COPPER ORES IN THIS COUNTRY 265 



black oxides and accumulations of native copper are of fre- 

 quent occurrence when opening the mines. 



The Coronado mines (Arizona) are an example of a rapid 

 depreciation of copper ores with depth. Surface ores assayed 

 6 to 45 per cent copper, and bodies of pure copper glance 

 cropping out at the surface s?.emed to indicate a large future 

 value underground. A tunnel 150 feet long was in solid 

 copper glance 1 to 2 feet wide, averaging 40 per cent metallic 

 copper. The vein is 5 to 15 feet wide, traceable through 

 the porphyry dike. The walls are decomposed and kaolinized. 

 Where the vein is not decomposed it is poor. 



Copper glance in the Coronado mines occurs massive, 

 but disappears at a depth of 150 feet, being replaced by yellow 

 copper pyrites, sparingly disseminated. Analysis shows from 

 11 to 21 per cent copper. In the granite the ore occurs also as 

 copper glance, but not continuously. The ores of the Bisbee 

 district, Arizona, are also in limestone at contact with 

 porphyry. 



Copper signs can be traced in the district for five miles 

 with occasional large iron caps or bodies of iron oxide which 

 may mark copper bodies below. In the southwest region it 

 is observed that when the outcrop is compact and not porous, 

 honeycombed, spongy, or decomposed, no copper is found 

 beneath. In the Silver Bear claim, oxidized copper ore 

 changed into copper glance and soon played out. 



The ore in the Globe mine is in a great continuous chimney 

 running 23 per cent copper. At the Black range the ore 

 is at contact of diorite and slate in a tableland of limestone; 

 below the surface zone of carbonates and oxides is one of 

 sulphurets carrying silver as well as copper. 



It has been noticed that in some mines rich deposits 

 of native silver are found, just at the line of demarcation 

 between oxidized and unoxidized ores. In the southwest 

 the successful mines are in a carboniferous limestone at con- 

 tact wdth eruptive diorites, felsites and porphyries. 



As long as the veins remain in the limestone they are 

 profitable, but on entering the eruptive, or acid rocks, be it 

 sandstone or porphyry, the veins narrow. The more and 

 thicker the limestone is the better and thicker the ore. 



