20 PROCEEDINGS OP THE NATIONAL MUSEUM vol.74 



SIMILAR DEPOSITS 



In some respects the iron ores of Cerro Mercaclo are unique in that 

 they are deposits of the first importance connected with extrnsivp 

 volcanic rocks of Tertiary age. There are, however, a few minor 

 iron-ore occurrences of this type, and some of the features of the 

 famous Kiirunavaara deposits of Sweden as described by Per Geijer, 

 Stutzer, and others are similar. Minor developments of hematite in 

 .some of the rhyolites of Tertiary age, especially when associated with 

 tin, are common in Durango and other parts of Mexico but are of 

 only mineralogical interest and are perhaps of somewhat different 

 origin. In this case the iron mineral is always specular hematite 

 8nd never martite. 



The iron-ore deposits of Barth, Nev., described by J. Claude 

 Jones ^* are remarkably similar to the Cerro Mercado. Like them 

 the ores are found in Tertiary eruptive rocks of anclesitic character. 

 The iron mineral is hematite. Apatite is present and there is a de- 

 velopment of biotite along the walls. The boundary of the ore and 

 the andesite wall is sharp ; that is, without gradation of ore to wall 

 rock, but the ore extends into the footwall as anastamosing veins, 

 cementing the included fragments of the andesite into a breccialike 

 mass. Where the veins are particularly abundant the fragments are 

 changed in color to a deep green and contain much biotite in an 

 indefinite matrix. Clusters of apatite radiate from the angles of 

 the andesite fragments and phlogopite is particularly abundant in 

 their vicinity. The ore is massive and compact hematite, through 

 which are disseminated euhedral crystals of apatite. It is somewhat 

 magnetic but little or no magnetite can be detected. 



This is an almost exact description of Cerro Mercado ores with the 

 augite of Cerro Mercado in place of the biotite of Barth. Such close 

 similarity argues for a close relationship. 



The martite deposits of Twin Peaks, Millard County, Utah, de- 

 scribed by Horace Patton ^^ are also closely related to those of Cerro 

 Mercado. The country rock is a light colored rhyolite with abundant 

 phenocrysts of quartz and some biotite and hornblende. It is a very 

 massive rock but only exceptionally is it fresh. It is cut in different 

 directions by sharply defined veins of martite which vary in width 

 from 1 inch to 7 or 8 feet. Associated with this martite are two 

 other minerals — apatite and augite. The apatite crystals, originally 

 white and quite transparent, are small, rarely over an inch or more. 

 The pyroxene occurs in slender crystals of a black color. Specimens 

 of these augite crystals now in the National Museum are exactly 



"Economic geology, vol. 8 pp. 247-263, 1913. 



•° Martite Crystals from Twin Peaks, Millard County, Utah. Colorado School of Mines, 

 Quart., vol. 2, pp. 7-13, 1907. 



