1048 A TREATISE ON METAMORPHISM. 



inquiry as to what extent magmatic segregation has produced the deposits 

 and to what extent subsequent metamorphic action has beeu influential. I 

 suspect it will be ascertained, as is so frequently true of ore deposits, that 

 in some of the cases cited the ores, instead of being due to a single process 

 at one period, are due to a combination of processes. Even if a first segre- 

 gation were made by direct igneous action, as maintained by Vogt, such 

 deposits may have been further enriched by metamorphic processes after 

 the first concentration. (See pp. 1235-1236.) 



Concerning- the occurrence of metallic nickel-iron in eruptive rocks, 

 and platinum in highly basic rocks, it may be merely remarked that such 

 deposits are not ores. It is well known that not only these, but various 

 other metals, such as copper, occur widely distributed in igneous rocks, and, 

 indeed, it has been shown that in such rocks, directly or indirectly, all ores 

 have their ultimate source. But before the term "ore" can be. properly 

 applied to such rocks it should be shown that there is at least a reasonable 

 chance that the economic conditions will be such that some metal may some- 

 time be exploited with profit. It follows that the occurrence of sparsely 

 disseminated metals of various kinds through igneous rocks has no bearing 

 upon the question of the segregation of ores by igneous processes alone. 



Spurr, holding that pyrrotite veins are due to magmatic segregation, 

 has applied the same explanation to the gold-quartz veins of the Yukon." 

 He says the pure quartz veins are produced at the final stage of crystalliza- 

 tion in connection with igneous rocks when the residue "is little more than 

 hot siliceous water, which contains, besides silica, small quantities of many 

 other rock elements which have not been taken up by the rock-forming 

 minerals." 6 Later, but with no further field study or additional evidence, 

 Spurr goes somewhat further in his views toward magmatic segregation. 

 Explaining' the origin of the Yukon and other gold-bearing quartz veins, 

 he says: 



Therefore it has been concluded that certain quartz veins in the Yukon district 

 (part, at least, of which are auriferous) have originated by a process of magmatic 

 segregation, which has separated them from other materials while in the state of 

 aqueo-igneous fusion (the condition of molten rock in general), and that they repre- 

 sent the siliceous extreme of that process. From this standpoint, they are a variety 

 of the igneous rocks. But it has been shown that as magmas become more siliceous 



"Spurr, J. E., Geology of the Yukon gold district, Alaska: Eighteenth Ann. Rept. U. S. Geol. 

 Survey, pt. 3, 1898, pp. 311-316. 

 & Spurr, cit. , p. 312. 



