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Quicksilver deposits in San Luis Obispo County and southwestern Monterey County, California
| Content Provider | Semantic Scholar |
|---|---|
| Author | Eckel, Edwin Butt Yates, Robert Giertz Granger, Arthur Earle |
| Copyright Year | 1941 |
| Abstract | Most of the deposits here described lie within an elongate area of about 75 square miles in northwestern San Luis Obispo County. Other deposits, most of them small, are scattered southeastward from southwestern Monterey County to the southern border of San Luis Obispo County. Quicksilver was first discovered in the region in 1862. Though mining since then has been intermittent, the output has been relatively large during or immediately after the periods of high quicksilver prices. Monterey County has produced very little quicksilver, but San Luis Obispo County, which ranks sixth among the quicksilverproducing counties of the State, produced 69,264 flasks between 1876 and the end of 1939, 70 percent of which came from the Oceanic and Klau mines. All but one of the known quicksilver deposits are in or closely associated with the Franciscan formation, of Jurassic (?) age. This formation, embodying the oldest and most widely distributed group of rocks in the mapped areas, consists mainly of highly contorted and metamorphosed shale, sandstone, and con glomerate and is overlain by Cretaceous and Tertiary sediments. Extrusive and intrusive igneous rocks, most of them basic in composition, were formed at several periods after the Francisca formation was deposited. Many of the intrusive bodies are now represented by serpentine. This part of the Coast Range province is characterized by numerous strong, complex, northwestward-trending fault zones, many of which have been intermittently active since late Jurassic time. Bodies of silica-carbonate rock ("quicksilver rock"), composed of dense quartz and mixed carbonates, were formed In many places by s'olutions that rose along the major faults and replaced the country rocks. Most of the Igneous rocks also are closely associated with these faults. The quicksilver deposits comprise not only irregular and discontinuous cinnabar-bearing veins but also rock masses that contain disseminated cinnabar. All are within or very near northwest-trending fault zones, and nearly all are intimately associated with silica-carbonate rock. Most of the ore shoots are small and irregular, though a few are several hundred feet in length and height and as much as 40 feet wide. The shoots are structurally controlled by local gouge zones or by changes in dip or strike of the enclosing vein matter. The quicksilver content of the ore has a 'wide range, but most of the ore mined in the past has probably contained 5 to 10 pounds of quicksilve to the ton. Ol 5 516 STRATEGIC MINERALS INVESTIGATIONS, 1940 Proved reserves of ore are very small at any given time, but some of the productive shoots in known mines have very likely not been exhausted, and further exploration will probably uncover new shoots in known mines or elsewhere. The geologic history and relations Indicate that the total future production may equal if not exceed the past production of nearly 70,000.flasks and that, with stable and profitable prices, an annual production of 1,000 to 3,000 flasks may well be attained. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://pubs.usgs.gov/bul/0922r/report.pdf |
| Alternate Webpage(s) | https://doi.org/10.3133/b922R |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
