192 ECONOMIC GEOLOGY 



The nickeliferous pyrrhotite of Sudbury, Ontario, is by some 

 authorities considered to represent a magmatic segregation. This 

 origin has been advocated by A. P. Coleman and others, but 

 the more recent investigations of W. Campbell have shown that 

 the ores were deposited from circulating solutions and therefore 

 of secondary origin. The hydrous sulphates of iron are always 

 of secondary origin and of minor commercial significance. 



Magnetite is often a primary mineral, solidifying along with 

 chromite as the first segregations of a periodotite magma. This 

 origin holds especially true for the magnetites and chromates 

 of northern Vermont and Megantic County, Quebec. Magne- 

 tite is often a product of contact metamorphism. According to 

 C. R. Van Hise, magnetite may be derived from marcasite and 

 pyrite or even from the oxidation of siderite in situ. The 

 mineral occurs as an accessory constituent in the rocks of all 

 classes, but it is obviously more abundant in the rocks rich in 

 the ferromagnesian minerals, as the diabases and the peridotites. 

 In the Lake Superior region and in Michigan, Minnesota and 

 Wisconsin, magnetite is found in the slates and cherts where the 

 mineral is not of igneous origin. 



According to C. K. Leith, the hematite of the Mesabi iron 

 district has been leached from a hydrous iron silicate, greenalite, 

 as FeO and developed magnetite where oxidation was partial. 

 Other silicates through metamorphism may yield magnetite. 



The carbonate, siderite, is always of secondary origin. It may 

 be found in the igneous rocks as an alteration product. Carbo- 

 nated waters can extract iron from silicate rocks or disseminated 

 hematite and deposit their load, in the presence of much car- 

 bonic acid or decaying organic matter, as siderite. If the air has 

 free access to these ferriferous waters the hydrated oxide, limon- 

 ite, would be produced. If the waters are muddy the silt goes 

 down with the iron compounds and the clay ironstones are the 

 result. The black band ores of the coal measures were once a 

 carbonaceous mud. In the presence of reducing agents as organic 

 matter the carbonate will remain as such, but in their absence 

 it will be oxidized and limonite may be the resulting product, 



According to F. W. Clarke, hematite can crystallize out from a 

 magma when the ferrous compounds are either absent or present 

 in quite subordinate amounts, for the ferrous oxide unites with 

 it to form magnetite. Hematite is therefore more common in 

 the acidic than in the basic rocks. It is found as a pyrogenic 



