CHAPTER II. 

 MINERAL VEINS. 



VEINS OF THE EARLIER ANDESITE. 

 PERIOD OF MINERALIZATION. 



The most important veins of the Tonopah district occur in the earlier andesite 

 and do not extend into the overlying rocks; hence, where the earlier andesite -is 

 not exposed at the surface the later rocks form a capping to the veins, and this 

 capping must be passed through before anything can be learned of the presence 

 or the nature of the veins beneath. This fact shows pretty plainly that the veins 

 were deposited before the eruption of the later andesite and immediately after 

 that of the earlier andesite, for the period of erosion between the two andesites 

 seems to have exposed the veins at the surface, indicating that they were formed 

 before this period or early in it. Indeed, there is every evidence that the veins 

 were formed by ascending hot waters succeeding and connected with the earlier 

 andesite intrusion, and that these waters had become inactive by the time of the 

 later andesites. 



NATURE OF CIRCULATION CHANNELS. 



The openings which afforded channels for these ascending waters were of the 

 nature of sheeted zones. The rock was complexly fractured, apparently soon 

 after cooling, and probably as a result of the stresses exerted by the still active 

 volcanic energy below. A major set of fractures extended in an east-west direc- 

 tion and zones of close-set parallel fractures attained a maximum thickness of 

 several feet. These became the chief channels of circulation. In places the 

 circulating waters divided into separate channels, which diverged and frequently 

 reunited, and many lateral channels were favorable to egress of the waters. 

 These channels, however, were apt to get poorer as the distance from the main 

 fracture zone increased. 



The conditions above stated are clearly shown by a study of the veins of 

 Mizpah Hill and vicinity (fig. 12). The circulation channel now occupied by the 

 Mizpah vein may be taken as a type of the main fracture zones, and the diverging 

 Burro veins, dwindling as they increase their distance from the master veins, 

 represent the lateral channels. The splitting and reuniting is shown by the 



structure of the veins at many points. 



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