PRIMARY GOLD DEPOSITS OF CALIFORNIA The primary gold deposits, as distinguished from the secondary auriferous gravels derived from the destruction of the former by erosion, are for the most part quartz veins containing free gold. They were formed shortly after the intrusion of granitic magmas into the compressed and folded pre-Tertiary sediments, and filled the fissures and joints in the granitic rocks and altered sediments. The deposits, which are of unusual persistence, are contained in a gold belt which begins as a narrow border along the foot of the Sierras in Tulare and Kern counties, widens toward the north, and in Butte and Plumas counties is over 60 miles broad. In the southern counties, from Mariposa to Eldorado, it contains the numerous and strong veins of the famous Mother Lode region. Farther north, 20 or 30 miles from where the Mother Lode veins disappear, occur the celebrated vein systems of the Nevada City and Grass Valley districts. The parts of the veins now exposed give evidence of having been formed at considerable depths. Thousands of feet of overlying rock masses, together with many former outcrops of veins, have been removed by erosion and their gold content deposited with the detrital products. Still farther north, in northern Plumas, Lassen and Tehama counties, the gold belt is covered by a wide sheet of Tertiary volcanic rocks. Above Redding it abruptly emerges from this blanket and continues northward, with an average width of 50 or 60 miles, to the Oregon line. In this area, comprising parts of Shasta, Trinity and Siskiyou counties, and generally known as the Klamath Mountains, gold-bearing veins and their derived gravels are widely distributed under geologic conditions similar to those of the Sierra Nevada. In the southern part of the state, gold-quartz veins, probably of the same age as those of Central California, appear in Los Angeles, Riverside, Inyo and San Diego counties. The veins occur in or near masses of metamorphic rocks, generally schists, in granitic rocks. With the exception of the Randsburg district in southern Kern County, just outside of Riverside County, gold production from southern California is relatively unimportant. At one time San Diego County contributed considerably to the annual California gold production, but has not done so in the last few years. The principal gold-quartz mining districts are the Mother Lode, the Grass Valley and Nevada City districts, and the Randsburg district in the southern tip of Kern County. Gold-bearing veins have been prospected and developed in practically every county along the eastern mountain ranges. With the exception of the above mentioned districts and certain scattered mines, production has not been commercially important. The ore occurrence in these more important dis tricts may be briefly described as follows. The Mother Lode. The Mother Lode is a belt of strong, closely spaced veins located in the center of the auriferous belt and extends over 100 miles in a northwesterly direction through Mariposa, Tuolumne, Calaveras, Amador and Eldorado counties. It parallels the axis of the range and the general trend of the formations, and consists of a series of parallel quartz veins, remarkable for their continuity, following more or less consistently a narrow belt of soft black slates. These slates are bounded laterally by harder slates, diabase (greenstone), amphibolitic schists, serpentines and occasionally granitic intrusions. The strike of these strata is northwest while the dip varies from 60 deg. to the northeast to 80 deg. to the southwest, but is most commonly 70 or 80 deg. to the northeast. The veins follow the slates very closely in strike, but cut them at acute angles on the dip, the average dip being between 50 and 70 deg. to the northeast. The main vein always carries a strong gouge either on the foot or hanging wall. The quartz bodies forming the ore shoots occur in lenticular masses from a few inches to fifty feet or more in thickness, from one hundred to one thousand feet in length along the strike, and have been mined to depths of over 4500 feet. They are irregularly distributed along the line of the main fissure; between them the |