ORE DEPOSITS 



111 



Workings and geology by U.S. Geological Survey 



Serpentine with no 

 veins. Rest of slope 

 in silica-carbonate 

 rock 



EXPLANATION 



Hilo, or narrow quartz- 

 dotomite vein, showing dip 

 Locally contain some cinnabar 



Vertical hilo 



O 



Slope 



50 FEET 



DATUM IS MEAN SEA LEVEL 



FIGURE 78. Map of La Ventura stope of the New Almaden mine where the localization of the ore is unusual In that It Is along northwest- 

 trending fractures rather than aloiig fractures with a northeast trend. 



contact into the silica-carbonate rock can rarely be 

 determined; but they generally taper gradually to be- 

 yond the limits of any ore, and in some places they 

 extend at least a hundred feet from the contact. Their 

 extent from the contact into rocks of the Franciscan 

 group is more variable. In some places, especially 

 where the rocks of the Franciscan group are hardened 

 by alteration, the hilos penetrate, though with much 

 diminished size, for a few feet into the country rock. 

 In other places they terminate exactly at the contact. 

 Exceptionally, as in the Water tunnel above the Cora 

 Blanca workings of the New Almaden mine, more 

 persistent veins with strikes similar to those of the 

 hilos are found in the rocks of the Franciscan group 

 at distances of several scores of feet from silica-car- 

 bonate rock, but in general the typical hilos are con- 

 fined to the silica-carbonate rock near its contact with 

 the rocks of the Franciscan group. 



Similar steeply dipping quartz-carbonate veins that 



strike N. 40 -50 W. are in a few places considerably >. 

 more abundant than the typical northeast-striking 

 hilos. They appear to represent a conjugate system 

 related to the typical hilo fractures, rather than a 

 local deviation from them, for they accompany, rather 

 than take the place of, the normal hilos. In at least 

 one stope La Ventura (fig. 78) this second system 

 of fractures probably had an important part in local- 

 izing ore. 



Swarms of hilo fractures were the dominant struc- 

 tural control for some ore bodies, particularly those 

 along steep contacts, as in the Randol area of the 

 New Almaden mine and in the central part of the 

 Guadalupe mine. (See figs. 93, 89.) The fractures 

 were also an important factor in the formation of 

 many of the other ore bodies of the New Almaden 

 mine, as is indicated by the elongation of many of 

 the stopes parallel to the trend of the hilos. (See 

 fig. 79 and pi. 4.) It should be emphasized, however, 



