MIZPAH VEIN. 



117 



On the 700-foot level, south drift, above the Siebert fault, there was encoun- 

 tered a higher body of Tonopah rhyolite-dacite containing some barren quartz, 

 and similar rock occurs on an east drift on the same level. An east drift on the 

 500-foot level ran into a mass of the same formation. 



VEIX STRUCTURE. 



The Mizpah vein is usually several feet wide. Its walls are always earlier 

 andesite, which is generally completely altered to quartz, sericite, etc. The vein 

 may be succinctly described as a sili- ^ S 



cified and mineralized sheeted zone in 

 the andesite. There are all stages of 

 transition from the sheeted altered an- 

 desite (PI. XX) to solid quartz. Both 

 extremes may be observed at many 

 places along the vein, and sometimes 

 not very far apart. More frequently 

 the vein is intermediate in character, 

 showing a variable amount of quartz in 

 the altered porphyry. Sometimes the 

 quartz forms parallel streaks or vein- 

 lets and sometimes it occurs reticulated 

 in the decomposed rock. Frequently 

 some of these small veinlets possess comb 

 structure, which shows that the3 r origi- 

 nated by deposition in open cavities; but 

 their frequently irregular branching and 

 their distribution indicate that these cavi- 

 ties were caused by solution by circulat- 

 ing waters and not by fracturing. Their 

 very existence proves that the main 

 zone did not originate by fracturing. 

 As a rule, however, even the small 

 veinlets give no evidence of having been deposited in cavities, but have evidently 

 been formed by a process of silicitication of the porphyry involving replacement, 

 the extreme of the process which has altered the andesite near the veins. This 

 profound alteration of the zone which became the vein was caused by close-set 

 parallel fractures, which marked this zone, and which afforded a favorable channel 

 for the silicifying and mineralizing solutions. 



The main premineral fractures had therefore the course of the present vein, 

 striking east and west and dipping steeply north. Frequently, also, the walls are 

 locally not parallel (fig. 18). 



Scale 

 10 



20 feet 



FIG. 18. Vertical cross section of Mizpah vein. Oddieand 

 McMann lease, showing diverging walls. 



