276 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



A METALLOGRAPHIC PROVINCE COEXTENSIVE WITH THE PETRO- 



GRAPHIC PROVINCE. 



In the paper above referred to the writer brought forward the idea of metal- 

 liferous provinces (perhaps better, metallographic provinces) characterized by the 

 presence of certain metals; and pointed out that these provinces may or may not 

 be closely identified with petrographic provinces, although they probabh T generally 

 are so, to a certain extent at least." 



Unquestionably the close relation between the Nevada mineral districts, Tonopah 

 and the Cornstock, with the far more numerous array in Mexico, and the individuality 

 of this group as compared with other known veins of North America, shows a 

 metallographic province, which in this case coincides with a portion of the petro- 

 graphic province previously mentioned. 



In this metallographic province ores occur in Miocene andesites in the great 

 majority of cases, and their formation followed soon after the eruption of these 

 rocks. In occasionally recurring cases (such as Silver City and De Lamar, Idaho, 

 and others) they appear in Miocene-Pliocene rhyolites, which succeeded the andesites. 



In general, however, the Miocene andesites of this province are, as Humboldt 

 noted, the metalliferous formation par excellence, and if the conclusions which have 

 been arrived at regarding Tonopah are correct (which coincide with a number of 

 similar conclusions concerning other districts reached by other authors), the ore is 

 due to the after actions of the eruptions in the shape of fumaroles, solfataras, and 

 hot springs. Moreover, since similar manifestations (of fumaroles, solfataras, and 

 hot springs) follow most volcanic eruptions, it is probable'that the metals deposited 

 by the after processes at this period arose from an unusual proportion of them in 

 the andesitic magma; indeed, the very definition of a metallographic province 

 implies this. The existence of such metallographic provinces is evident; and the 

 theory of their origin, as propounded by the writer, is like that long entertained 

 by many petrographers for the origin of petrographic provinces namely, that 

 the}' are formed by magmatic segregation. * 



ORIGIN OF SHOOTS OR BONANZAS IN THE VEINS OF THIS 

 METALLOGRAPHIC PROVINCE. 



Light is thrown upon the origin of the shoots, chimneys, or bonanzas in 

 this class of veins by the studies of the influence of cross fractures on their 

 formation in Tonopah, and the similarity between these bonanzas and those at 

 Silver City and De Lamar, Idaho, the Cornstock and Pachuca (fig. 78). At De 

 Lamar the shoot or chimney form is evident, some of the bonanzas having been 



"Trans. Am. last. Min. Kng., vol. 33, p. 33f>. 



fcSpurr, J. E., Trans. Am. lust. Mln. Eng., vol. S3, p. 336. 



