964 THE GEOLOGY OP MINNESOTA. 



[The iron ores. 



hand, it is quite certain that in some instances the ore is almost or quite free from 

 titanium, and usually carries less than the standard amount for ilmenite, and in 

 many of the Keweenawan diabases there is no evidence whatever of the presence of 

 ilmenite, while the cubic crystalline form plainly points to magnetite. 



In the original greenstones of igneous origin, these minerals are frequently 

 seen, not only as minerals, but in the form of diffused leucoxene. In the secondary 

 greenstones, when unmetamorphosed, i. e., the elastics either of the Kawishiwin or 

 of the Upper Keewatin, they are very rare, but magnetite in scattered minute crys- 

 tals has been noted in intimate association with hematite in the jaspilyte ores. 

 When, however, these ores and the rocks containing them are converted to crystal- 

 line schists, the Coutchiching, so-called, or the gneisses and mica schists in general, 

 magnetite is the sole form of iron ore that has been observed. 



In the Keweenawan gabbro are large masses of magnetite which usually, so far 

 as observed, but not always, carry titanium. Similar iron ore is disseminated 

 through the adjacent gabbro rock in crystals and small masses that vary largely in 

 size and structure. In but few cases in the gabbro, and none in the muscovadyte, 

 has this magnetite been seen to present unequivocal evidence of being of primary or 

 original date, as if crystallizing from a cooling magma amongst the first phenocrysts. 

 On the other hand, it has exhibited in many cases clear proof of its secondary, or at 

 least of its late, origin (Nos. 1, 1C, 5, 6). The original forms of magnetite constitute 

 but a small moiety, and are of microscopic dimensions as crystals. The secondary 

 masses are large, constituting ore bodies that are of promise in economic value. 

 The original crystals are widely distributed in general through the body of the 

 gabbro, or are absent; the secondary masses are usually associated with other 

 evidences of contact relations, and are especially frequent, so far as observed, in 

 association with muscovadyte. The large masses are, in general, believed to be the 

 result of transformation of older jaspilyte lodes existing in the greenstone from 

 which the gabbro itself was derived, while the original minute crystals, having inde- 

 pendent cubic outlines, were probably from magnetite (or ilmenite) originally 

 distributed as an essential ingredient in the mass of the same greenstone. In neither 

 case has the iron ore been transferred in any noteworthy amount from the position 

 it occupied in the original greenstone.* It has been subjected to entire recrystal- 

 lization and has acquired, perhaps, some chemical characters which it did not possess 

 before. 



The following figure, drawn from rock No. 1C, shows some relation between the 

 supposed original crystals of magnetite and the secondary accretions. The crystal- 



*In Bulletin vi, where these ores were discussed, a distinction was made between titaniferous and non- titaniferous magne- 

 tites from the gabbro, and it was assumed that the former was indigenous in the gabbro, and the latter masses had been derived 

 from the Animikie as foreign inclusions. Such distinction seems, however, not well supported by field evidence, and cannot be 

 maintained from any other evidence. There is reason to believe that both belong to the gabbro. 



