MINES 



149 



here the contact is vertical. The maximum width of 

 the body on the surface is about 220 feet, and on the 

 Upper Santa Mariana tunnel level it appears to be 

 only about 160 feet. From the relations of the ser- 

 pentine bodies exposed on the crest and south slope of 

 Mine Hill, and from the known extent of the nearby 

 serpentine mass in the underground workings of the 

 San Francisco-Washington area, it is concluded that 

 the serpentine bodies explored in the two areas are 

 not connected. 



Silica-carbonate rock has replaced the serpentine at 

 almost every place where it is exposed on the surface, 

 though a narrow core of serpentine is preserved at the 

 road just northwest of the Washington shaft. Under- 

 ground only the margins appear to have been con- 

 verted to silica-carbonate rock, for considerable ser- 

 pentine was encountered in crosscuts traversing the 

 inner part of the mass. 



Ore bodies 



The ore in the Santa Mariana area was irregularly 

 scattered, for the cinnabar occurred in small veinlets 

 distributed irregularly through the silica-carbonate 

 rock. Cinnabar seen in the Mariana Road tunnel was 

 associated with veinlets of quartz and dolomite, but 

 these were too small and irregular to be typical hilos, 

 and the mineralization at that place was not obviously 

 related to any contact. On the other hand the one 

 mapped stope in the workings, which is in the south- 

 eastern part of the Upper Santa Mariana tunnel level, 

 was apparently near the northeast contact of the ser- 

 pentine, which suggests that the contact may there 

 have had a part in localizing its ore. See pi. 5.) Else- 

 where small raises and winzes, in some of which small 

 ore shoots were mined, appear to have been driven 

 along contacts. 



Suggestions tor further development 



Judging from the general distribution of the work- 

 ings in and along the serpentine, it would seem that 

 the Santa Mariana area has been fairly well prospected 

 down to the 200 level. As a 200-foot interval of un- 

 explored ground lies between the lowest Santa Mari- 

 ana workings and the 400 level extending northwest 

 from the Washington shaft, and as the closest work- 

 ings below that are on the 800 level, some additional 

 exploration beneath the Santa Mariana tunnels may 

 be justified. Another possibility is a stripping opera- 

 tion on the large surface area of silica-carbonate rock, 

 but the work done here in 1941 and 1942 indicates 

 that the quicksilver content of this rock is too low to 

 make such an operation pay, except when the value 

 of quicksilver is exceptionally high. 



SAN PEDRO-ALMADEN AREA 

 Location and extent of workings 



The San Pedro-Almaden area contains a group of 

 surface and underground workings on the north slope 

 of Mine Hill, near the crest of the ridge and about 

 midway between the America and San Francisco 

 shafts. (See pi. 3 and fig. 92.) The San Pedro work- 

 ings comprise an opencut and several relatively short 

 adits; the adits are about 1,000 feet in aggregate length 

 as shown on available maps, though company records 

 give their extent in 1865 as 1,608 feet. The Almaden 

 shaft is a few hundred feet northeast of these work- 

 ings, and the levels from it, which in part extend 

 under the San Pedro workings, are known as the 

 Almaden workings. The 500 level from the shaft 

 contains about 1,400 feet of workings, the 600 level 

 about 1,200 feet, and the 700 level about 300 feet. 

 (See pi. 4.) The 1400-level crosscut from the Santa 

 Isabel shaft to the Washington incline passes 700 feet 

 directly below the deepest Almaden workings, but as 

 the geology exposed in the crosscut cannot be pro- 

 jected with assurance across this 700-foot gap, the 

 1400 level is not further considered with the San 

 Pedro-Almaden area. 



History and production 



The ores of the San Pedro area were exposed at the 

 surface, and they probably were mined from open- 

 cuts, at least intermittently, between the early 1850's 

 and 1865. Available records indicate that both the 

 San Pedro and the nearby San Ramon tunnels were 

 begun in 1865, and from then until 1874 ore was mined 

 from this area at the rate of less than 200 tons per 

 year. After a 10-year period of inactivity small 

 amounts of ore were recovered between 1884 and 1893. 

 The total recorded production of the San Pedro work- 

 ings was about 1,000 tons of ore, which probably 

 yielded some 1,500 flasks of quicksilver. During 

 World War II the outcrops were mined with a power 

 shovel and yielded 2.68 pounds of mercury per ton. 

 With the postwar price decline the ore became too 

 lean to make continued operation profitable. 



The Almaden shaft was begun in August 1888, and 

 ore was mined from the shaft on the three levels until 

 January 1891. Although hilos containing cinnabar 

 were cut on both the 500 and 600 levels (pis. 9, 10), 

 they were not sufficiently numerous to form minable 

 ore at a time when the price of quicksilver was only 

 $45 a flask, and the only recorded production through 

 the shaft was 3 tons of "fair ore." 



Geology 



The San Pedro-Almaden workings explore a serpen- 

 tine sill of northwesterly strike which seemc to be an 



