160 



GUIDEBOOK OF THE WESTERN UNITED STATES 



AtCobre (pronouiiced co^brajj Spanish for copper) is the junction 



with the Nevada Northern Raihoad^ which since 1906 



access to the Ely or Robinson copper 



Cobre. 



Elevation 5,922 feet, 

 Omaha 1,138 miles. 



has 



given 



districts/ 140 miles to the south, and a number 

 of other less well-known districts, including Cherry 

 Ci*eek and Egan Canyon.^ 



West of Cobre the railroad crosses a number of scarcely perceptible 

 divides. The old town of Toano, opposite milepost 643, is now 

 represented only by a few fallen and deserted stone buildings. These 

 were built from blocks cut from the sandstone of the Humboldt for- 



* The first ailniiig locations in tlie 

 vicinity of Ely were made in 1867, tlu-ee 

 years after tlie organization of the Eureka 

 mining district, in the same year in which 

 bonanza silver orets were discovered in the 

 "WTiite Pine district, 60 miles to the west. 

 Early operations disclosed a few deposits 

 of lead -bearing ores carrying precious 

 metals to the value of $10 to $40 a ton. 

 Occasionally small bonanzas were found, 

 and shallow deposits of rich copper ore 

 were mined. 



The present copper industiy^ of the dis- 

 trict is the outgrowth of explorations that 

 began about 1901. The aggregate quan- 

 tity of low-grade sulphide ore developed 

 is perhaps 80 million tons, in which the 

 mean- copper content is a little over IJ 

 per cent. In 1906 extensive reduction 

 works were built at McGill, on the east 

 side of Steptoe Valley, about 25 miles 

 from the mines. 



The sedimentary rocks of the district, 

 comprising limestones, quartzites, and 

 shales, range in age from Ordovician to 

 Pennsylvanian. They have been dis- 

 turbed by folding and especially by 

 faulting and have been invaded by 

 masses of ierneous rocks 



(monzonite 



porphyr^O • 



The ore, like the greater part of that at 

 Bingham, Utah, consists of monzonite 

 porph>T}% greatly altered (metamor- 



ose- 



carrying disseminat 



p>Tite and chalcop^iite, and varpng 

 amounts of chalcocite. Masses of por- 

 phjTy which, through metamorphism, 

 had been almost uniformly charged with 

 grains of pyrite and chalcopyrite became 



subject to erosion and oxidation. As the 

 rock was gradually worn down, surface 

 waters attacking the metallic sulphides 

 and charged with copper derived from 

 them soaked downward into the rock and 

 deposited the dissolved copper by chem- 

 ical reaction with the pyrite and chal- 

 copyrite in the rock. In this way a part 

 of the rock was gradually converted into 

 ore by addition of the copper sulphide. 

 Superficial examination of ore samples 

 shows a white to gray rock specked 

 through and through with a black min- 

 eral, which is the rich copper sulphide 

 chalcocite. On close inspection it is 

 found that this mineral occurs mainly as 

 films or coatings on grains of the pale- 

 yellow iron mineral pyrite or the deeper 

 yellow copper-irou sulphide chalcopyrite. 

 The oxidized capping or overburden has 

 an average thickness of about 100 feet. 

 The underlying ore blankets are from 

 15 ^to 500 feet thick. Up to the present 

 time comparatively little underground 

 mining has been done, though caving 

 methods were employed in the Veteran 

 mine. The Ruth ore body, estimated to 

 contain 8 to 10 million tons of ore car- 

 rying over 40 pounds of copper to the 

 ton, may be mined in a similar "way. 

 ^\'here the overburden is shalloAV the ore 

 is mined by steam shovels, and between 

 1908 and January, 1914, nearly 12 million 

 tons of ore averaging about 38 pounds of 

 copper to the ton had been produced in 

 this way, and in addition some 20 million 

 tons of overburden had been removed. 



^ On the west side of Steptoe Valley, 93 

 miles south of Cobre, are the Cherry Creek 

 and Egan Canyon mines, in a low pass 



