PRECIOUS METALS 



67 



of tuffs and breccias of Tertiary age. These are cut by dikes of 

 phonolite, latite-phonolite, syenite, dolorite, and even the more 

 basic rocks. (See Fig. 46.) 



The ore bodies occur in two forms. (1) Lodes or veins, and 

 (2) irregular replacement deposits. The veins are exceedingly 

 narrow fissures incompletely filled. They are essentially in the 

 volcanics and present a radical appearance. They are short and 

 nearly vertical (Fig. 47). Some of the most productive fissures 

 have been only a few hundred feet in length. In fact the entire 

 field is circular in form with a radius of 2 or 3 miles. 



The lodes may occur in both the eruptives and the irruptives. 



Scale 

 10 



20 feet 



FIG. 48. Sheeted zone and flats of the Apex vein, Ajax mine, Cripple 

 Creek district, Colorado. (After W. Lindgren and F. L. Ransome, U. S. 

 Geological Survey.) 



In the former case they favor the breccias and in the latter the 

 granites, as shown in Fig. 48. The fissures seem to have been 

 formed by compressive stresses associated with the cooling igneous 

 rocks. The fissures are particularly small and narrow and may 

 occur in any rock in the series. (See Fig. 49.) 



The replacement deposits usually occur in the granite. The 

 principal gold ore is the telluride, petzite or calaverite which 

 upon roasting brings the gold to the surface forming beautiful 

 museum specimens. Pyrite is associated with the tellurides. 

 Near the surface and in the oxidized zone in general the gold 

 appears as brown, spongy gold while the tellurium has been con- 

 verted into tellurites. 



The common gangue minerals are quartz, fluorite and dolomite 



