By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Related: Nevada County, California Mines
The gold-quartz mines of the Grass Valley-Nevada City district in Nevada County have been the most productive in the State, and the Empire mine in the same district was in continuous operation from 1850 to 1940, which at that time was the longest period of operation for any gold mine in the country (Logan, 1941, p. 375).
Placer production has also been significant, especially in the early days; however, no consistent records have been kept. Tertiary gravels on San Juan Ridge, North Columbia, Sailor Flat, Blue Tent, Scotts Flat, Quaker Hill, and Red Dog-You Bet still contain tremendous reserves, but these are for the most part undeveloped because of the curtailment of hydraulic mining.
The total gold production of the county from 1849 through 1959 was 17,016,000 ounces, including Lindgren's (1896, p. 26) estimate of $133,800,000 in gold. Since 1903, when placer and lode production have been reported separately by the U.S. Geological Survey (1904-24) and U.S. Bureau of Mines (1925-34, 1933-66), 286,655 ounces of placer and 7,119,353 ounces of lode gold have been mined in the county.
GRASS VALLEY-NEVADA CITY DISTRICT
Because the towns of Grass Valley and Nevada City, which form the center of the Grass Valley-Nevada City district, are only 4 miles apart and because of their similarity in geology, distribution and mineralogy of veins, and common history and development, they are discussed together.
The initial rush to this area took place in 1850 after rich gravels were discovered near the present site of Nevada City. In October of the same year, the first lode discovery was made at Gold Hill on the outskirts of Grass Valley (Johnston, 1940, p. 19). Almost immediately the towns of Grass Valley and Nevada City were founded; they grew rapidly, prospered, and became permanent communities.
By 1851 quartz mines had been developed at Nevada City, but these early ventures failed. The miners turned to working the rich Tertiary gravels on a large scale by introducing hydraulic methods (Lindgren, 1896, p. 19).
The discovery at Gold Hill was followed by discoveries of veins at Ophir Hill, Rich Hill, and Massachusetts Hill, and by 1867 most of the major mines of the district had been located. Grass Valley, where 1,600 men were working and 284 stamps were crushing the ore, was then one of the leading camps in California (Johnston,1940, p. 19).
Meanwhile new discoveries of lode deposits were made at Nevada City, and quartz mining became dominant there after 1880. Some of the important mines at Nevada City are the Champion, Providence, Canada Hill, Hoge, and Nevada City. The Empire, North Star, and Idaho-Maryland mines have accounted for more than two-thirds of the production of Grass Valley and have exerted considerable influence on the economic life of the community.
Indeed, in 1938 and 1939 the Idaho-Maryland had the largest gold output of any mine in the State. In recent years activity has declined in the district, and in 1957-58 most of the production was from cleanup operations at the Empire and North Star mines.
Production of the Grass Valley-Nevada City district is difficult to determine accurately because of the incompleteness of the early records. Of the estimated $113 million worth of production from 1849 to 1893, at least 60 percent was credited to the lode mines (Lindgren, 1896, p. 28). From 1903 through 1958, Nevada County produced 7,119,353 ounces of lode gold and 286,655 ounces of placer gold.
The reports of Lindgren (1896) and Johnston (1940) have been drawn upon for the following discussion of the geology of the district.
At the Grass Valley camp, the oldest rocks are schists and slates of the Calaveras Formation, of Carboniferous age, and relatively unaltered clay slates of the Mariposa Formation, of Jurassic age. Igneous rocks in the area consist of large masses of diabase, porphyrite, and amphibolite schist of Carboniferous to Jurassic age and serpentine, gab-bro, diorite, granodiorite, and quartz porphyry, of Jurassic and Cretaceous age. Andesite flows of Tertiary age cover large areas east and southeast of the town of Grass Valley.
The most characteristic geological feature of the Grass Valley camp is the elongate body of Cretaceous granodiorite which is 5 miles long and 14 to 2 miles wide. The ore deposits are gold-quartz veins in the granodiorite mass and in the serpentine, porphyrite, and diabase wallrock. The veins in the granodiorite, porphyrite, and diabase, in the central and southern part of the camp, strike north or slightly northwest, parallel to the contact of the intrusive. Veins in the northern part of the camp are north of the granodiorite and are in serpentine. These strike predominantly east.
Quartz is the principal vein material, and it appears in several textural types representative of successive stages of mineralization. Comb quartz, milky quartz, ribbon quartz, and brecciated quartz are the most common varieties. Ankerite and calcite are common gangue minerals but are less abundant than quartz. The principal sulfides are pyrite, galena, and sphalerite; also present, but less common, are arsenopyrite and chalcopyrite.
Gold occurs along cracks and grain boundaries in the sulfides and in brecciated quartz. Commonly bounding the ore shoots in the granodiorite are vertical or steeply dipping fractures, called crossings, that strike northeast, normal to the long axis of the granodiorite body.
The Nevada City camp, about 4 miles northeast of Grass Valley, is at the south end of a large body of granodiorite that extends northward into Butte County. This mass is separate from the stock at Grass Valley but is probably of the same age. A narrow belt, 400 to 1,500 feet wide, consisting of argillites, quartzites, and mica schists of the Calaveras Formation, is in contact with the southern end of the granodiorite and crosses the Nevada City camp in a northwesterly direction. Masses of porphyrite, diabase, and amphibolite schist, of Carboniferous to Jurassic age, form irregular lenses and bands southwest of the Calaveras Formation.
North of the town of Nevada City are several ridges of Tertiary gravels capped by andesite lava flows. In general, the veins of Nevada City are miner-alogically similar to those at Grass Valley, the main difference being the larger amount of coarse gold in the Grass Valley veins. The Nevada City veins are concentrated in the vicinity of the granodiorite-country rock contact and they are arranged into two systems: (1) a system that trends west-northwest with steep dips to the north or south, and (2) a system that trends north with medium eastward dips and contains the most productive veins.
MEADOW LAKE DISTRICT
The Meadow Lake district, 35 miles east of Grass Valley, is a relatively minor lode district and has been inactive since 1905. Production, which began in 1863, totaled at least 10,000 ounces of gold, but records are fragmentary.
Bedrock consists of granodiorite intruded into and bordered on the east by diabase and eruptive rocks (Wisker, 1936, p. 192-194). Most of the veins are in the granodiorite, a few are in the diabase. There are two productive vein systems: the major one trends N. 25° W.; the other, composed of smaller veins, ranges from N. 45° W., to N. 85° W. Some of the larger veins are remarkably persistent and can be traced for as much as 6,000 feet on the surface.
High amounts of pyrite, arsenopyrite, and chalcopyrite are present in the veins. Quartz is not as abundant as it is in the typical California gold-quartz veins. Free gold is associated with pyrite and rarely with quartz.
TERTIARY PLACER DISTRICTS
The Tertiary placer districts include the placer deposits in the following areas: San Juan Ridge, North Columbia, Sailor Flat, Blue Tent, Scotts Flat, Quaker Hill, and Red Dog-You Bet.
Most of the placer production of Nevada County before 1900 came from hydraulic mining of the Tertiary gravels from these localities. The production of individual areas is not known, but Lindgren (1911, p. 133) estimated that their aggregate production through 1909 was at least $60 million in gold. After passage of the California Debris Commission Act of 1893, many operators closed down, even though huge reserves of auriferous gravels still remain in these areas. In recent years, production from these areas has been sporadic and small scale.