1046 A TREATISE ON METAMORPHISM. 



is more than this, being 8.66 per cent in the Duluth gabbro of Minnesota." 

 Therefore to produce an ore body by maginatic segregation in such a rock 

 as this the iron need be increased only about seven times. The production 

 of ores of the less common metals by direct magmatic segregation is an 

 entirely different matter. In the original igneous rocks the amounts of 

 copper, nickel, cobalt, gold, silver, etc., are usually so small as not to be 

 detected by assay. In order to produce ores of these metals segregation 

 must take place many fold, probably in all cases more than a hundredfold, 

 and in many cases probably a thousand or many thousand fold. Those 

 who maintain that an ore deposit of any of these less common metals is 

 produced wholly by magmatic segregation should determine the ratios 

 between the amount of metal in the original igneous rock and in the ore, 

 and show that the igneous processes have segregated the metal to that 

 extent. 



These considerations apply to the other cases of ore deposits held by 

 Vogt to be due to magmatic segregation and to the gold-ore deposits 

 referred by Spurr to the same processes. Thus to segregate chromite from 

 peridotite and the sulphides of the less common metals from igneous rocks 

 by magmatic processes alone would require an amount of segregation 

 which is yet to be proved. Of the sulphides ores that of nickel is the one 

 which has most strenuously been held to be due to magmatic segregation. 

 Of nickeliferous pyrrhotites, I have seen deposits at only one locality, 

 near Sudbury, Canada, where they are associated with norite. I shall not 

 deny that a first segregation of these deposits along the border of the 

 norites may have taken place in part as a consequence of magmatic differ- 

 entiation. Walker 6 states that in certain cases near the center of the 

 norite masses there is comparatively little pyrrhotite, and that in passing - 

 toward the borders the amount steadily increases until the ore deposits 

 are reached. But so far as I have seen the deposits there is a great differ- 

 ence between the nickeliferous norite and the deposits which are mined, a 

 difference between a comparatively small and a high percentage of sulphide. 

 It is these high-grade products which I hold were certainly to a large 

 extent precipitated from circulating solutions. The increase in the amount 



« Winchell, A. N., Gabbroid rocks of Minnesota: Am. Geol., vol. 26, 1900, p. 374. 

 i> Walker, T. L. , Geological and petrographical studies of the Sudbury nickel district, Canada: 

 Quart. Jour. Geol. Soc. London, vol. 53, 1897, p. 52. 



