ORE DEPOSITS 



107 



same geologic structure that they might be thought of 

 as a single ore body with barren patches; if they are 

 so grouped, the composite body was larger than the 

 North Randol ore body. 



In the Guadalupe mine the two ore bodies mined 

 through the older workings, south of Guadalupe 

 Creek, were considerably larger than any found in 

 the newer part of the mine. They were mined in the 

 Thayer and Dore labores ("labor" is Spanish for 

 "stope"; plural, "labores") which measure 300 by 350 

 feet and 400 by 450 feet, respectively, and they are 

 said by Wagoner 7 to have had a thickness of about 

 25 feet. In the Senator mine the largest ore body 

 had an average length of about 175 feet, an average 

 width of perhaps 20 feet, and a pitch length of about 

 800 feet. 



GRADE OF THE ORE BODIES 



The average grade of the ore from a given ore body 

 in the New Almaden district depends, as it does almost 

 anywhere, upon how much of the lower grade or bar- 

 ren material surrounding or mixed with the rich ore 

 was sent to the furnaces. As the ore bodies were at 

 first mined very selectively, and at a later date had 

 their margins stripped to such an extent that hardly 

 any cinnabar can now be seen in some of the old 

 stopes, it is possible to give information on both the 

 average grade of the richest portions mined in the 

 early days and the grade of the entire ore body. The 

 richest ores ever mined from the New Almaden mine 

 were taken out in its earliest days, and during the 

 first 7 years of recorded production the annual aver- 

 age content of quicksilver determined by furnace re- 

 covery was always above 20 percent. To obtain such 

 exceedingly rich ore required not only selective min- 

 ing but extensive cobbing and hand sorting, so that 

 the average for the ore bodies as a whole was much 

 less than 20 percent ; this figure does indicate, how- 

 ever, that the ores mined from near the surface of 

 Mine Hill were extremely rich. Further indication 

 of their richness is given by the nuggets mined from 

 a placer deposit below the original outcrops of ore in 

 the New Almaden mine area; these nuggets have an 

 average cinnabar content of 75 percent, or about 65 

 percent quicksilver. 



A- somewhat closer indication of the average grade 

 of the ore found in the large and exceptionally rich 

 ore bodies is furnished by the Santa Rita ore body, 

 which yielded 25,300 tons of ore averaging a little 

 more than 10 percent of quicksilver. This ore body 

 was reported to have been less rich than the North 



Ardilla body which adjoined it, but for the latter no 

 exact figures are available. Perhaps the best idea of 

 the average overall grade of the New Almaden ore 

 bodies can be obtained from the following figures. 

 From the time that mining began in 1846 until the 

 end of April 1896, when 942,447 flasks had been re- 

 covered, the average grade of all the ore treated had 

 been 4.57 percent, or only a little less than 100 pounds 

 of quicksilver to the ton. Subsequent mining of 

 lower grade ores has diminished this figure somewhat, 

 but probably the average grade of all the ore mined 

 up to the end of 1948 is not below 4 percent. 



To get an indication of the richness of the ore bod- 

 ies in the New Almaden mine as compared with those 

 of other quicksilver mines in California, we may com- 

 pare the yield of quicksilver per linear foot of horizon- 

 tal workings in this mine with that for some other 

 California quicksilver mines. For the New Almaden 

 mine this yield, based on the total extent of all hori- 

 zontal workings including all the nonproductive adits, 

 is 5.9 flasks, which is higher than that of any of the 

 other mines, as shown in table 16. 



TABLE 16. Approximate yield of quicksilver per linear foot of 

 workings in some California quicksilver mines 



At the Guadalupe mine the ore bodies mined from 

 the large stopes to the south of Guadalupe Creek are 

 estimated by Wagoner 8 to have averaged a little less 

 than 5 percent quicksilver. The ore bodies of the 

 Senator mine, exploited between 1910 and 1925, aver- 

 aged only about 0.5 percent quicksilver. 



The material taken from the opencuts of Mine Hill 

 during World War II was even leaner, and yielded 

 only a few pounds of quicksilver to the ton. Although 

 it was profitably mined because of the prevailing high 

 price of quicksilver, and hence was ore according to 

 the usual definition, it contained only scattered cinna- 

 bar crystals and veinlets, and was not comparable to 

 the material that constituted the true ore bodies of 

 the New Almaden district. 



Wagoner, Luther, 1881, Unpublished report on the Guadalupe mine. 



8 Wagoner, Luther, 1881, op. cit. 



