272 GEOLOGY OF TON OP AH MINING DISTRICT, NEVADA. 



lavas. The ores were discovered in 1863. The total production to 1899 was 

 313,448 ounces gold and 10,540,000 ounces silver. The deposits are normal 

 fissure veins, chiefly in rhyolite. In one type the principal ore minerals are 

 small quantities of argentite and chalcopyrite, with a gangue of quartz and ortho- 

 clase (adularia). The proportion of gold to silver by weight averages 1:120. 

 In the other type scarcely any sulphides are ordinarily visible, though occasion- 

 ally pyrite, argentite, and pyrargyrite occur. The gangue is quartz, pseudo- 

 morphic after calcite or barite. The relation of gold to silver by weight is about 

 1 : 10. At De Lamar there is a strong silicification of the country rock near 

 the veins, with the formation of abundant pyrite and marcasite, and a little 

 sericite. Farther away from the veins the country rock is softer and more 

 pyritized. The veins strike northwest and dip southwest, both strike and dip 

 varying considerably. The system comprises ten veins, 20 to 80 feet apart. The 

 strike of these veins is such that parts of the group are like some of the radii 

 of a circle, as is the case at Tonopah, and each vein may join and fork in the 

 manner of linked veins, both horizontally and vertically. The width of the veins 

 is from 1 to 6 feet, averaging 3 or 4 feet. The rich ore occurs in large, contin- 

 uous bodies extending from the surface to a depth of a 1,000 feet, dipping 

 gently (20-30) southeastward along the plane of the vein. They are generally 

 about 200 feet long arid ordinarily 1 to 6 feet thick. 



In other veins the ore bodies do not extend so deep, and, while having often 

 a generally definite course, are so irregular and discontinuous as to constitute 

 irregular bonanzas rather than definite shoots." No considerable ore shoots have 

 been yet found below 1,000 feet, though the veins remain strong. Cerargyrite, 

 pyrargyrite, and argentite occur locally (the latter being common to nearly all 

 the mines), as well as polybasite, proustite, native gold and silver. 



Besides occurring in rhyolite, some of the veins are also in granite and basalt. 



The rock alteration and the ore deposition are considered to have been accom- 

 plished by ascending hot waters, whose nature is indicated by the silicification of 

 the rhyolite and the formation of adularia, chlorite, and epidote. The period of 

 formation is post-Miocene. The veins extend along the strike sometimes for a 

 mile or so, but average less; they die out on botli ends. The ore at present mined 

 at De Lamar goes $14 in gold and $2 in silver; in 1872 the average value of the 

 ore mined was from $12 to $t>0 per ton in different mines. 



The districts of Silver City and De Lamar just described are similar to Tonopah 

 in that the ores occur in Tertiary volcanics, and are probably in both cases post- 

 Miocene in age; to a striking degree in the character of the ores and gangue 

 materials; in the structural character of the veins, which form a group knit together 



a Op. cit., p. 152. 



