336 



University of California Publications. 



[Geology 



in the vicinity of the porphyry. At the surface, and often foi 

 considerable depths below the surface, these ores are now in the 

 form of the carbonates, malachite and azurite. But these car- 

 bonates are regarded as the result of the alteration of chalco- 

 pyrite in situ. In other localities, particularly at the Morris 

 Shaft near Pilot Knob and at a pit near the road between Copper 

 Plat and Pilot Knob, the quartz blout at depth carries consid- 

 erable garnet, with pyrite and chalcopyrite. In the Copper Plat 

 Mine on the 300-foot level there is an extensive body or so-called 

 "dyke" of garnet rock with nests of pyrite and chalcopyrite. 

 The garnet rock is for the most part thoroughly decomposed to 

 a greenish or brownish material which, when moist, has a cheesy 

 consistency and on drying on the dump, slacks in the air. All 

 gradations may be observed between the perfectly fresh garnet 

 rock and the decomposed material. 



It thus appears that the only traces of primary copper ores 

 in the district are in the form of chalcopyrite associated with 

 garnet, and that certain of these occur in the quartz blout either 

 as the original chalcopyrite or altered to carbonates. 



On this basis the hypothesis may be favorably entertained 

 that the secondary copper ore of the Ruth Mine has been de- 

 rived from the downward leaching of copper from bodies and 

 disseminated particles of chalcopyrite in the course of superficial 

 oxidation of the blout in which they were originally deposited. 

 There are at present vast bodies of blout at the surface imme- 

 diately above the Ruth Mine, but these are only remnants. The 

 greater part of the blout has been removed by erosion. The 

 product of this leaching was deposited as chalcocite in the por- 

 phyry at a time when the water level Avas much higher than at 

 present. And finally, when in consequence of the subsidence of 

 the ground water to lower levels the zone of oxidation extended 

 into the porphyry, this chalcocite was in turn taken into solu- 

 tion and redeposited as chalcocite at still lower levels. This, 

 leaving out of account for the moment the source of the chalco- 

 pyrite, appears to be the history of the copper deposits in the 

 Ruth stated broadly. 



The modifying factors are the structural features. These 

 fall into two categories. (1) The thoroughly sheared and in- 



