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Title: The Tertiary Gravels of the Sierra Nevada of California by Waldemar Lindgren (1911) Collection Type: Books and Periodicals Source(s): Google Books # of Pages: 301 Download PDF: User Login Required Download OCR Text: User Login Required

Date Scanned: 2024-07-14 06:26:22 Item Sourced From: PDF First Created: 2025-03-17 05:53:20 Last Updated: 2025-03-17 05:53:20

Original Held At: Searls Historical Library Location: 161 Nevada City Hwy, Nevada City, CA 95959

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GOLD OF THE TERTIARY GRAVELS. 73 In limestone areas the bedrock is extremely irregular (Pl. XI, A) and solution has produced holes which in places may be 50 or 75 feet in depth. Accumulation of rich gravels often takes place in these cavities. A soft bedrock is considered advantageous because of its property of catching the gold driven across its surface in the moving gravels. Sometimes the gold will work down into the soft mass for a depth of 1 to 2 feet. On the other hand, a hard and smooth bedrock is less efficacious as a gold catcher, and serpentine is said to be especially unfavorable in this respect. The steeply dipping ridges made by alternate strata of slate serve to catch the gold, but at many places it is held to be more advantageous if the strike of the slates runs parallel to the channel than if they cross it. In many parts of the United States gold-bearing gravels rest on clays or tuffs above the true bedrock, and this means, as a rule, several epochs of gold concentration. In the Tertiary rivers of California such secondary pay streaks and false bedrock are of comparatively rare occurrence. Gold is not retained on the surface of sand and gravel, and during the deposition of the gold-bearing gravels proper such thick clay beds were not ordinarily formed on account of the generally steep grade of the watercourses in a region of accentuated topography. Later, during the epoch of the rhyolitic eruption, such tuffs and clays were frequently deposited, but at that time there was little opportunity for the accumulation and concentration of gold in the wide flood plains. Some notable occurrences of false bedrock are mentioned in the detailed descriptions. An excellent example is that of the upper channel 150 feet above bedrock between Mayflower and Bath, on the Forest Hill divide. This channel was 225 feet wide and 5 feet deep and yielded $4.50 a ton. The lower channel, only 75 feet wide, was richer, averaging in the drifting ground $7 a ton. Another excellent example is found in the three pay streaks of the Excelsior mine near Placerville, which has been mentioned above. MINERALS ACCOMPANYING GOLD IN THE TERTIARY GRAVELS. Comparatively few useful minerals are found with the gold in the Tertiary gravels, but naturally the concentration which sorted out the gold from the bedrock also accumulated in the sands and gravels such heavy minerals as may be contained in the rocks. In the sluice boxes which are used for the washing of gravels these heavy minerals accumulate, and from the prevalence among them of magnetite and ilmenite they are usually referred to as ‘black sands,” The minerals occurring in the gravels may be divided into those of detrital origin and those which have been formed by chemical action within the gravels themselves. DETRITAL MINERALS. As stated above, magnetite and ilmenite are the most common of the minerals which accompany the gold, and their derivation is easily found in the basic rocks, like diabase, gabbro, and allied greenstones, which occupy so much space in the gold-bearing region. The granodiorites also furnish a considerable amount of magnetite. Most of the ilmenite is doubtless derived from the basic rocks mentioned. The Tertiary volcanic rocks are also rich in these constituents and channels traversing them are likely to contain an exceptional amount of black sand. A number of detailed determinations of the quantity of these minerals present were made in the examination of the black sands by D. T. Day at Portland in 1905,' and the mineralogical classification was carried out by Charles H. Warren, of the Massachusetts Institute of Technology. From the results it appears that magnetite largely prevails, but that chromite is also present in considerable quantity, as was indeed to be expected from the occurrence of large areas of serpentine in the gold belt. The black sand of Oroville contains, for instance, 1,400 pounds of magnetite, 250 pounds of chromite, and 150 pounds of ilmenite to the ton; this is the average black sand from dredging operations. At Cherokee, Butte County, 1 Day, D. T., and Richards, R. H., Useful minerals in black sands of the Pacific slope: Mineral Resources U.S. for 1905, U. S. Geol. Survey, 1906, pp. 1175-1258.