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University of California Publications. 



[Geology 



depth. Some of these pass into a massive greenish or brownish 

 rather soft rock, evidently a decomposition product, and then 

 penetrate a garnet rock, or garnet and quartz, in which is usually 

 found fresh chalcopyrite and pyrite, although in no great quan- 

 tity. This is true of the Morris shaft near Pilot Knob, and of 

 a shaft sunk on the Emma Nevada claim. A similar occurrence 

 was found in the third level of the mine at Copper Flat, the 

 gradation from the fresh garnet rock with some chalcopyrite 

 into an olive brown or greenish decomposition product being 

 perfectly apparent in the abundant material on the mine dump. 



It is not proposed at this place to discuss the genetic rela- 

 tions of this garnet rock and its decomposition product to the 

 quartz blouts ; but it is well to note that the field evidence indi- 

 cates that they are associated to the extent that there are bodies of 

 garnet rock with chalcopyrite in the midst of the porphyry in 

 the vicinity of, or beneath, the blout. 



THE PORPHYRY AS A COPPER ORE. 



Certain portions of the porphyry mass, notably at the Ruth 

 Mine and at Copper Flat, have been so mineralized as to con- 

 stitute an ore of copper. In considering the characteristics of 

 this ore and its genesis, it will be first necessary to recognize 

 the fact that the porphyry, considered as an ore, is, like most 

 metalliferous deposits, separable into two zones: the zone of 

 oxidation near the surface, and the unoxidized zone beneath 

 this, extending to depths at present unknown. The porphyry 

 of both zones may constitute a copper ore but each in a different 

 way. At the Ruth Mine the porphyiy of the oxidized zone is 

 almost wholly if not quite devoid of sulphides. It has a pre- 

 vailingly yellowish color, but is in many places of a deep red 

 color due to the abundance of iron oxide. It carries never more 

 than a small fraction of one per cent, of copper, and frequently 

 none can be detected. The passage from the oxidized zone to 

 the unoxidized is very abrupt and sharp, and, below the divid- 

 ing line, the porphyry throughout the mine is bluish white, and 

 is usually well sprinkled with crystals of pyrite and chalcocite 

 of small size, probably averaging half a millimeter in diameter. 

 The immediate appearance of these two sulphides at the divid- 



