ORE DEPOSITS 



12f) 



are somewhat younger than (lie period of igneous ac- 

 tivity repivsenti-il by this bed, and as cinnabar is found 

 in fractures in similar rock to the north, the genetic 

 relation between the ores and the igneou, ac'ivily is 

 not so clear as Becker would have one believe. The 

 ore-forming fluid obviously rose from a deep-seated 

 source, and possibly this source was the same magma 

 chamber in which the magma previously formed, but 

 such n relationship is not proved by the available 

 evidence. 



RELATION TO MINERAL SPRINGS 



The association, and supposed genetic relationship, 

 between quicksilver ore and hot springs has been em- 

 phasized in textbooks on ore deposits; but so far as 

 we know there are no hot springs in or near the New 

 Almaden district. Mine surveyors' records, moreover, 

 contain no mention of hot waters, or even of excessive 

 heat in the deeper parts of the New Almaden mine, 

 though they repeatedly observed the abundance of 

 leases, ("old springs, on the other hand, giving off 

 abundant carbon dioxide are found in at least three 

 places in the area. One of these, near the mouth of 

 Soda Spring Canyon on the west edge of the district, 

 is far removed from any known ore deposit. The 

 other two are close to Mine Hill. Near the junction 

 of Deep (itilch and Almaden Canyon is a mineral 

 spring whose water, naturally charged with carbon 

 dioxide, was at one time bottled and sold as "Almaden 

 Vichy water" (Hanks and Irelan, 1888, p. 73); and 

 when a pit dug for mining cinnabar nuggets in this 

 area became filled with water, gases bubbled to the 

 surface in several places. There are other mineral 

 springs in the vicinity of the Soda Springs tunnel, 

 south of the Knriquita mine, and here the springs 

 have built up a thin but striking accumulation of 

 calcium carbonate tufa. A similar area of tufa lies 

 about a mile southwest of the summit of Mine Hill, but 

 no active springs were noted in it. 



lu the deeper workings of the New Almaden mine 

 carbon dioxide was encountered in such abundance 

 that it drove the miners from the working face many 

 times, and in some of the deepest levels it is reported 

 to have been under siillicieut pressure to have "burst 

 the face out." It was so abundant in the 1400-level 

 crosscut driven from near the bottom of the Santa 

 Isabel shaft toward the American mine that when the 

 crosscut was abandoned it was bulklieaded. and the 

 gas was piped to the surface, compressed into metal 

 containers, and sold for commercial use. These min- 

 eral springs and the continued evolution of carbon 

 dioxide gas may all be attributed to the last dying 

 stages of the magmatic activity in the district. 



SUMMARY OF ORIGIN OF THE QUICKSILVER ORE BODIES 



The quicksilver ore bodies of the New Almaden dis- 

 trict are believed to have formed (hiring the Pliocene 

 epoch. They resulted mainly from the replacement 

 of silica-carbonate rock by cinnabar, but in minor part 

 from replacement of other rocks and the tilling of open 

 space by cinnabar or quartz-dolomite veins containing 

 cinnabar. The cinnabar was deposited in the silica- 

 carbonate rock from rising alkaline hydrous solutions 

 following fracture zones, which in most places are best 

 developed near the contacts between the silica-carbon- 

 ate rock and rocks of the Franciscan group. The rich- 

 est ore bodies resulted from spreading out and slowing 

 down of these solutions in structural traps formed by 

 a capping of the relatively impervious alta, but some 

 fairly good ones were formed under alta along steep 

 contacts where hilo fractures were abundant. Some 

 ore bodies, moreover, were formed above alta rather 

 than below it. The depth of deposition extended from 

 near the surface to about 2,600 feet, and the tempera- 

 ture range through which the ores were deposited can 

 be no greater than 25 to 280C and is more likely to 

 be from 50 to 150C. Erosion of the primary ore 

 and redeposition has resulted in the formation of an 

 unusual placer deposit in Almaden Canyon about a 

 mile downstream from the surface exposure of the ore. 

 The various stages of rock alteration, mineralization, 

 erosion, and redeposition that resulted in the primary 

 and placer ores are summarized on plate 2. 



OTHER MKTALXJC DKPO8ITS 



COPPER 



A gossan zone containing oxidized copper minerals 

 has been prospected on the northeast slope of Fern 

 Peak, at an altitude of 1,050 feet and about 0.7 mile 

 S. 15 E. of the Hacienda. The gossan, which is only 

 about 8 feet wide, crops out between serpentine and 

 greenstone of the Franciscan group and consists largely 

 of ocherous rock cut by veinlets of pyrite-bearing 

 sugary and vuggy quartz. The rock contains some 

 scattered crystals of cuprite, and is locally veined and 

 coated with thin crusts of malachite. 



Two shafts, one now filled and the other now inac- 

 cessible but open for at least 75 feet, were sunk many 

 years ago in the gossan zone, and 500 feet soul Invest 

 of the shafts are :\ caved adits driven to explore the 

 zone at depth. These adits all begin in serpentine, 

 but the lowest, which is apparently the longest, had 

 some oxidized copper ore on its dump, indicating that 

 it reached the gossan zone. We have no knowledge 

 either of the history of these very old inaccessible 

 workings or of what they revealed. Probably the 

 metal sought was copper, but the gossan may also con- 



