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GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



History and production 



The San Antonio mine was worked at least as early 

 as 1848 (U.S. Supreme Court, 1857, p. 47, 53), and it 

 produced some quicksilver in 1849. Very little else is 

 known of its early history. It is reported in company 

 records to have yielded "much good ore before 1865," 

 but this is perhaps erroneous. In 1908 and 1909, when 

 the management of the New Almaden mine was at- 

 tempting to develop ore in several of the "outside" 

 mines, a lower tunnel was cleared out and several 

 hundred feet of new workings were driven from it. 

 This work apparently failed to expose any ore, and 

 the mine was soon abandoned. In 1915, under W. H. 

 Landers, the two more extensive adits shown on the 

 accompanying map (fig. 97), were driven in order to 

 develop some ore found by a lessee during the previous 

 year. Development was handicapped by shortage of 

 miners, mining equipment, and funds, but was car- 

 ried on at a relatively slow pace until Landers re- 

 signed in April 1917. Some ore was revealed by this 

 work, and, to avoid the long haul to the New Almaden 

 furnaces at the Hacienda, plans were made to either 

 install a concentrator at the mine or haul its ores 

 through the 200 level Senator tunnel to the Senator 

 furnaces. Apparently a little ore was actually mined 

 and hauled to the New Almaden furnaces, but neither 

 of the other plans was ever carried out. 



Geology 



A comparatively thin tabular body of serpentine, 

 together with bordering graywacke and shale of the 

 Franciscan group, are explored by the more recent 

 workings of the San Antonio mine. The serpentine 

 body is well exposed on the surface, where it trends 

 N. 20 W. from Guadalupe Creek to the crest of Los 

 Capitancillos Ridge; its dip, in the few places where 

 it can be determined, is consistently at low angles to 

 the northeast. For much of its exposed length it has 

 been converted to silica-carbonate rock, though above 

 the uppermost mine workings its lower side is un- 

 altered. In the mine workings it has a core of serpen- 

 tine, with a shell of silica-carbonate rock along both 

 sides. (See fig. 97.) A second, less extensive, body 

 of silica-carbonate rock overlies the more conspicuous 

 one from the crest of the ridge down about half way 

 to the canyon floor. This body has been explored by 

 a few shallow workings. 



Ore bodies 



Very little ore has been mined from the more recent 

 workings, but according to company records almost all 

 the silica-carbonate rock penetrated by the two adits 

 driven in 1915-17 was "low-grade ore." The best ore 

 was said to have been found along the upper side of 



the sill-like body, but the silica-carbonate rock along 

 the lower side also is said to have contained some ore 

 on the lower level. The "low-grade ore'' in the upper 

 adit was reported to contain more than 3 pounds of 

 quicksilver to the ton, and the records say, regarding 

 the place in the lower adit where the 2 crosscuts branch 

 to the north and south, "daily samples are being taken 

 here and we have some returns as high as 0.35 per- 

 cent." 



Suggestions for further development 



No high-grade ore bodies have been found in the 

 workings driven during the last 50 years, and it seems 

 unlikely that the records of "rich ore" mined in the 

 early days are authentic. However, if most of the 

 silica-carbonate rock penetrated by the recent work- 

 ings contains as much as the reported 3 pounds of 

 quicksilver to the ton, the mine could be worked at a 

 profit during times of high prices of quicksilver such 

 as prevailed during World War II, provided a furnace 

 were installed nearby. 



SAN MATEO MINE 



Location and extent of workings 



The San Mateo mine, which is one of the small "out- 

 side" mines on the New Almaden property, is near the 

 base of the south slope of Los Capitancillos Ridge, 

 just below the Guadalupe Dam. (See pi. 1.) The 

 distribution of the more recent workings of the mine 

 is shown in figure 98, but the positions of the older 

 workings, driven in the 1860's and 1870's, are not 

 indicated because accurate maps showing these work- 

 ings are not available. Probably most of these lost 

 workings were in the vicinity of the ones shown on 

 the map, and certainly the position of one of the very 

 old stopes is indicated on the map by a surface depres- 

 sion lying between the main adits. The ore bodies 

 found in the mine were scattered within an area about 

 175 feet in diameter, and according to an old cross 

 section they were mined through a vertical interval 

 of at least 120 feet. Access to the larger stopes was 

 first gained through an adit driven northward for a 

 little less than 300 feet, and after this adit was caved 

 the same area was reached through an adit driven east- 

 ward for 450 feet. 



History and production 



The small San Mateo mine has attracted so little 

 interest that only fragments of its past history are 

 available. The date of its discovery is unknown, but 

 the mine is reported to have yielded some "rich ore" 

 in the 1860's and 15 tons of ore in the 1870's. From 

 1890 to 1901, under C. C. Derby, part of the workings 

 shown in figure 98 were driven, and in the first of 



