420 



THE VERMILION IRON-BEARING DISTRICT. 



IRON-OXIDE BODIES IN THE GABBRO. 



It has been known for a long time that there were iron-oxide masses 

 in northeastern Minnesota within the area which is underlain by the great 

 Keweenawan gabbro mass. One of these, which occurs on Mayhew Lake, 

 was seen and described by Chauvenet in 1883 and 1884," and since that 

 time there have been a number of brief mentions made of these gabbro 

 iron-oxide masses in the various papers on the geology of Minnesota to 

 which reference has been made in this chapter. The iron oxide occurs in 

 bodies of varying size and the existence of these bodies has aroused a 

 considerable interest among investors. 



CHAKACTEK, OCCUERENCE, AND ORIGIN OF THE IRON-OXIDE BODIES. 



These bodies consist of titaniferous magnetites, and have, according to 

 H. V. Wiuchell,'' the following composition: 



Analyses of iron-oxide hod/ies. 



Constituents. 



Silica, SiO^ 



Alumina, AI2O3 



Titanium binoxide, TiOj 



Magnetic oxide of iron 



Protoxide of iron 



Chromium sesquioxide, CrO 



Magnesia, MgO 



Lime, CaO 



Phosphoric acid 



2.02 

 2.68 

 12.09 

 80.78 



Metallic iron. 



Tr. 

 .03 



100. 00 



58.48 



II. 



20.90 

 1.75 

 2.23 



70.29 

 2.01 



2.63 

 Tr. 



None. 



99.81 

 52.46 



In view of the fact that a great deal of money has been spent in this 

 district in exploring for nickel ore, which has been reported as occurring in 

 the titaniferous magnetites in the gabbro, the following statement by Grant 

 is valuable: 



Nickel does occur in similar situations," and is frequently found as an impurity 

 in one of the gabbro minerals (olivine), but as j^et no reliable reports of nickel ore 



« W. M. Chauvenet, U. S. Geol. Survey, manuscript notes. 

 » Geol. and Nat. Hist. Survey of Minnesota, Bull. No. 6, 1891, p. 141. 



"This evidently has reference to its occurrence in other districts; not in Minnesota, however. 

 Clements. 



