CALCITIC VEINS OF ARARAT MOUNTAIN. 101 



elsewhere (see p. 182), the earlier andesite vein has been reopened and a new vein 

 of barren jaspery quartz formed along the hanging wall. This is probably due 

 to waters of the rhyolite-dacite period of mineralization. In the case just men- 

 tioned the new quartz is barren as compared with the old. It is evident, however, 

 that such solutions must have dissolved a great deal of the gold and silver contained 

 in the earlier veins, and naturally may have reprecipitated it elsewhere. In this 

 case the ores might be reprecipitated in a concentrated form. This very likely 

 has been the case in the Montana Tonopah, where, as described (see p. 171), the 

 original vein has been reopened and in the fissure thus formed minerals similar to 

 those in the older vein, but richer in gold and silver, have been precipitated in 

 crustified form. It is very likely that this was the work of the waters of the 

 rhyolite-dacite period, of the same kind and character as those to which the 

 barren quartz hanging-wall portion of the vein in the Tonopah Extension is 

 due. 



Again, it is natural that such waters may have dissolved some of the metallic 

 contents of the older veins and, instead of precipitating them within these veins, 

 may have carried them out and deposited them elsewhere, as, for example, in the 

 veins of the rliyolite-dacite, forming bunches of high-grade ore in these usually 

 barren veins. This may be the explanation of the comparatively small amount 

 of rich ore found in the rhyolite-dacite veins, as, for instance, in the Desert Queen 

 and the MacNamara. These are practically the only cases of high-grade ore in 

 the district in veins of this period, and in both cases the veins are in the vicinity 

 of rich earlier andesite veins and the ores have a character altogether similar to 

 that of the earlier veins. Outside of the earlier andesite vein region, the veins 

 in the rhyolite-dacite have been found to be frequently large, but typically are 

 low grade or barren. 



THE CALCITIC VKINS OF ARARAT MOUNTAIN. 



THE RHYOLITE OF ARARAT A VOLCANIC PLUG. 



The top of Ararat Mountain is composed of white rhyolite like that of Mount 

 Oddie. On the southwest side this is intrusive into the later andesite, and on 

 the other sides into the glassy Tonopah rhyolite-dacite, which is itself intrusive 

 into the later andesite. The area of white rhyolite is broadly ellipical in outline, 

 with the longer axis of the ellipse, as in the case of most of the other hills on 

 the map, lying in a general east-west direction (PI. XIV). 



The contact, as is shown by the Wingfield tunnel and the Boston Tonopah 

 shaft and in other places, pitches steeply all around. The rhyolite is then in the 

 nature of a volcanic column or plug which has been forced up into the older 

 rocks, and which probably occupied the vent of an old volcano, now removed by 

 erosion. 



