ART. 23 DURANGO MINERALOGY AND GEOLOGY FOSHAG 23 



While Chrustschoff -^ does not accept this idea entirely, still he 

 believes that the deposits were due to some form of eruption, prob- 

 ably a fluid aqueous mass, that rose through channels to spread out 

 about their openings. 



Birkenbine '^- in his description of the occurrence has little to say 

 resardino: its genesis. His ideas are embodied in the following 

 citation : 



I incline to the belief that the Cerro de Mercado is formed of one or more 

 immense veins or lenses of specular iron ore, standing nearly vertical, th<i 

 fragments of which have, by the action of the elements for ages been thrown 

 down to form the slopes of the mountain as a talus. 



Manuel Rangejl -^ describes the ore body as follows : 



The mineral part presents the form of a very strong dike, ramifying in the 

 western part, with two small branches to the north and includes admixture of 

 rhyolite. Erosion operating primarily on the inclosing riJck, ha? disintegrated 

 it into blocks of diverse shapes that have been deposited on the talus slopes 

 of tlie hill ; the part of the mineral so uncovered has suffered on its part, the 

 slow and continued action of the weather and has been disintegrated into 

 fragments of variable size which have formed as talus deposit, an apron of 

 mineral which gives to Cerro Mercado the appearance of a mountain made 

 up only of ferruginous minerals. The illusion disappears through a close ex- 

 amination of the structure of the mountain. In effect, in the base one sees 

 enormous banks of rhyolite ; higher, the fragments of rhyolite, covered on some 

 of their faces with crystallized minerals of iron in small octahedrons, are 

 mixed with pieces of mineral, whose proportion increases with height untl' 

 it constilutes the entire deposit forming the crest of the hill. 



Since Seiior Rangel wrote this he has continued exploratory work 

 on the deposits and has demonstrated that the ores do not continue 

 downward but that the form is that of dipping tabular bodies ; hence 

 the inference that the ore bodies are of the nature of a dikelike intru- 

 sion, I believe, Senor Rangel is now willing to modify. 



In the report of Cerro Mercado by Leopoldo Salazar-Salinas and 

 his coworkers,-* there are two somewhat distinct conceptions ad- 

 vanced. According to the interpretation of A. R. Martinez- 

 Quintero -^ the pyroxenitic rocks that surroimded the iron-ore bodies 

 are intrusive pyroxenites, and he refers the source of the iron to a 

 batholith of which this rock is a basic border phase. 



According to ISIartinez-Quintero : 



The intrusive is probably a batholith with a basic border phase forming the 

 contact ; the visible portion of the intrusion being a pyrfixenite whose principal 

 constituent is the hedenber.uite-diopside. which in narrow crystals is encountered, 

 among other places, in the differentiated dikes of the Cerro de Fray Diego. 



^ Eineges ueber den Cerro del Morcado boi Ilurango in Mexico, Wiirzburg, 1878, Abs. in 

 Zeit. Krys.. vol. 3, pp. 632-634, 1879. 



"Amer. Inst. Min. Engrs. Trans., vol. 13, pp. 189-209, 18S5. 



"^ Criadero de flerro del Cerro de Mercado. Inst. Geol. Mex. Bol. 16. pp. 3 14, 1002. 



■•" El Cerro do Mercado. Inst. Cool. Mox., Hoi. 44. 1923. 



•■» Salazar-Salinas. Inst. (ieol. Mex., Bol. 44, 1923, p. 41. 



