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7. TERTIARY GRAVELS
Gold-bearing gravels of Tertiary age are abundant in the central Sierra Nevada region of California and to a lesser extent in northwestern California and southwestern Oregon. A few are found in northeastern Oregon and central Idaho. In California, many miles of these ancient river channels have been mined and according to figures offered by (Clark 1965, p. 44), seven of the California deposits have together produced gold amounting to more than 232 million dollars. The total production is not on record but it is known to far exceed this figure. Although few, if any, of the remaining Tertiary deposits can be mined at the present price of gold, the possibility of mining some at a future date is real and must be given serious consideration. Because of this future potential and today's sporadic but continuing mining efforts, it is important that the mineral examiner have a clear concept of this special type of placer. The subject of Tertiary channels is too extensive for review here and, for this reason, the reader is directed to the following selected references:
Tertiary Channels (Clark, 1965, pp. 39-44). This article contains a brief history of mining operations and descriptions of the Tertiary channels of the Sierra Nevada region. Contains indexed map with list of principal deposits and bibliography.
California's Gold-Bearing Tertiary Channels (Jenkins & Wright, 1934). A condensed but excellent discussion of the origin of California's Tertiary gravels and related geology. Highlights factors in gold accumulation and discusses methods of geologic exploration and proposed geophysical techniques as aids to future exploration work.
Tertiary Gravels of the Sierra Nevada of California (Lindgren, 1911). A classic on Tertiary gravels and related subjects. Most writers on placer subjects have drawn heavily from this source of information and as a result, quotations from and references to Lindgren's work are prominent in the literature.
The Auriferous Gravels of the Sierra Nevada of California (Whitney, 1880). Discusses in considerable detail, the concepts (circa 1879) of origin and deposition of Tertiary gravels. The book is best suited to advanced study. Also contains maps and excellent descriptions of important Sierra Nevada placer mining districts and offers some of the best available data on individual mining operations of the day. Includes chapters by W.A. Goodyear and W.H. Pettee.
The Ancient River Beds of the Forest Hill Divide (Browne, 1890). One of many excellent articles describing Tertiary gravel deposits and related mining operations that have been published by the California State Mining Bureau.
Tertiary Gold-Bearing Channel Gravel in Northern Nevada County, California (Peterson and others, 1968). A report on studies of the San Juan Ridge Tertiary channel in Nevada County, California, made as part of the Department of the Interior's Heavy Metals Program. It describes the geology and physical characteristics of a 15-mile section of this channel and also discusses geophysical investigations of the channel gravel carried out by personnel of the United States Geological Survey. Seismic, resistivity, gravity, magnetic, electromagnetic, and induced polarization geophysical methods were applied and evaluated. 22 pp., maps, iIIus.
Gold Resources in the Tertiary Gravels of California (Merwin, 1968). One of a series of reports presenting results of investigations conducted by the United States Bureau of Mines under the Department of the Interior's Heavy Metals Program. It reviews the history of hydraulic and drift mining in California's Tertiary gravels and the reasons for cessation of large-scale mining of these deposits. The report discusses general geology, the distribution of gold within the gravels, past production, and gold reserves. 14 pp., illus., bibliography.
Most of the foregoing references are of a technical nature and primarily of interest to engineers and geologists.
The term "Tertiary gravel" is used broadly to designate extensive gold-bearing gravel deposits laid down in ancient streams approximately 50 million years ago. These often-rich channels were subsequently buried under as much as 1500 feet of younger gravels and volcanic material which effectively sealed and preserved the original placers. The present Sierra Nevada range and its Tertiary deposits have since been elevated Ly mountain-building disturbances and, today, the dissected channels are typically found as lava-capped segments high above the present streams. Parts of the Tertiary deposits were worked extensively by the hydraulic method during the latter half of the 1800's and in many places, the richer bedrock gravels were exploited by tunneling operations known as "drift mines", some of which followed the ancient channels for miles along their subterranean course. Parts of California's Tertiary channel system remain buried and unexplored; these are believed to contain a large gold reserve, available for mining at some future date.
