128 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



property; all the rest of the ridge is in the extensive 

 New Almaden mine property. The latter contains not 

 only the famous New Almaden mine at its eastern end, 

 but also half a dozen other mines, which are included 

 under the collective term "New Almaden Mines." The 

 district also includes a second mineralized belt lying 

 along the north slope of the Santa Teresa Hills and 

 containing two small mines with relatively insignifi- 

 cant production. 



NEW ALMADEN MINES 



The New Almaden mine property, owned in 1948 

 by Richard H. L. Sexton and Eric H. L. Sexton, of 

 Philadelphia, Pa., includes, from east to west, the 

 New Almaden, America, Providencia, Enriquita, San 

 Antonio, San Mateo, and Senator mines. This prop- 

 erty was originally part of several Spanish land 

 grants, and consequently, although it is more exten- 

 sive than most mining properties, it is not subdivided 

 into mining claims. It originally included 8,580 acres, 

 or more than 13 square miles, but has been reduced 

 by the sale of nonmineralized portions to less than half 

 that size. About 3,361 acres lying along Los Capitan- 

 cillos Ridge contain all the mines. A smaller separate 

 acreage lying on the steep north slope of the Sierra 

 Azul was formerly used as a source of mine timber 

 and wood for the reduction furnaces, but it is now 

 valuable as a source of water. 



The hundred-year history of the New Almaden 

 Mines is treated at length in the historical section 

 beginning on page 176. A brief summary statement 

 indicating the past production and present status of 

 mines follows. 



The value of the cinnabar-bearing quicksilver ores 

 cropping out on Mine Hill was first recognized in 

 1845, and the development of the New Almaden mine 

 was begun immediately thereafter. During the next 

 50 years enough ore was found beneath Mine Hill to 

 make the New Almaden one of the world's great quick- 

 silver mines. By 1905, however, the ore bodies had 

 largely been exhausted. Since then, only small-scale 

 attempts to find new ore bodies underground in the 

 mine have been made; but production has been con- 

 tinued, at a low and declining rate, by the reworking 

 of old dumps and the stripping of submarginal ore 

 from the walls and floors of old slopes. During World 

 War II production was temporarily increased by large- 

 scale surface mining of low-grade ore. The other 

 mines on the New Almaden property were also partly 

 developed before 1870, but, because of the abundance 

 of ore in the Mine Hill area, they remained little ex- 

 ploited until production from the New Almaden mine 

 began to decline, when they were reopened. This re- 



vival of the so-called outside mines resulted in a sig- 

 nificant production from the Senator mine, but the 

 others yielded little ore. The outside mines were not 

 reopened during World War II, and they remain 

 largely inaccessible. By 1948 the part of the New 

 Almaden mine that had been reopened during the 

 wartime boom had also again become largely inac- 

 cessible. 



The total production of all the New Almaden mines 

 to the end of 1945 was 1,046,198 flasks of quicksilver. 

 Of this total, 93 percent was recovered before 1900, 

 and more than 95 percent came from the famous New 

 Almaden mine proper. The decline of the production 

 of the mines and the decrease in grade of the ore 

 treated are shown graphically in figure 91. 



NEW ALMADEN MINE 



The New" Almaden mine proper includes the inte- 

 grated workings and unconnected exploratory adits 

 underlying an area of about li/ 2 square miles on Mine 

 Hill. (See pi. 3.) In altitude these workings, as 

 shown by plate 4, range from about 1,750 feet above 

 sea level to 643 feet below sea level, and thus span a 

 greater vertical interval than those of any other quick- 

 silver mine in the world. In the aggregate length of 

 its horizontal workings, which is approximately 33 

 miles though it is commonly reported to be several 

 times as great, the mine probably holds first place 

 among the world's quicksilver mines. 



The top of Mine Hill (1,750 feet above sea level 

 before mining began) was the datum for nil the older 

 maps, and levels were designated in hundreds of feet 

 below this point. The major adits lie on the 800 

 level, and the deeper workings, extending down to 

 1,600 feet below this level, could be reached only 

 through shafts. Some of these were internal, but 13 

 surface shafts, of which the deepest is 1,519 feet deep, 

 surround the apex of the hill. Most of the workings 

 above the 850 level, or about 40 percent of the total, 

 were accessible during the examination leading to this 

 report. 



The surface geology of the area underlain by the 

 New Almaden mine is shown on plates 5-10. In the 

 latter the geology for the workings above plate 3, and 

 the geology exposed on the levels as shown on the 850 

 level was reduced, and, in places, generalized from maps 

 originally made by Survey geologists at a scale of -H) 

 feet = 1 inch. The geology shown on the lower lev- 

 els was plotted from records of company surveyors 

 or from unpublished maps by S. H. Christy. 



Even a cursory inspection of these maps and the 

 sections shown on plate 11 will show that the geology 

 is complex, and the subsurface neology could not have 

 been even approximated from the surface exposures. 



