94 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



sulphide zone are argentite and pyrargyrite, the latter always coating cracks or 

 cavities, with probably chalcopyrite. Possibly the copper of the secondary 

 chalcopyrite is formed by the action of copper solutions from above on primary 

 pyrite, but galena or blende have not been noted as secondary sulphides, and at 

 best are rare. Moreover, the secondary silver minerals argentite and pyrargyrite 

 are more abundant than secondary pyrite and chalcopyrite, and all these usually 

 occur on cracks in rich primary sulphides and not in barren or low-grade ore, 

 suggesting the derivation of the secondary minerals from this rich ore by lateral 

 secretion rather than an exotic origin. 



FORMATION OF GYPSUM BY OXIDIZING WATERS. 



Gypsum frequently occurs as veinlets or incrustations in both the earlier and 

 the later andesites where these are altered. It is more rare in the earlier 

 andesite, which has become highly silicified, and is abundant in the later andesite, 

 which has developed a large amount of calcite as a decomposition product. This 

 association with calcite suggests derivation from it, and the proximity in many 

 of these cases of partly oxidized pyrite indicates that the sulphuric acid derived 

 from the pyrite has wrought the change. The surface waters containing oxygen 

 would decompose the pyrite and form limonite (which is found near the surface) 

 and sulphuric acid. The latter would decompose the calcite (which itself was 

 formed by hydrothermal processes from the calcareous silicates of the andesite) 

 and produce gypsum and carbonic acid. 



In the Fraction workings, at a depth of 400 feet and in the West End and 

 the MacNamara (the latter at 280 feet), fissures were tapped which contained a 

 heavy odorless gas that put out lights and necessitated temporary interruption 

 'of work. This gas was immediately dispersed by the ventilation, indicating that 

 the fissures were reservoirs and not outlets. The writer has not been able to 

 collect any of the gas, but in all these cases it was encountered near calcareous, 

 pyritiferous, and gypseous andesite, and it is likely that it may have been carbonic 

 acid, the final result of the reactions indicated, which accumulated in cavities. 



SECONDARY SULPHIDES. 

 PYRARGYRITE, ARGENTITE, AND NATIVE SILVER. 



Wherever observed macroscopically, pyrargyrite (ruby silver) and to a great 

 extent, also, argentite (silver glance) coat crevices which cut the primary ore and 

 are evidently of secondary deposition. These minerals were found in comparative 

 abundance in the Fraction, in the unoxidized ores on the 237- and 300-foot levels; 

 on the 237-foot level native silver occurred also, coating cracks, and also plainly 



