152 



ECONOMIC GEOLOGY 



those at Bisbee. Mi neralogically the condi tions differ more widely . 

 The geological section embraces igneous and sedimentary rocks 

 ranging in age from pre-Cambrian to Quarternary. Post-Creta- 

 ceous granitic and dioritic porphyries cut all the older formations. 

 The ores are dependent upon the porphyries for their existence. 

 The ores occur in the porphyry, or close to its contact, or along dikes 

 of the porphry as it cuts the sedimentaries. These fissure veins 

 traversing the granite, porphyry, and clastic rocks carry, in their 

 unoxidized portions, chalcopyrite, pyrite and sphalerite, while in 

 the oxidized portion the leaching out of the copper and its down- 



FIG. 91. The Old Dominion mine and smelter from the west, Globe, 

 Arizona. (After F. L. Ransome, U. S. Geological Survey.) 



ward transference has given rise to secondary chalcocite. (See 

 Fig. 90.) 



(3) The Globe District: According to H. Ries, the ore bodies 

 occur as lenticular replacements in limestones and fault lodes, 

 or fissure zones in diabase. The ores in the upper levels are of the 

 oxidized type. At the lower levels they are enriched. Some 

 bodies of primary ore with commercial significance have been 

 developed. (See Fig 91.) 



(4) The Jerome District: The rocks of this district are pre- 

 Cambrian metamorphics. The ores are of the same age and re- 

 place a schist that has been formed by the intense shearing of the 

 basic porphyry. The ores are bornite and chalcopyrite with a 



