COMPARISON WITH ^SIMILAR ORE DEPOSITS ELSEWHERE. 273 



by branches; in the general character of the alteration of the wall rock; and in the 

 occurrence of the rich ores in irregular bonanzas. The chief difference is that the 

 wall rocks are mainly rhyolite and not andesite. 



RELATION OF THE DESCRIBED DISTRICTS TO TONOPAH. 



Of all the described ore deposits of North America, therefore, Tonopah appears 

 to be most closely related to many of the Mexican silver veins, and also to the 

 Comstock in Nevada and the Silver City-De Lamar veins of Idaho. With Pachuca, 

 as is seen, the relation is intimate, and Ordonez's description of the veins of this 

 district would do, with a very little change, for a report on the Tonopah veins. 

 The chief difference is in the occurrence of manganese silicate in depth at Pachuca. 

 which has not been found at Tonopah, " and also the less content of gold, with 

 the absence of ruby silver. Ruby silver, however, occurs in the cognate and 

 contiguous Real del Monte district; also gold in considerable quantity occurs 

 with silver in some of the Mexican districts of this type. Those enumerated bv 

 Aguilera* all occur in hornblendic andesite. 



This group of veins is characterized by the following features: They occur 

 in Tertiary volcanic rocks of similar character in the different localities, being 

 chiefly Miocene andesites or rhyolites. They constitute strong masses or frequently 

 branching and "linked" veins of quartz, which have as gangue essentially quartz, 

 with frequently a little calcite, while adularia, barite, rhodochrosite, or rhodonite 

 may also be present in limited amount. The ore is characteristically a silver- 

 gold one, silver being usually predominant in the values in vaiying proportions, 

 though the relative value may be -reversed, and in some extreme cases either 

 metal may occur with little admixture of the other. In any case the abundance 

 of silver or gold, or both, in reference to lead, zinc, iron, etc., is characteristic. 

 Silver sulphides, especially argentite, also stephanite and polybasite (with ruby 

 silver) and gold, probably largely in the free state, are distinguishing features 

 in the great majority of cases. Tellurides c and selenides may also be present. 

 Pyrite, blende, chalcopyrite, and galena are usually present in varying quantity. 

 Where they become predominant the vein becomes relatively low grade. 

 Tetrahedrite, stibnite, and bismuthinite'' are also known to occur. The wall 

 rocks are characteristically much altered to quartz, sericite, chlorite, calcite, 

 epidote, pyrite, and sometimes adularia, etc. Frequently the rocks nearest the 

 veins are chiefly altered to quartz and sericite, those farther away to the softer 

 "propylitic" alteration, consisting of calcite, chlorite, pyrite, epidote, etc. 



a Since the above was written manganese carbonate has been found in the sulphide ores at Tonopah. See p. 89. 



6 Aguilera, J. G., Trans. Am. Inst. Min. Eng., vol. 32, p. 519. 



oAt Goldfield, Nev., and Jalisco and Tepic in Mexico (Trans. Am. Inst. Min. Eng., vol. 32, p. 601). 



<iAt Goldfleld. See Bull. U. S. Geol. Survey No. 260, p. 138. 



16843 No. 4205 18 



