By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
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Placers were discovered in Boise County in 1862 about 25 miles northeast of Boise in Boise Basin, an area of about 300 square miles. The placer operations led to the discoveries of lodes at the heads of streams, and some of these lodes were mined as early as 1863 (Anderson, 1947, p. 176). The lodes were never developed to sustain any extended yield; first one district would be active for a few years, then another.
Placers, on the other hand, had a less erratic history and remained highly productive through the 1890's. In the early 1900's they were worked by dredges, and some time later, by large-scale hydraulicking (Ballard, 1924, p. 31-32).
The Boise Basin is divided into many mining districts. In this report that part of the basin that includes the Idaho City, Moore Creek, Gambrinus, and Centerville camps is referred to as the Boise Basin district. The Pioneerville (Summit Flat, Grimes Pass) and Quartzburg (Gold Hill, Granite, Placerville) districts are considered separately.
In general, bedrock in the mineralized parts of Boise County consists of quartz dioritic and quartz monzonitic facies of the Idaho batholith, which are cut by groups of porphyry dikes of Tertiary age. Certain areas are covered by Tertiary lake beds, by basalt lavas of the Columbia River Basalt of Miocene age, or by Quaternary alluvial deposits (Anderson, 1947, p. 129).
Recorded production in the county began in 1863 (Jones, 1917, p. 86). Total gold production for the county from 1863 through 1959 was 2,891,530 ounces, about 95 percent of which came from the Boise Basin.
Boise Basin District
The Boise Basin (Idaho City, Moore Creek, Gambrinus, Centerville) district is in the central and southern part of the Boise Basin.
All the districts in the Boise Basin have a common history related to the original placer discoveries in 1862 and subsequent development of both placer and lode gold mines. The first placer discoveries in Boise County were made in this area in 1862. Most of the county's gold production came from the rich placers during the first few years of mining.
Estimated production from 1863 to 1896 from the Idaho City camp was valued at $44,651,-800 (2,167,500 ounces) (Lindgren, 1898, p. 655). The district produced 129,038 ounces from 1939 through 1958; its total production was about 2,300,000 ounces, mostly from placers.
Lode mines in the Gambrinus area were active from time to time. Two of the most productive were the Illinois and Gambrinus with outputs valued at $225,000 and $263,000 respectively (Lindgren, 1898, p. 685-686).
Most of the Boise Basin is underlain by a quartz monzonite facies of the Idaho batholith of middle Cretaceous age (Anderson, 1947, p. 130-132). In the Gambrinus area, many thin aplite dikes, dikes and stocks of diorite and granodiorite, and several lamprophyre dikes, all of early Tertiary age, cut the quartz monzonite. Near Idaho City and Centerville, patches of lake beds are interstratified with basalt lava and volcanic ash. This sequence is considered lower Miocene (Anderson, 1947, p. 153).
Alluvial deposits of two ages, Pleistocene and Recent, cover much of the district. The younger deposits are more restricted to the present stream valleys, whereas the older deposits cover low ridges and form terraces over a considerably wider area. The younger gravels were the source of most of the placer gold (Anderson, 1947, p. 156-159).
The lode deposits, which are mainly in the Centerville and Gambrinus area, are of early Tertiary (?) age. They consist of fissure fillings in fracture zones in the quartz monzonite. The fissures were formed by reverse faults, in contrast with the fissures formed by horizontal movement, which characterize the lodes of Miocene age in the Pioneerville and Quartzburg districts (Anderson, 1947, p. 181).
The vein mineralogy is simple, consisting of quartz and small amounts of pyrite, arsenopyrite, sphalerite, tetrahedrite, chalcopyrite, galena, and stibnite. Gold occurs with quartz or with the sul-fides (Anderson, 1947, p. 183).
Pioneerville District
The Pioneerville (Summit Flat, Grimes Pass) district is at about lat 44°00' N. and long 115°50' W., near the settlements of Grimes Pass and Pioneerville, in the northern part of the Boise Basin. This has been predominantly a lode-mining district; its mining history is closely associated with that of Boise Basin. The district was most active before 1920.
The Golden Age mine produced ore worth $200,000 between 1895 and 1920, and the Mammoth mine, $472,000 in the early days (Ballard, 1924, p. 75-76, 95). Most of this was in gold, although it included considerable silver and some lead. Only 3,340 ounces of gold was produced from 1939 through 1959. Total production for the district from 1895 through 1959 was about 25,000 ounces.
The Pioneerville district is at the north end of the "porphyry belt" discussed by Ross (1933a, p. 330-333) and Anderson (1947, p. 191). The country rock is quartz monzonite of the Idaho batholith which is cut by a zone of northeast-trending dikes of dacite porphyry, rhyolite, granophyre, granite porphyry, and diabase. These were intruded along preexisting shear zones in the quartz monzonite.
The ore deposits are closely associated with the porphyry dikes and were emplaced in fissures that resulted from later movements along the old shears. The principal metallic minerals of the veins are pyrite, tetrahedrite, chalcopyrite, arsenopyrite, galena, and sphalerite. These occur in a gangue of sericitized dike rock and quartz monzonite, quartz lenses, and some calcite.
Native gold occurs most abundantly in quartz or in or near the bismuth minerals galenobismutite, bismuthinite, and tetradymite. Some deposits, characterized by abundant miargyrite and pyrargyrite, are mined for silver alone. Electrum, containing about equal amounts of silver and gold by weight, is the chief ore mineral at the Comeback mine (Anderson, 1947, p. 195-203).
Quartzburg District
The Quartzburg (Gold Hill, Granite, and Placer-ville) district is in T. 7 N., R. 4 E., near the town of Quartzburg.
Soon after the initial placer mining rush to the Boise Basin, lode mining began in the Quartzburg district. The Gold Hill mine, discovered in 1863, was worked almost continuously until 1938 (Anderson, 1947, p. 176). Other important producers were the Mountain Chief and Belshazzar mines.
Ross (1941, p. 20) mentioned a total of $8 million (about 400,000 ounces) in gold from this district; however, the district was virtually idle from 1940 through 1959. Production since 1932 must have been combined with production reported from other districts because this district does not appear in the annual volumes of "Minerals Yearbook."
The Quartzburg district is at the southwest end of the "porphyry belt" that crosses the northern part of Boise Basin. The country rock is quartz monzonite of the Idaho batholith which was cut by northeast-trending shear zones, which in turn were intruded by porphyry dikes during Miocene time (Anderson, 1947, p. 129-150; Jones, 1917, p. 89-97; Ross, 1933a, p. 330-331).
The gold lodes are fissure veins and small stockworks in or along the dikes and in adjacent quartz monzonite. The deposits are extensively oxidized, and most of the early production came from this easily treated ore which had high gold content. The hypogene minerals are pyrite, galenobismutite, arsenopyrite, native gold, sphalerite, tetradymite, pyrrhotite, stibnite, chalcopyrite, and either tetrahedrite or tennantite. Gangue consists primarily of altered host rock and quartz (Ross, 1933a, p. 339-341).
