4:2 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



rocks, in which places the alteration has been accomplished mainly by hot-spring 

 action succeeding the intrusion. Secondary quartz, pyrite, and sometimes siderite 

 are the chief results, with exceptionally epidote and adularia and very rarely a 

 little calcite. The quartz ma}' form veinlets in the rock as viewed under the 

 microscope, and these silicifications may increase in importance till they form 

 large quartz veins. Vesicles lined with chalcedony were noted in one instance. 



Some of the Tonopah rhyolite-dacite presents, when altered by the processes 

 above referred to, a close resemblance to certain highly altered and silicified phases 

 of the earlier andesite. Field work, however, seems to leave little doubt as to the 

 nature of such types, as they can be traced into unequivocal rhyolite-dacites. 



These altered and silicified rhyolite-dacites, especially those which contain no 

 quartz phenociysts, differ from the similarly altered earlier andesites in the scarcity 

 and smallness of the phenociysts, in the predominating stout, blunt form of the 

 feldspars, which indicates orthoclase where the alteration is so great that no 

 determination can be made, and in the absence or scarcity of apatite. 



Distinction between the northern and the southern areas. The general character 

 and relations of the Tonopah rhyolite-dacite differ considerably north and 

 south of an east-west line across the middle of the area mapped. This line, 

 probably a fault line (see PI. X[), runs up the main gulch along which the road 

 to town passes and into the town. To the north the dacite is always intrusive, 

 as its contacts prove. To the south the petrographic characters are in general 

 the same as to the north, but the geologic relations are more complicated. In 

 many cases the dacite is evidently intrusive, while in other places it occurs in 

 sheets that alternate with pumiceous tuffs and have all the appearance of flows. 

 Under the microscope also new features present themselves and indicate that 

 manv of these rocks are probably fragmental. In thin sections of such rocks 

 the autoclastic glassy dacite has been finely broken mechanically and the 

 fragments are intersected by dense kaolinic matter into which iron has infiltrated. 

 The material seems to be an unassorted accumulation of angular fragments, 

 which resulted from a shower of dacitic ash 'and lava fragments during and after 

 explosive eruptions. 



The southern part of the area mapped has in general been depressed below 

 the northern half by faulting, and here internal faulting has been much more 

 active than in the other part (see p. 47). This depressed tract exactly corresponds 

 with the area of intermingled Tonopah rhyolite-dacite dikes, flows, and tuffs. In 

 the relatively elevated northern portion of the area mapped the surncial formations 

 have been largely worn away, and only the intrusive portions of the Tonopah 

 rhyolite-dacite are left, while in the southern portion the corresponding flows and 

 tuffs, as well as the feeding dikes, remain. 



