Van Horn — Silver, Copper, and Cobalt Minerals. 23 



Art. III. — A new Occurrence of Silver, Copper, and Cobalt 

 Minerals in Mexico ; by Frank R. Van Horn. 



Introduction. 



The minerals which are the subject of this paper were found 

 at the Veta Rica mine, Sierra Mojada, Coahuila, Mexico. 

 The specimens were presented to the Case School of Applied 

 Science by Mr. R. B. Cochran, who was at the time superin- 

 tendent of the Compania Metallurgica Mexicana at the mine 

 mentioned. Our thanks are due Mr. Cochran for some very 

 interesting as well as valuable specimens. The Sierra Mojada 

 is a mining region in the extreme western part of the state of 

 Coahuila, about 494 miles south of El Paso, Texas. It is 

 reached by the Mexican Central and Mexican Northern rail- 

 roads via Escalon. The district was a silver-lead camp up to 

 1893, and is still so, although since that date large amounts of 

 silver-copper ores have been found. 



Although ore has been mined during most of the period 

 since it was discovered in 1878, very little has been published 

 about the locality. Up to 1911, as far as the writer is aware, 

 there have appeared but two articles, which were written in 

 1886 by Chism,* and in 1901 by Malcolmson.f Both of these 

 papers cover the mining and metallurgical methods employed 

 in the region, and little, if any, attention has been paid to the 

 mineralogy, although the geology has been briefly considered. 

 In 1911, a new occurrence of the rare mineral pearceite was 

 described from this locality.;}; The last and most recent article§ 

 on the district appeared in 1912, and discusses the geology and 

 economic geology in the light of more recent developments. 

 Since, therefore, the previous literature on the region is not of 

 a mineralogical nature, it has seemed to the writer that it 

 might be fitting to discuss rather briefly the mineralogy of the 

 district. 



The ore minerals are found at or near the contact of a Cre- 

 taceous limestone with what has been called a " porphyritic 

 breccia," although it much resembles a decomposed rhyolite ; 

 or rhyolite tuff. Full details about such features, as well as 



* Sierra Mojada, Mexico, by Eichard E. Chism, Trans. Amer. Inst, of Min. 

 Eng., xv, 542, 1886. 



f The Sierra Mojada, Coahuila, Mexico, and its Ore Deposits, by James W. 

 Malcolmson, Trans. Amer. Inst. Mining Eng., xxxii, 100, 1901. 



% A New Occurrence of Pearceite, by Frank R. Yan Horn and C. W. Cook, 

 this Journal, xxxi, 518, 1911. 



§ The Occurrence of Silver- Copper- and Lead Ores at the Veta Rica Mine, 

 Sierra Mojada, Coahuila, Mexico, by Frank R. Yan Horn, Bull. 68, Trans. 

 Amer. Inst. Mining Eng., p. 867, August, 1912. 



