Il6 NEW YORK STATE MUSEUM 



bodies. The Great Export pit with its broad lens of ore, rolHng 

 walls, pegmatite dikes and horses of country rock seems almost a 

 physical counterpart of some of the Gellivare mines. 



In an illuminative chemical and petrographical study of the 

 Grarigesberg area Dr 11. Johansson^ has found much to support the 

 view of a magmatic origin for the ores. By reconstructing from 

 chemical analyses a theoretical magma representing the average 

 composition of the ore-bearing formation, he shows that it corres- 

 ponds to a fairly acidic alkali granite with a predominance of soda 

 over the potash element. The complex is characterized by rather 

 strongly contrasting rock types from a chemical standpoint which 

 correspond to the cleavage products of such a magma under a 

 discontinuous process of differentiation. As extremes of the series 

 we have on the one hand the granulite and gneiss group with 68 

 per cent or more of silica, and on the other hand the amphibolites 

 with 50 per cent silica, with only few intermediate types. The ores 

 that separated out during the differentiation show consistent rela- 

 tions to their inclosing rocks. The skarn ores are peculiar to the 

 soda granulites ; the apatite ores come in the plagioclase gneisses 

 and granulites of more basic composition ; and the quartzose ores 

 are found with the potash granulites. 



The ores of Grangesberg, as illustrated in the Export mines, 

 which are much the largest of the group, are granular mixtures of 

 magnetite and apatite. The latter amounts perhaps to rather more 

 than 5 per cent of the whole. In some smaller mines of the vicinity 

 hematite is the chief ore, and it occurs in one part of the Export 

 mines, constituting the western half of the large northern deposit. 

 It usually carries some magnetite and apparently grades into the 

 latter. An explanation for its presence, which has been noted also 

 in connection with the Gellivare deposits, is not easily found. Tin: 

 hematite is not a produce of later weathering, at least that derivation 

 does not appear probable. The occurrence of hematite as a primary 

 constituent of igneous rocks may be conceded, but its accumulation 

 in quantity in the deep-seated zone by magmatic differentiation 

 would hardly be expected. 



The included bands of country rock arc a curious feature of the 

 larger ore bodies. They trend generally in the direction of the 

 strike but may send off branches and coalesce with the foot or 

 hanging wall so as to form a network separating the ore into 



^ Die eiscnerzfiihrende Formation in dcr Gegend von Grangesberg. Geol. 

 For. For. Stockholm, 1910. 



