California Gold Production Summary
The most productive gold-bearing region in the State is in the central part of the Sierra Nevada on the west slope (fig. 9). This region contains the Tertiary channel gravels, the Quaternary stream deposits, the Grass Valley-Nevada City lode district, the Alleghany and Downieville districts, and the complex vein system of the Mother Lode, East Belt, and West Belt. The Mother Lode and the placer deposits are exploited in many individual mining districts in numerous counties. The history and production of each of these districts are distinctive, but the geology is much the same. To avoid needless repetition of the geology of the Mother Lode in each of the five counties it traverses and the geology of each Tertiary placer district, this material is outlined in the following paragraphs.
The auriferous gravels of California are of two general types: buried placers of Tertiary age and normal stream placers of Quaternary age. The gold was derived from the many gold-bearing veins, icluding those of the Mother Lode, in the mountains that had been formed at the close of Jurassic time by the intrusion of the Sierra Nevada granitic batholith (Lindgren, 1911, p. 9-11), most of which was emplaced in Late Cretaceous time (Curtis and others, 1958, p. 10). A long period of erosion followed during Tertiary time when the mountains were nearly leveled, and gold from the eroded parts of the veins was concentrated in stream channels. Some of the resulting Tertiary placers were extremely rich. The Tertiary drainage system consisted of six main streams (Lindgren, 1911, p. 33-37). One of these, the Jura River, flowed northward in Plumas County; the others flowed westward. The names of the major westward-flowing streams, not to be confused with their present-day counterparts, are the Yuba in Yuba and Nevada Counties, the American in Placer and El Dorado Counties, the Mokelumne in Calaveras and Amador Counties, the Calaveras in Tuolumne and Calaveras Counties, and the Tuolumne in Tuolumne County. Near the end of the Tertiary Period, much of the region was covered with volcanic debris, composed chiefly of andesite. The old drainage pattern was obliterated, but a new one soon developed on the new volcanic surface as the present Sierra Nevada range was uplifted. During Quaternary time the new streams cut deep canyons through the volcanics, exposed and eroded parts of the old Tertiary channels, and reconcentrated some of the gold in the gravels of the new streams. Only scattered remnants of the Tertiary channels which are higher than the chan¬nels of the present streams are now found preserved beneath ridges of resistant volcanic rocks. Rich Quaternary placers have been mined very successfully in numerous districts along the present-day Feather, American, Yuba, Mokelumne, and Merced Rivers, and low-grade Quaternary placers have been profitably dredged in the central valley where the gradient of these rivers is nearly level and only fine-grained gold is concentrated.
Outstanding among the lode deposits of the Sierra Nevada is the Mother Lode system of gold deposits, a strip of mineralized rock 1 to 4 miles wide that extends 120 miles along the lower western flank of the Sierra Nevada. From near Georgetown in El Dorado County it extends southward to Mormon Bar, 21 miles southeast of Mariposa, in Mariposa County. The five counties it traverses El Dorado, Amador, Calaveras, Tuolumne, and Mariposa are often referred to as the Mother Lode counties.