The Sierra Nevada, the dominant mountain range in California, is approximately 400 miles long, with steep multiple scarps on its eastern flank and a gentle western slope. It has been the source of the bulk of the state's gold production and contains the richest and the greatest number of districts.
The main mass of the Sierra Nevada is a huge batholith of granodiorite and related rocks that is intrusive into metamorphosed rocks of Paleozoic and Mesozoic age. The metamorphic rocks occur largely along the western foothills and in the northern end of the range. They are complexly folded and faulted and consist of a number of major rock units. The principal units are the slates, phyllites, schists, quartzites, hornfels, and limestones of the Calaveras Formation (Carboniferous to Permian); the Amador Group (Middle and Upper Jurassic) of metasedimentary and metavolcanic rocks; the Mariposa Formation (Upper Jurassic), much of which is slate; schists, phyllites, and quartzites of the Kernville Series (Jurassic or older) in the southern Sierra Nevada; and a vast amount of undifferentiated pre-Cretaceous greenstones and amphibolites.
In addition, there are numerous intrusions of basic and ultra-basic rocks, many of which are serpentinized. The serpentine bodies apparently have been structurally important in the localization of some goldbearing deposits and often are parallel to or occur within the belts of gold mineralization. Also, there are numerous dioritic and aplitic dikes that are closely associated with gold-bearing veins.
Much of the gold mineralization is In the belt of metamorphic rocks that extends along the western foothills and in the northern end of the range, although some important districts are in granitic rocks. Some arc associated with small intrusions or stocks related to the Sierra Nevada batholith. The richest as well as the largest number of lode-gold deposits are in the northern and central portions of the range. In the Butte-Plumas County area at the northern end of the Sierra Nevada, the gold belt is nearly 70 miles wide. Continuing south it narrows and dies out almost completely in the Fresno-Tulare County area but appears again in Kern County in the southern end of the range. There are a few widely separated districts along the steep eastern flank of the range.
The most productive lode-gold districts in the northern end of the Sierra Nevada have been the Alleghany, Crescent Mills, Downieville, Forbestown, Graniteville, Grass Valley, Johnsville, Nevada City, and Sierra City districts. In the central portion the most productive and best-known districts are in the Mother Lode gold belt. Although the entire foothill region of the Sierra Nevada is sometimes loosely termed the "Mother Lode Country," technically the Mother Lode is a 120-mile-Iong system of linked or en echelon gold-quartz veins and mineralized schist and greenstone that extends from the town of Mariposa, north and northwest to northern E1 Dorado County (see fig. 4).
The most production portion of the Mother Lode has been the la-mile segment between Plymouth and Jackson in Amador County. Other major sources of gold in the Mother Lode have been the Angels Camp, Bagby, Carson Hill, Coulterville, Georgetown, Greenwood, Jacksonville, Jamestown, Kelsey, Mount Bullion, Nashville, and Placerville districts.
Although the terms "East Gold Belt" and "West Gold Belt" have been arbitrarily coined to describe the gold deposits east and west of the Mother Lode, each contains extensive systems of gold-bearing veins (see fig. 4). Unfortunately few systematic studies have been made of these belts. The principal sources of gold in the East Gold Belt have been the Grizzly Flat, West Point, Sheep Ranch, Soulsbyville, Confidence, Clearinghouse, Hite Cove and Kinsley districts. The most important in the West Gold Belt have been the Ophir, Shingle Springs, Hunter Valley, Hodson, and Hornitos districts. To the southeast in Madera and Fresno Counties there are some gold districts, but they have been much less productive than those to the north.
In the southern Sierra Nevada, in Kern County, considerable quantities of lode gold have been mined in the Cove district and from scattered areas to the west and south that include the Keyesville, Clear Creek and Loraine districts. Gold has been mined from a few districts along the east flank of the Sierra Nevada, the most productive having been the Bishop Creek district, Inyo County, and the Homer, Mammoth and Jordan districts in Mono County. Appreciable quantities of by-product gold have been recovered from the Sierra Nevada copper belts in the western foothills (see separate section below) and the Plumas County copper districts. Some has been recovered from tungsten mines on the east flank of the Sierra Nevada.