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GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



FIOUBE 73. Polished surface on rich ore formed by replacement of silica-carbonate rock, which, In turn, had replaced 

 sheared serpentine. Neutral gray on left Is cinnabar; rest Is silica-carbonate rock. 



completely fill the openings, it commonly shows crys- 

 tal faces. How widespread this type of ore may have 

 been is unknown; virtually none remains in the stope 

 walls in the accessible parts of the mine, and, judg- 

 ing by museum specimens and the ores found on dumps 

 and in the fill in stopes, it was probably rare. 



In summary, the writers, who have seen none of the 

 real ore bodies in the district, believe from a study of 

 the available information that the phenomenally ricli 

 ore bodies were all composite that cinnabar replaced 

 silica-carbonate rock along closely spaced fractures, 

 which later were filled with dolomite, quartz, and a 

 little cinnabar. Possibly a few of the thinner and 

 flatter blankets of ore are so little veined that they 

 could properly be termed "replacement bodies"; other 

 ore bodies of lower grade apparently contained only 

 the late cinnabar-bearing veins and should be classed 

 as fissure deposits. 



SIZE OF THE ORE BODIES 



The size of the ore bodies mined in the district 

 varies between wide limits. Not only were large ore 

 bodies followed from level to level, but it was cus- 

 tomary also, because of the general richness of the 

 ore, to mine out any small pockets or even single rich 

 veins that were cut in mine development. Thus, only 

 the upper limit to the size of the ore bodies deserves 

 particular attention. The most extensive single con- 

 centration of ore found in the district was the North 

 Randol ore body of the New Almaden mine. This had 

 a strike width of about 200 feet and an average thick- 

 ness of about 15 feet, and was mined down the dip 

 for a distance of 1,300 feet; its total volume was 

 therefore about 4,000,000 cubic feet. The great cen- 

 tral ore bodies in the same mine including the Santa 

 Rita, North Ardilla, La Ventura, Sacramento, liuenos 

 Ayres, and others were so closely spaced along the 



