.m:t. 23 DUEANGO MINERALOGY AXD GEOLOGY FOSHAG 25 



new evidence in hand one can readily determine that such ore bodies 

 as that of the west side of the hill of Cordon Kangel and of Picachos 

 Sur and Norte are also not dikelike as they appear to be, but por- 

 tions of a larger tabular form broken up into segments by faulting. 

 One may point also to the occurrence of ore as schlieren in the tuffs 

 of the Socavon de la Cueva as further evidence contrary to the forma- 

 tion of the ore in dikes. Soler -' has attempted to explain tliTs tabular 

 form of the ore as a faulted body of segregated ore ; that is, the mag- 

 netite is a magmatic segregation in tabular form that has been 

 faulted into segments. The difficulties of accepting such a mode of 

 origin are, of course, obvious, chief of which is the occurrence of 

 ore in the tuft's and it does not explain the occurrence of pure hema- 

 tite in siliceous rocks such as rhj'olite. 



There are but three genetic j)i"ocesses that merit serious considera- 

 tion : Firstly, an origin through intrusion of a magnetite magma, 

 invading the rhyolite, latite, and tuff; secondly, a replacement of 

 these rocks by iron-bearing solutions; and thirdly, fumarolic activity. 

 All these ideas are open to criticism and objection but it is believed 

 that the final choice must lie between them. 



The association of magnetite and apatite immediately suggests 

 that the ores are the result of magmatic segregation of some sort. 

 That they can not have segregated from such rocks as rhyolites and 

 tuffs in which they are now found is quite evident. They must 

 have had their origin in some deeper source. The numerous anas- 

 tamosing veinlets are also suggestive of intrustion, as is also the 

 further fact that, while the wall rocks are altered, the alteration is 

 not as great as one would ordinarily expect from aqueous solutions, 

 but might be accounted for by the smaller amounts of solution accom- 

 panying a dikelike intrusion. So far the evidence points satisfac- 

 torily to intrusion but when we come to consider the manner in 

 which a magma of such composition may form we are confronted 

 with difficulties. If our present ideas on magmatic differentiation 

 are correct a magnetite-apatite body could form by the separation 

 and segregation of the crystals of these minerals into definite bodies. 

 But the difficulty arises in that this body of magnetite and apatite 

 must be brought again to a state of fusion in order to traverse and 

 inject rocks as readily fusible as itself without contamination. 



The suggestion that the iron ores of Cerro Mercado may be the 

 result of fumarolic action rests upon the observations of E. G. 

 Ziess -^ of the formation of magnetite at the Katmai region of 

 Alaska. About some of the hotter fumaroles there formed great 



^ Report to the Cia. Fundidora do Kicrro y Acero de Montorrey, S. A., pp. 30-35, 1925. 

 =« The Fumarolic Incrustatlous in the Valley of Ten Thousand Smokes. Nat. Geol. Soc. 

 Tech. Papers. Katmai Series vol. 1, No. 3. 



