Reviews 



The Origin of the Magmatic Sulfid Ores. By C. F. Tolman, Jr. 

 and Austin F. Rogers. Leland Stanford Junior University 

 Publications, 1916. Pp. 76, figs. 7, pis. 20. 



After reviewing the literature bearing on the modes of origin of the 

 various magmatic ore deposits, the authors have proposed as their 

 thesis that "the magmatic ores have in general been introduced at a 

 late magmatic stage as a result of mineralizers and that the ore minerals 

 replace the silicates. This replacement, however, differs from that 

 caused by destructive pneumatolytic or hydrothermal processes in that 

 quartz and secondary silicates are not formed at the time the ores are 

 deposited." 



The authors follow the position taken by Bowen in his recent work 

 establishing the process of fractional crystallization as the dominant one 

 during magmatic differentiation. After studying suites of specimens 

 from Sudbury, Ontario, Elkhorn, Montana, Ookiep, South Africa, and 

 Plumas County, California, the conclusion is reached that the ores have 

 been introduced by pneumatolytic means after the formation of the 

 rock-bearing silicates. The authors show clearly that the sulphides are a 

 late magmatic product, that they surround the silicates, cut them with 

 well-defined veinlets, embay them, and penetrate cleavage cracks and con- 

 tacts with other minerals. The absence of metallic silicates makes it 

 clear that the ores were not introduced as molten material, while the 

 replacement of early formed minerals indicates the presence of mineral- 

 izing solutions. The complete absence of reaction rims shows that the 

 replaced material was removed by the same agents which introduced 

 the ores. Selective replacement is shown by the preservation of the 

 original graphic texture of the rocks in the ores. There is also evidence 

 of the alteration of pyroxene to hornblende prior to the introduction of 

 the ore minerals, suggesting the presence of aqueous vapor. The small 

 amounts of hydrothermal alteration present appear to be related to a 

 post-magmatic stage. 



The authors conclude that the temperatures involved in the deposi- 

 tion of the ores did not exceed 300 C. to 400 C, but it is unsatisfactory 

 so to limit the temperature without further data than are here presented. 



594 



