250 GEOLOGY OK TONOPAH MINING DISTRICT, NEVADA. 



SCARCITY OF EPIDOTE AS AN ALTERATION PRODUCT. 



Epidote, so common in similarly altered rocks elsewhere, is rare in the later 

 andesite at Tonopah, and where found is often in positions suggesting that the 

 conditions of alteration ma}- have been abnormal. For instance, bowlders of later 

 andesite in explosive volcanic ash and breccia not far from the contact of the Golden 

 Mountain dacite, east of Mizpah hill, show feldspar and biotite phenocrysts largely 

 altered to epidote. Also rare epidote was noted in one or two specimens from the 

 Halifax shaft. In a shaft sunk to "a depth of 60 feet in decomposed later andesite, 

 just west of the Siebert shaft dump, a specimen was collected which carried rather 

 abundant epidote. This, however, is exceptional, and the typical alteration seems 

 to be illustrated by the detailed descriptions and analyses given. 



COMPOSITION OF ALTERING WATERS. 



The waters which produced the widespread and often profound alteration of 

 the later andesite were then, as it seems, highly charged with carbonic acid and 

 sulphur and contained magnesia and iron. Since the3 r did not attack the lime in 

 the rocks, it is probable that they contained also this element in considerable 

 quantity. In the rock alteration observed they changed their composition chiefly 

 by the acquirement of the alkalies and silica. They were not ordinary cool ground 

 waters, but clearly hot-spring waters. The extensive carbonation and sulphura- 

 tion show this, as well as the formation of sericite and talcose material, uralite, 

 chlorite, serpentine, zeolites, etc. Thorough as their work was, their effects were 

 not so intense as in the case of the waters which affected the earlier andesite 

 in the vicinitj' of the veins, where the most insoluble elements were attacked. 

 Moreover, the chemical composition of the waters was evidently quite different. 



PERIOD OF ALTERATION OF LATER ANDESITE. 

 ALTERATION MAINLY ANTECEDENT TO FAULTING. 



The last and most altered specimen, No. 4, is, as already noted, the type in the 

 Montana Tonopah shaft between depths of 90 and 278 feet. Specimens taken at 

 various intervals show the persistence of this general type of alteration down to the 

 Mizpah fault, which was encountered at 376 feet. Immediately beneath the fault, 

 however, and in the rest of the workings, the earlier andesite was encountered, 

 completely altered to the quartz-sericite phase. In the Mizpah mine, also, it was 

 noted that earlier andesite altered to quartz and sericite was separated sharply by 

 the Mizpah fault from later andesite marked by the strong development of car- 

 bonates and sulphides. The indications are, therefore, that the faulting was not 

 only subsequent to the alteration of the earlier andesite (as is shown by the fact 

 that it faults the quartz veins), but that it was subsequent to the alteration of the 



