122 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



waters has been freer and the result greater. In these portions the channels must 

 have been more open, and since the main vein zone was a single set of fractures, 

 with fairly uniform conditions, the difference in degree of openness, influencing 

 circulation, must have depended largely on the cross fractures. In other words, it 

 appears likely that whore these cross fractures were most numerous rude east-dipping 

 columns or chimneys, speaking general!}', were fonhed, in which the circulat- 

 ing solutions were relatively concentrated. 



POSTMINEEAL FAULTS AND FRACTURES. 



Postmineral fractures and faults are com- 

 mon. Besides the great faults mentioned 

 there are continually encountered in the mine 

 many minor ones (figs. 22 and 23) which may 

 prove puzzling to the miner. Small faults 

 are very numerous in the workings in the 

 vicinity of the great Mizpah fault. These 

 faults usually strike north-south and dip 

 east, though they may have other attitudes. 

 Numerous postmineral fractures, along which 

 there has been no movement, have the same 

 general north-south strike and easterly dip, 

 while others have a variety of positions (fig. 

 24). Postmineral fractures parallel to the 

 vein are always present. In other . words, 

 the postmineral fractures in general have the 

 same directions as the premineral fractures, and stress subsequent to the ore 

 deposition has reopened the old wounds, which had been more or less completely 

 healed by the vein formation (fig. 25). 



VEIN COMPOSITION. 



The quartz of the vein is fine and cloudy. Poor quartz and rich quartz are 

 often much alike in appearance, save for a purplish tinge in the latter. Under 

 the microscope this tinge is seen to be due to disseminated particles of argentite. 

 This mineral is found from the outcrop of the vein downward, through all the 

 oxidized zone. Silver chloride is also very abundant, though usually, like the 

 other metallic minerals, it is determinable only microscopically. Orange and 

 yellow amorphous minerals were also observed, and surmised to be the combinations 

 of silver with chlorine, bromine, and iodine, and chemical examination of the speci- 

 mens showed the presence of all these elements. Free gold is sometimes observed, 



Scale 



z feet 



FIG. 22. Sketch of faulted quartz veinlets in ancles. 

 ite, 300-foot level, Mizpah, just south of the Valley 

 View shaft. 



