108 



GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 



FIGURE 74. Extremely rich replacement ore from the New Inclined shaft area of the Guadalup* mine. 



LOCALIZATION OF THE ORE BODIES 



The ore bodies in the district are not distributed at 

 random ; most of them are in certain rocks and in cer- 

 tain structural environments. The following section 

 discusses the influence of these controlling factors, 

 which were responsible for the restricted distribu- 

 tion of the known ore bodies and which should be 

 considered in seeking new ones. 



LITHOLOGIC CONTROL 



In the New Almaden district at least 99 percent of 

 the quicksilver ores mined were in silica-carbonate 

 rock. This rock contained not only the greatest num- 

 ber of ore bodies but also the largest and richest. 

 Therefore, even though ore bodies have been found in 

 other rocks in the district, exploration for new ore 

 bodies in silica-carbonate rock is most likely to be 

 rewarded. There are probably several reasons why 

 the silica-carbonate rock was the most favorable for 

 ore deposition. The rock is exceptionally brittle, and 

 it generally forms a thin shell between incompetent 

 serpentine and sheared rocks of the Franciscan group: 



it was therefore the rock most likely to be fractured 

 by even mild deformation. Hence it provided the 

 channels for the migration of mineral solutions and 

 openings for the deposition of ore minerals. And. 

 because of its mineral content, it \v;is especially sus- 

 ceptible to replacement by cinnabar-bearing solutions. 

 The small bodies of ore in other rocks of the dis- 

 trict, however, are locally of economic importance ;i^ 

 well as of scientific interest. Apart from silica-car- 

 bonate rock, only rocks of the Franciscan group and 

 serpentine occur along the mineralized belt in the 

 district; only these, therefore, have had a good op- 

 portunity to become mineralized, (i ray \vacke and 

 shale contained the ore bodies of the San Mateo mine, 

 and also some of the ore mined in the Yellow Kid 

 workings in the southern part of the Harry area of 

 the New Almaden mine. Altered greenstone or tuff 

 contained the ore mined in the upper Cora Blanca 

 and Ixis Angeles workings of the New Almaden mine. 

 Chert, which is the host rock for ore bodies else 

 where in the Coast Ranges (Bailey, 1946, p. 217, 219- 



