By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Related: Sierra County, California Mines
Sierra County has two major gold-producing areas: the Alleghany and Downieville districts, and the Sierra Buttes district. The total recorded gold production from 1880 through 1959 was about 2,161,000 ounces, most of it from lode mines. If the estimated gold output before 1880 is considered, the total production would be about 3 million ounces. Large placer production has come from both Tertiary and Quaternary gravels, but the exact amount is not known.
ALLEGHANY AND DOWNIEVILLE DISTRICTS
The Alleghany and Downieville districts, about 5 miles apart in the southern part of Sierra County, are considered as one area for the purpose of this report. The lode mines have been productive since the early 1850's, and their total production through 1959 was 1,590,990 ounces.
The earliest record of mining was in 1852 when the Tertiary gravels were worked at several localities by both drift and hydraulic methods. Drift mining of the rich gravels was on a large scale to 1888, after which production declined (Ferguson and Gannett, 1932, p. 25-26). About $10 million (485,000 ounces) in gold was produced from the drift mines, and between $2 million (97,000 ounces) and $4 million (194,000 ounces) was produced from the hydraulic operations.
Lode mines developed slowly because of the rich yields of the placers in the early years, but by 1898 they were the chief source of gold in the districts. The ore has been rich but spotty; therefore, except for a few mines, lode mining has had alternating periods of prosperity and inactivity (Ferguson and Gannett, 1932, p. 27).
The Sixteen to One mine, the most productive in the Alleghany district, had a total output of about $9 million in gold to 1928 (Ferguson and Gannett, 1932, p. 106). Lode mining has continued to flourish in the districts. Production of more than 17,000 ounces was reported in 1958, and an undisclosed amount was produced in 1959. Total minimum production through 1959, including the estimated early production from the placers, was about 2,173,000 ounces.
The following summary has been abstracted from the detailed geology by Ferguson and Gannett (1932, p. 5-24). The oldest rocks in the area are quartzite, slate, greenschist, and conglomerate. These rocks make up five sedimentary formations that can be correlated with part of the Calaveras Formation of Carboniferous age.
The rocks dip steeply and crop out as belts that trend north-northwest. They are intruded by gabbro, now partly serpentinized, and younger granitic rocks. Auriferous gravels of Eocene and Miocene age, andesite breccia of Miocene (?) age, and some Pleistocene and Recent gravels overlie the older rocks with marked unconformity.
The rocks were affected by two periods of metamorphism and deformation. The first period took place between Carboniferous and early Triassic time and resulted in folding, faulting, and regional metamorphism. The second period was at the close of the Jurassic and was accompanied by intrusions of basic rocks first and then granitic rocks, by the development of a complicated fault pattern, and by mineralization.
The principal veins in the Alleghany and Downieville districts strike northwestward and dip gently eastward. They follow minor reverse faults that cross all pre-Tertiary rocks except the larger serpentine masses. Other veins have similar strikes but dip steeply to the west (Ferguson and Gannett, 1932, p. 29-31).
There were four stages of mineralization: (1) chloritization and serpentinization of the wallrock, (2) deposition of quartz and minor amounts of arsenopyrite, pyrite, albite, oligoclase, and barite, (3) replacement of quartz by gold, and of wallrock by ankerite and sericite, and (4) deposition of veinlets of fine pyrite and calcite in the veins and country rock, and of drusy coatings of these minerals on the quartz (Ferguson and Gannett, 1932, p. 38-39).
SIERRA BUTTES DISTRICT
The Sierra Buttes district is 10 to 12 miles east of Downieville.
Lode mining began in the 1850's but no record could be found of any early placer mining in this district. The most important mine was the Sierra Buttes, whose total production of between $15 and $17 million in gold made it the largest single gold producer in Sierra County (Averill, 1942, p. 44). The mine was closed in 1938, and though cleanup operations were conducted in 1941, no further activity had been reported through 1959. Total gold production for the district was about 825,000 ounces; nearly all production was from the Sierra Buttes mine.
Most of the country rock in the district consists of altered quartz porphyry and rhyolite porphyry bordered on the west by bands of slate, quartzite, and limestone of the Calaveras Formation of Carboniferous age that trend north-northwest (Logan, 1929, p. 155-156). The metasediments are probably older than the porphyries. The ore deposits are gold-bearing quartz veins.