COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE. 269 



OTHER SIMILAR MIXERAL, DISTRICTS IX MEXICO. 



The deposits of Pachuca are similar in many respects to many other Mexican 

 ores. J. G. Aguilera remarks concerning the ores of Mexico in general: 



"The silver deposits proper are found in eruptive rocks. A very few are found 

 in sedimentary rocks, and in these the silver is accidental and variable in quantity. 

 Where silver veins occur in sedimentary rocks it is evident that they are related to 

 and dependent upon andesitic Tertiary eruptive rocks."" 



"The majorit\ r of the silver- veins of Mexico are in hornblende- and 

 pyroxene-andesite. As examples of fissure veins in eruptive andesitic rocks, we mav 

 mention the following: In Zopilote, Tepic, the veins have a northwest course, and 

 consist of quartz, blende, and pyrite, sulphides of silver, and small amounts of 

 galena. At Topia the veins extend northeast-southwest, and contain galena, blende, 

 a very small amount of pyrite, argentite, and pyrargyrite with a gangue of quartz 

 and calcite. At the mines of Tecatitliin, Jalisco, the veins strike about N. 40 W., 

 and dip 45 to the southwest. The gangue is quartz with a little calcite, carry- 

 ing sulphides and antimonides of silver, pyrite, and chalcopyrite. At Chinipas, 

 Chihuahua, the veins occur in diorite and hornblende-andesite. The strike is 

 northeast, or in some cases northwest. The vein filling is quartz with argentite and 

 pyrite, oxides of iron, and dendritic manganese. At Ajijic, Jalisco, the veins are in 

 hornblende-andesite, with an east-west strike; there is an oxidized zone, and as depth 

 is reached complex sulphides are encountered. At San Sebastian and Los Reyes, 

 Jalisco, the veins have a quartz gangue with some calcite, complex sulphides, and 

 tellurides of silver and gold, a very little galena, blende, and pyrite. The veins of 

 the Rosario mines and San Nicolas del Oro mine, Guerrero, are in hornblende- 

 andesite; their course is northwest, or in some cases northeast, and they contain an 

 oxidized zone. Below this is the sulphide zone, containing argentite, ruby silver, 

 pvrite, and a small amount of chalcopyrite. The gangue is quartz, carrying gold. 

 Some of the veins of Sierra de Tapalpa, San Jose del Amparo, and Rosario, etc., 

 have a north-south course, and dip west; the gangue is quartz with some barite. In 

 the oxidized zone they contain the carbonates of copper, and beneath this grav 

 copper and stibnite occur. At Tlalchapa, Guerrero, the lodes have a northwest- 

 southeast course, dipping to the northeast. The vein-filling is quartz with argentite, 

 pvrite, and blende; occasionally the vein quartz contains calcite and, in addition to 

 the minerals named above, galena and chalcop\ T rite. At the mines of Chacoaco, 

 south of Fresnillo, the veins extend nearly north and south, and contain quartz with 

 marcasite and pyrite. Some of the veins strike northeast-southwest, and contain 

 quartz, pyrite, and sulphides of silver. The veins of Real del Espiritu Santo are 

 found in augite-andesite. 



"In the pyroxene-andesites may be found the deposits of Pachuca, Real del 

 Monte, El Chico, Tepenene, Capula, Santa Rosa, in Hidalgo; the mines of Santo 

 Domingo, in Jalisco; and some of the mines of Noxtepec, Guerrero. Among the 

 veins in andesite may be mentioned those of the following mines: San Pable Analco, 

 which in the oxidized zone somewhat resembles those of Pachuca; the California 



"Trans. Am. Inst. Min. Eng., vol. 32, p. 513. 



