DURANGO MINERALOGY AND GEOLOGY FOSHAG 



17 



ore and during the hydrothcrnial stage that resulted in the forma- 

 tion of the sepiolite, quartz,. calcite, and barite. It is possible that 

 the brecciation of the ore was due to this conversion of magnetite 

 to hematite, since there is a small diminution of volume involved 

 in this change. Many of the broken surfaces seem quite fresh, others 

 are somewhat altered. Few, if any, show a development of goethite 

 upon them. 



ORES 



The ores of Ccrro Mercado are as already stated essentially all 

 martite, although other minerals of iron are encountered in minor 

 amounts. The ores as mined contain 65 to 68 per cent of iron and 

 while they are quite high in phosphorus, they are practicall}^ wanting 

 in sulphur and titanium. Manganese is present in essentially negli- 

 gible amounts. Its exceptionally high content of iron means that 

 there is practically no gangue mineral in the ore and indeed an 

 inspection shows nothing more than a very small amount of silica and 

 apatite. 



The chief workings are at the southwestern corner of the hill where 

 a cliff of iron ore rises from the valley. At this place, called Penascos 

 de la Industria, the iron ore is hard, has numerous cavities lined 

 with octahedrons of martite, but carries no other minerals except very 

 minor amounts of goethite, and apatite. The goethite lines the cavi- 

 ties and coats the martite while the apatite is found as small white 

 needles. The ore is dark steel gray in color and of the highest grade. 



Martite ore, Penaacos de la Industria 

 Forest A. Gonyer, analyst 



SiO= 0. 32 



TiOo None. 



Fe.03 93. 64 



AUO. 4. 04 



FeO 1. 15 



C:iO None. 



MgO 0. 72 



MnO Trace. 



P.Os None. 



H.0 . 31 



100.18 



The locality on the west flank of the Pieacho Socavon 4, called 

 Cueva de la Marmaja, is also being actively worked and produces 

 high-grade ore. Here the mineral is a friable sandy mass made up 

 almost entirely of small black octahedrons of martite. The softer 

 material is a black sand that can easily be picked down, while the 

 somewhat more compact ore is spongy and friable. Occasional hard 

 lenses owe their superior resistance to a siliceous cement. In color 

 the ore is dark bluish to irreenish black which sometimes show numer- 



