VEINS OF TONOPAH RHYOLITE-DACITE. 99 



greater ratio of gold to silver in them as compared to that in the earlier andesite 

 veins. In the earlier andesite veins the gold averages about two-fifths of the value, 

 the silver three-fifths, while in the rhyolite-dacite veins the gold is likely to exceed 

 this amount and sometimes occurs with practically no silver, although the proportion 

 is very changeable. Very often again the proportion of gold and silver is the same 

 as in the earlier andesite veins. 



AGE OF TONOPAH RHYOLITE-DACITE VEINS. 



These veins are younger than the Tonopah rhyolite-dacite, in which they 

 usually occur. In the mine workings referred to above this lava is a deep-seated 

 injection corresponding in age and composition to a great mass of surface breccias 

 and tuffs in the southern half of the area mapped. Even in the lower part of the 

 white tuffs or lake beds which succeeded the deposition of the volcanic ejectamenta 

 of this period there are intrusive sheets of the rhyolite-dacite. In this portion 

 of the tuffs occur the elliptical outcrops of the pipe-like deposits, formed by hot 

 springs in the hills west of Siebert Mountain. Thus the period of this mineral- 

 ization was, in broad terms, contemporaneous with the volcanic activity of the 

 Tonopah rhyolite-dacite period, and very likely persisted for some time after- 

 wards. These veins are plainly the results of ascending hot waters, and represent 

 the effects of the Tonopah rhyolite-dacite eruption. They have the same relation 

 to these eruptions that the earlier andesite veins had to the eruptions of the 

 earlier andesite. 



The characteristic lack of definition and persistence in these veins as compared 

 with the veins in the earlier andesite shows that at the time they were formed no 

 definite fracture zones were available as channels, so that the ascending waters had 

 to force themselves up along irregular courses. This means that the faulting now 

 so characteristic of the district had not begun at the time of this mineralization, 

 and therefore that this mineralization ceased before the beginning of that period 

 of rhyolite and dacite injections and eruptions which is marked by the rhyolite 

 and dacite necks that form the hills around Tonopah. The mineralization is then 

 probably the same in time, nature, and origin as that at Gold Mountain, 4 miles 

 south of Tonopah," and very likely similar to that in the newly discovered camp 

 of Goldfields, about 28 miles south of Tonopah. 



GENERAL RESTRICTION OF VEINS TO RHYOLITE-DACITE. 



At first it seems strange that in underground workings like the West End* 

 the MacNamara, etc., these rhyolite-dacite veins do not extend into the earlier 

 andesite in which the rhyolite-dacite is intrusive. The fact that such veins 



a Bull. U. S. Geol. Survey No. 218, p. 87. 



