterward at Atlanta. The 1870's and 1880's were years of intense lode-mining activity, but by the early 1900's many of the mines closed. Gold mining in Elmore County, with the exception of that in the Atlanta district, continued to decline, although by 1956 the Atlanta district also ceased significant production. Gold production for the county from 1889 through 1942 was 381,396 ounces (Staley, 1946, p. 19) and from 1943 through 1959 it was 60,300 ounces. A total of 441,696 ounces was produced through 1959.
The Atlanta (Hardscrabble, Middle Boise, Yuba) district is in T. 5 N., Rs. 11 and 12 E., in rugged, mountainous country 65 to 70 miles east of Boise.
Gold was first discovered in the district in 1864, in veins on Atlanta Hill, but extreme inaccessibility of the area hindered its development for almost 10 years (Ballard, 1928, p. 7-8). The greatest activity was in the 1870's and 1880's. Ballard (1928, p. 10) reported that the Monarch Mining Co. shipped ore valued at $700,000 in the period 1876-78. Mining declined after the 1880's but was rejuvenated in 1929 (Ross, 1941, p. 51) and increased steadily until 1956.
The total gold production of the Atlanta district is difficult to ascertain because figures for both gold and silver were combined in the early reporting. According to Ross (1941, p. 51), total metal production to 1936 was valued at $6 million, but the amount of this in gold was not stated.
From 1932 through 1959 the district produced 243,175 ounces of gold. If at least half of the early production worth $6 million was in gold, most of which was from lode deposits, total district gold production may have been 385,000 ounces. Placers were worked in the district, but their yield was not great (Ballard, 1928, p. 9).
The country rock in the district is granite of the Idaho batholith which was fractured and intruded by aplite and porphyry dikes. These fractures and dikes are arranged in a zone about 1.5 miles wide that trends northeastward. The ore deposits are in veins filling the fissures (Ballard, 1928, p. 13-18). The chief vein is the Atlanta on which most of the workings of the district are located. Stephanite and pyrargyrite are silver minerals characteristic of the ores that were mined in the early days. Ore mined more recently contains auriferous pyrite (Ross, 1941, p.51).
The Featherville district is in T. 3 N., R. 10 E., along the South Fork of the Boise River.
This placer district has only a partial production record. From 1922 to 1927, dredging operations produced a total of 32,777 ounces of gold (Ross, 1941, p. 48). No additional production data were noted except for 1951 when 33 ounces was reported.
The Neal district is in Tps. 2 and 3 N., R. 5 E., about 15 miles southeast of Boise, near the Arrow-rock Dam. This district, discovered in 1889, was noted chiefly for its lode mines, but small amounts of gold were also produced from intermittently worked placers. Since 1911 the district has been virtually inactive.
Estimates of the early production vary widely. Lindgren (1898, p. 699) noted that about $200,000 worth of gold was produced from the district between 1889 and 1896. Ross (1941, p. 49) later estimated that $2 million in gold was produced, but much of this could have been produced from 1906 to 1911, when the district was fairly active.
The predominant country rock is granodiorite of the Idaho batholith. It is cut by numerous dikes of porphyry and lamprophyre, and the veins seem to be closely associated with the lamprophyre dikes. Vein minerals consist of quartz, pyrite, gold, galena, sphalerite, arsenopyrite, and local garnet, in altered granitic rock (Ross, 1941, p. 49).
PINE GROVE DISTRICT
In the Pine Grove district, which is in Tps. 1 and 2 N., Rs. 9 and 10 E. near the town of Pine, gold and silver have been produced from lode mines and on a small scale from placers.
The principal mine was the Franklin, which produced $750,000 in gold and silver before it was closed in 1917 (Ross, 1941, p. 49). No other details on history or production are available.
The country rock in the Pine Grove district is granite, presumably of the Idaho batholith (Ballard, 1928, p. 18-19). This is cut by numerous northeast-trending dikes of diorite, rhyolite, and diabase. The veins occupy a zone 1.25 miles wide that trends north-northwest. The upper parts of the veins are oxidized and contain free gold and iron and manganese oxides. Primary ore contains pyrite, galena, sphalerite, and some arsenopyrite and chalcopyrite in a gangue of quartz.
ROCKY BAR DISTRICT
The Rocky Bar district is in T. 4 N., R. 10 E., near the town of Rocky Bar. Placers discovered in 1862 on Bear Creek produced from 1863 until about 1873 (Ballard, 1928, p. 22). The gold in the placers was traced to lode deposits which were quickly developed; the principal mines were the Elmore and Pittsburg. According to Ross (1941, p. 47), the quartz mines of the district produced about $2 million in gold and silver and the placers, about $2 million in gold to 1882.
Ballard (1928, p. 25) estimated a total production of ore worth from $3.25 to $3.75, million, presumably in gold, from the Elmore and Pittsburg mines alone. About 1900 the district became inactive and remained so except for small operations-in 1938 and 1939, which yielded no gold.
Granitic rock, part of the Idaho batholith, forms the bedrock here, as in most of the other gold districts of Elmore County. Numerous aplite, rhyolite, diorite porphyry, and mafic dikes, all trending east-northeast, cut the granite (Ballard, 1928, p. 22). The major veins strike eastward and consist of quartz gangue and auriferous pyrite. Small amounts of galena and sphalerite are in some veins.
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