USEFUL METALS 



147 



silver camp. It maintained this position until about 1880 when 

 it became a copper camp. It will always remain in the literature 

 of mining geology as distinctively a copper camp (Fig 86). 



The primary ore was chalcopyrite and pyrite. It is the enor- 

 mous deposits of secondary chalcocite that have been the large 

 producers of the metal. Other copper minerals appearing as ores 

 are bornite, enargite, covellite and tetrahedrite. The veins are 



FIG. 86. Geologic map of the western half of Butte, Montana, district. 

 (By permission of the Macmillan Company, from Hies' Economic Geology.) 



quite largely replacement deposits along fissures in the sheeted 

 granite. The country rock consists of two types of granite. One 

 is a dark hornblendic granite or quartz monzonite known as the 

 Butte granite. The other is an acid granite or better an aplite 

 termed the Bluebird granite. These granites are intersected 

 by dikes of quartz porphyry. Dikes of both intrusive and extru- 

 sive rhyolite intersect the copper veins. 



