ART. 23 DURANGO MINIJRALOGY AND GEOLOGY FOSHAG 21 



similar in habit and twinning to those of Cerro Mercado. The 

 martite veins are sometimes open fissures with the two walls lined 

 with crystals. In other eases the veins are brecciated, the fissure 

 partially filled with rhyolite fragments now coated with martite 

 crystals. In the occurrence of the martite with rhyolitic rocks, the 

 presence of apatite and augite, and in the brecciated nature of some 

 of the veins with the breccia fragments coated with martite crystals, 

 this deposit is similar to those of Cerro Mercado. 



The resemblance between Cerro Mercado and Kiirunavaara, Fin- 

 land, has been suggested by Per Geijer ^^ but since definite data at 

 that time were lacking no real conclusions could be drawn. Like 

 Cerro Mercado the ore deposits of Kiirunavaara are sheetlike bodies 

 in acid volcanic rocks, here largely syenites and quartz porphyries; 

 whether extrusive or intrusive is still a mooted question. Lund- 

 bohm, Backstrom, and Geijer regard them as flows, while Stutzer 

 and Daly believe them to be intrusive rocks. The ore mineral is 

 magnetite, and only to a small extent is it mixed with hematite. In 

 the exposures on " Professorn," however, hematite is abundant and 

 the ore is unusually porous and is believed by both Stutzer and 

 Geijer to be secondary. 



Minerals other than magnetite are quite rare in the entire field; 

 the most abundant of these extraneous minerals being apatite. The 

 amounts vary from masses of pure magnetite to others of pure 

 apatite but the main mass of ore does not average over 1 to 2 per 

 cent. Pyroxene and hornblende are present in much smaller 

 amounts. Sulphur seldom exceeds 0.05, titanium varies between 0.04 

 and 0.80 but may reach 1.36 in Luossavaara. Other minerals noted 

 are calcite, quartz, biotite, tourmaline, asbestos, and siderite. The 

 tenor of the ore is very high, varying from 65 to 70 per cent. 



As a rule the contacts between the ore and the comitry rock are 

 sharp and often without noticeable alteration of the inclosing rock. 

 Often, hovs'ever, the country is penetrated for a distance of several 

 meters by a complex system of anastamosing veinlets of ore to such 

 an extent that the rock appears brecciated. About the borders of 

 the ore in many places the wall rock is altered to a skarn of amphi- 

 bole or to a diopsidic pyroxene. The hornblende is, without doubt, 

 the result of the uralitization of preexisting pyroxene.^^ 



The analogies betAveen the ore deposits of the Cerro Mercado and 

 Kiiruna district may be listed as follows : 



1. The association of the ore bodies with acid volcanic rock. 



2. The high iron content and puritj' of the ores. 



3. The abundance of apatite in the ores. 



" Igneous Rocks and Iron Ores of Kiirunavaara, Luossaraara, and Iiiolluvaara. Per 

 Geijer. Stockholm, 1910. 



" Per Geijer. Inst. Gcol. Mex., Bol. 44, 1923, p. 246. 



