CHAPTER IX. 

 COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE. 



It is often advisable to study an ore deposit or a mining district not by itself 

 alone, but also in comparison with others. Similar districts often present informa- 

 tion, through their likeness or dissimilarities, concerning the nature, origin, and 

 future possibilities of the district under examination. 



VEIXS OF PACHUCA AXD REAL DEL MOXTE, IX MEXICO. 



Among the nearest anajogies to Tonopah yet described anywhere in the world 

 are the contiguous mining districts of Pachuca and Real del Monte, described by 

 Aguilera and Ordonez. " 



These celebrated districts are 62 miles north of the City of Mexico, on opposite 

 slopes of the Pachuca Mountains, which bound the great valley of Mexico. The 

 mines support the city of Pachuca, which contains 35,000 people, most of whom are 

 actually engaged in mining. The ore deposits were discovered in 1522, and have 

 been worked almost continuously to the present day. Pachuca is the most important 

 mining district in Mexico, and is estimated to have produced since its discovery 

 3,500,000 kilos of silver. b 



The geology is similar to that of numerous other mineral regions of Mexico. 

 The whole Pachuca Range is formed of Tertiary andesites, rhyolites, and basalts. 

 The andesites are of Miocene age and have a varied appearance, due to alteration, 

 the normal type being green and propylitic. The feldspar (labradorite) has often 

 been transformed to sericite, calcite, chlorite, epidote, and clayev products; the 

 pyroxene to chlorite, viridite, and epidote. The rocks are silicined near the veins, 

 so as often to resemble dacites or rhyolites, this alteration being due to the influence 

 of hot solutions during the formation of the veins. Rhyolites cover the andesites, 

 occurring as flows and dikes. The last eruptions were of basalt. The veins strike 

 east and west. Secondary veins branch out from the smaller ones, and splitting and 

 reuniting are common phenomena. The veins are more remarkable for constancy 

 and extension than for thickness. They seldom exceed 20 feet in thickness, while 

 they have a length of from 2 to 10 miles. 



"Boletin del Institute geo!6gico de Mexico, Nos. 7, 8, 9: Trans. Am. Inst. Min. Eng., vol. 32. pp. 224-241. 

 f> About 112,000,000 ounces, valued at 8145,600,000 (1 oz.-about 81.30). 



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