ORE DEPOSITS 



95 



has been to produce in some places a steplike intrusive 

 contact, which partly parallels the shearing in the alta 

 and partly follows these faults that run across the 

 grain of the alta. The cross-cutting faults continue 

 on into alta, but do not extend into the serpentine. 

 In many places it appears likely that the wallrocks 

 were fractured before the intrusion of the serpentine, 

 and the intrusive was responsible only for differential 

 movement along preexisting fractures. 



Faults of this type are best observed at scattered 

 localities in the underground workings of the New 

 Almaden mine, where nearly complete exposure in 

 three dimensions aids in determining their true rela- 

 tionships. Good examples may be noted in the north- 

 ern part of the Harry area on the 600 and 700 levels, 

 where step faulting has raised the alta side of the in- 

 trusive contact at intervals of a few feet, and devel- 

 oped mullions extending down the dip of the fault 

 contact. Faults of this kind are also well exposed 

 between the 600 and 700 levels in the New World stope, 

 where the offsets exceed 10 feet in a few places, and 

 in the Giant Powder and Ponce stopes of the Santa 

 Rita area, where the normal faults may be seen to 

 veer off from the silica-carbonate rock and die out in 

 the Franciscan wallrocks. 



TENSION FRACTURES 



The thousands of tension fractures in the silica- 

 carbonate rock of Los Capitancillos Ridge, and to a 

 minor extent in similar rock in the Santa Teresa Hills, 

 are small, but economically important structures in 

 the New Almaden district. During the quicksilver 

 mineralization they were open, but during the waning 

 stages of mineralization they became filled with dolo- 

 mite and quartz. They are generally no more than 1 

 or 2 inches wide, although a few are as much as 6 

 inches wide. Individual fractures generally extend 

 less than a hundred feet along the contact between 

 the silica-carbonate rock and the Franciscan rocks, 

 and the distance to which they extend into the Fran- 

 ciscan rocks is generally much less. Although indi- 

 vidual fractures are not very long, they are commonly 

 grouped in swarms which may extend for hundreds of 

 feet. The most striking feature of these fractures is 

 their prevalent N. 30 E. strike and steep dip, which 

 is maintained regardless of the attitude of the larger 

 structural features in which they occur. The strikes 

 recorded for thousands of these fractures in various 

 parts of the district do indeed range from due north 

 to N. 50 E., but a large proportion of them are be- 

 tween N. 25 E. and N. 35 E. The dips may be as 

 low as 60 in either direction, but most are within 

 15 of vertical. 



Nearly all the fractures are filled with vein mate- 

 rial, which consists mostly of dolomite and quartz but 

 locally contains pyrite, hydrocarbons, and cinnabar. 

 Veins of this type are so prevalent throughout the 

 silica-carbonate rock of Mine Hill that the early min- 

 ers gave them a name, "hilos" (Spanish for threads, 

 and comparable to our term "stringers"), and regarded 

 them as guides to ore bodies or integral parts of them. 

 These hilos in most places do not contain enough cin- 

 nabar to constitute ore, but the fractures were the 

 principal channelways for the ore-forming solutions. 

 Hilos are conspicuous in many of the accessible parts 

 of the mines at New Almaden and are very numerous 

 in part of the Guadalupe mine, particularly the Water 

 tunnel stopes. Scattered hilos have also been observed 

 in the silica-carbonate rock bodies of the Santa Teresa 

 Hills, including those explored by the Santa Teresa 

 and Bernal mines, but here they are somewhat less 

 numerous and less uniform in attitude than in Los 

 Capitancillos Ridge. 



ORE DEPOSITS 



The economically important ore deposits of the 

 New Almaden district are those that contain quick- 

 silver, but small amounts of ores of chromium, man- 

 ganese, and copper, bearing no direct genetic relation 

 to the quicksilver ores, have been noted. The recorded 

 production of quicksilver from the district between the 

 recognition of the quicksilver ores in 1845 and the end 

 of 1945 was 1,137,727 flasks, with a value of about 

 $55,000,000. A few hundred pounds of chromium also 

 has been produced, but only the quicksilver deposits 

 have been fully studied, and they alone are compre- 

 hensively described in this report. 



Even though the New Almaden district has yielded 

 far more quicksilver than any other in the United 

 States, in 1948, when the first draft of this report was 

 written, the mines were inactive and many of the 

 workings were inaccessible. Furthermore, in contrast 

 to many other districts in the California Coast Ranges, 

 this district did not respond to the stimulus of high 

 prices during the two world wars by again reaching 

 its former high level of production, although the 

 mines were active during these periods. The study 

 leading to this report indicates that, in spite of the 

 large amount of exploration already done, the district 

 should not be regarded as worked out, for undiscov- 

 ered bodies of quicksilver ore probably remain along 

 the mineralized belt. Because the history of the ex- 

 ploration and development of the individual mines 

 has direct bearing on the possibility of recovering 

 additional ore from them, pertinent historical facts 

 have been included with the individual mine descrip- 



