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THE TERTIARY GRAVELS OF THE SIERRA NEVADA OF CALIFORNIA - More excerpts
The following excerpts, presented in 1911, are relevant for the analysis of the proposed new shaft in our Red Star property. The shaft will pass through the tertiary gravel west of the productive Ruby Mine.
THE TERTIARY GRAVELS OF THE SIERRA NEVADA OF CALIFORNIA
by Waldemar Lindgren
USGS Professional Paper 73
RELATIVE VALUE OF QUARTZ GOLD AND PLACER GOLD
Observations in all parts of the world have shown that placer gold is always finer than the gold in the quartz veins from which the placers were derived. The explanation is that the silver alloyed with the gold is dissolved by the action of surface waters. The purity of the gold becomes greater as the size of the grains diminishes, the explanation being, of course, that the proportionate amount of surface exposed to the action of solutions is greater in the finer gold. The average fineness of the gold of Nevada County is given as 855; of Placer County, 792; of Plumas County, 851; of Sierra County, 858; of Calaveras County, 835; of Tuolumne County, 804.
In Sierra County gold from quartz mines varies from 622 to 883.
DEPOSITION OF PLACER GOLD FROM SOLUTIONS
In spite of the fact that the geologic conditions indicate so clearly a direct derivation of the gold from quartz veins, there have always been a number of adherents of the view that placer gold is formed by chemical depositions in the gravels.
A. Liversidge made an extensive examination of nuggets from various sources in order to ascertain whether they bear any evidence of segregation in water. He concludes that they are derived entirely and directly from veins and that “any small addition they may have derived from meteoric water” is quite immaterial and may be neglected.
The long exposure during gradual accumulation and the long rest of the gravels in channels exposed to percolation of atmospheric waters since Tertiary time has evidently produced a great enrichment in the fineness of the gold. The average grade in the main Tertiary channels is clearly much over 900.
The productiveness of a channel is best measured by its yield per linear foot. The Ruby Gravel Mine, in Sierra County, in which the channel was from 50 to 300 feet in width, was worked for a distance of 3,850 feet and yielded at the rate of $465 a linear foot. The cost is stated to have averaged $240 a foot.
The gold content of the gravel varies, of course, enormously. In general it may be said that the upper gravels, sands, and clays are very poor; and although more gold is contained in the lower gravels it is only within a few feet of the bedrock that the rich material begins to appear. By far the greatest part of the gold is ordinarily contained in the gravel within 3 feet of the bedrock, and in many places within the last foot above the bedrock.
THE COLFAX QUADRANDLE
GOLD-BEARING AREAS AND PRODUCTION
The largest and richest masses of Tertiary gravel known in the Sierra Nevada are found in the quadrangle and derived their contents from a great number of gold-quartz veins.
It is impossible to obtain exact data regarding the total amount of gold produced in this quadrangle. That part of Nevada County, which is contained in it, has certainly produced $60,000,000. The part of Placer County, which is contained in it, has surely produced, at the very least, the same amount. To this must be added the production from Minnesota, Alleghany, and Forest, in Sierra County, which is large. It is thought that $150,000,000 is a very conservative estimate of the total. These figures are only the rudest approximations, but they serve to convey an idea of the astounding richness of the region.
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