146 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



The rocks beneath the main sill are not well enough 

 exposed by the workings of the area to justify any 

 statement concerning their distribution or structure. 



Ore bodies 



Three major ore bodies and several smaller satellite 

 bodies were mined in the South Randol area. They 

 have been inaccessible for more than half a century, 

 but the little available information about them sug- 

 gests that they were similar in character to the other 

 ore bodies of the New Almaden mine. The principal 

 ore mineral was cinnabar, though an isolated occur- 

 rence of native mercury accompanying cinnabar on 

 the 1700 level is reported. The ore minerals occurred 

 in and along steep hilos which trend nearly north 

 through the silica-carbonate rock. The most exten- 

 sive ore shoot plunges northward, parallel to the hilos 

 and almost exactly down the dip of the upper surface 

 of the serpentine sill, from about the 1000 level to the 

 1600 level, or about 900 feet. (See pi. 4.) No exact 

 information as to how far the ore extended back from 

 the contact is available, but probably the distance 

 varied between 5 and 15 feet. A second large irregu- 

 lar ore shoot was found on the upper surface of a thin 

 finlike serpentine body which lapped over the shoot 

 just described. It extended down the dip from about 

 the 1450 level to the 2000 level; it had a maximum 

 strike length of about 300 feet on the 1700 level, but 

 tapered rather abruptly above and below. Apparently 

 both of these ore shoots were localized by a single set 

 of fractures, which were later filled to form the hilos. 

 Another ore body lying farther west along the upper 

 surface of the main sill had a strike width of 300 feet 

 on the 1500 level, and it extended from the 1200 to 

 the 1550 levels, having a dip length of about 350 feet. 



SAN FRANCISCO ARIA 

 Location and extent of workings 



The San Francisco area includes surface cuts, un- 

 derground levels, and stopes clustered about the San 

 Francisco and Washington shafts, which are high on 

 the south slope of Mine Hill. (See fig. 92 and pi. 3.) 

 The large surface cut, which is known as the San 

 Francisco opencut, was made during World War II. 

 Its upper end, from near the top of the hill, is 1,725 

 feet above sea level ; its lower end is at an altitude 

 of 1,500 feet, equivalent to the underground 300 level. 

 The principal underground workings, which are much 

 older, extend from the 300 level down to the 1100 level, 

 but the stopes extend only to the 900 level. 



The San Francisco workings are somewhat isolated 

 from the rest of the New Almaden mine and were 

 developed largely through separate adits and shafts, 

 but for better ventilation and economy in handling 



ore they were ultimately connected with the central 

 stope area of the mine on four levels. (See pi. 4.) 

 Access to the San Francisco workings could formerly 

 be gained through several high-level adits on the south 

 slope of .Mine Hill, through the 300-level San Cris- 

 tobal tunnel from the north slope of Mine Hill, 

 through the Main tunnel on the east slope, and through 

 the San Francisco and Washington shafts. In 1944, 

 when the workings were mapped by R. R. Compton, 

 G. Donald Eberlein, G. W. Walker, and A. C. Waters, 

 the only means of entrance were the San Cristobal 

 tunnel, the 600 level Santa Rosa drift (pi. 10). and 

 the 800 level Day tunnel from the central stope area. 

 All the workings below the 800 level were flooded, 

 and some others were largely inaccessible, so that only 

 a little more than half of the 20,000 feet of level work- 

 ings in the area could be studied. Nevertheless, at 

 least parts of nearly all the stopes were accessible for 

 study, and, with the aid of the records of the company 

 surveyors, an adequate conception of the unusually 

 complicated geology of the area is believed to have 

 been obtained. 



History and production 



Ore is reported to have first been found in the San 

 Francisco area in the "Upper San Francisco tunnel" 

 in 1864. It seems likely, however, that this explora- 

 tory tunnel was aimed at deeper exploration beneath 

 unrecorded surface ores that were removed at an 

 earlier date. With the discovery of ore in the upper 

 tunnel, an adit known as the "Lower San Francisco 

 tunnel" was soon begun, and by 1866 the San Fran- 

 cisco branch from the Main tunnel was driven to con- 

 nect with the San Francisco workings. (See section 

 C-C', pi. 11.) Production from this "outside mine" 

 reached a yearly peak of more than 2,000 tons of 

 "average grade ore" in 1867, and in 1868 the internal 

 San Francisco shaft was put down from the level of 

 the Main tunnel (300 level) to connect with the Santa 

 Rosa drift from the central stope area of the New 

 Almaden mine. The ore bodies that had been found 

 on the 300 and 400 levels were not large, but were rich 

 enough to encourage further development. In 1871 

 the Warren stope ore body was found near the 500 

 level, and by the end of 1873, after 9,197 tons of good 

 ore had been mined from the Warren and higher 

 stopes, the mine seemed to be nearly exhausted. 



In June 1874 the extensive New World ore bodies 

 (fig. 81) were found near the 600 level. During the 

 next 4 years these were followed downward to and 

 below the 800 level, and the D:iy tunnel, on the 800 

 level, was extended to serve this part of the mine. 

 To search for the continuation of the New World ores 

 at greater depths the Garfield (Washington) shaft 



