The Johnsville, in the east half of T. 22 N., R. 11 E., in south-central Plumas County, is primarily a lode district, but it has produced some placer gold from Tertiary and possibly also from Quaternary gravels.
The Plumas Eureka mine, the largest lode mine in the district, began operations in 1851 and had a total estimated yield of $8 million in gold to about 1925 (Averill, 1937, p. 118). In more recent years the district has been only sporadically active; during 1933-59, only 2,009 ounces of lode gold and 1,413 ounces of placer gold were reported. Its total gold production was about 393,000 ounces.
The bedrock in the district consists of a broad northwest-trending band of metasedimentary rocks which belong to the Calaveras Formation of Carboniferous age (Averill, 1937, map) and which are intruded by small bodies of andesite of Carboniferous age. The gold deposits are in the quartz veins in the metasedimentary rocks.
LA PORTE DISTRICT
The La Porte district, in T. 21 N., R. 9 E., in southwest Plumas County, was the center of hydraulic mining of gravels of the Tertiary Yuba River in the 1850's and 1860's. After the enactment of debris control laws, the gravels were worked by drift mines on a much-reduced scale. During 1932-59 the district produced 9,347 ounces of gold. Total production from 1855 through 1959 was about 2,910,000 ounces. A small amount of production was reported in 1957, but none was reported in 1958-59.
The channel near La Porte is about 500 feet wide. Gold was concentrated on the amphibolite bedrock or in the lowermost 2 feet of the gravel. The main channel has been traced a distance of 10 miles to the northeast, and it ranges from 500 to 1,500 feet in width and from 14 to 129 feet in depth. The channel is offset near La Porte by numerous faults which have an aggregate displacement of about 500 feet (Lindgren, 1911, p. 105-108).
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