236 University of California Publications in Geology [Vol. 13 



and as it penetrates the hornblende is said to be later than the period 

 of uralitization. Tbe saussurite is cut by veins of clear quartz carry- 

 ing pyrrhotite. The fresh rocks carry no ore. 61 He concludes that 

 the "world group" of nickeliferous pyrrhotite deposits belong more 

 with true contact deposits, and are in no way magmatic segregations? 2 

 Dickson's conclusions as to the relation between the silicates and 

 sulphides are supported by Campbell and Knight. They studied 

 polished surfaces of the ores and believe that the order of formation 

 of the various constituents, beginning with the earliest, is (1) magne- 

 tite, (2) silicates, (3) pyrrhotite, (4) pentlandite, (5) chalcopyrite. 63 

 They conclude that the basic rocks were more or less fractured and 

 ore-bearing solutions came in and replaced the rock matter wholly or 

 in part by pyrrhotite. Later another period of straining and breaking 

 was followed by deposition of pentlandite and chalcopyrite. They 

 finally state that the foregoing explanation has been rejected by men 

 of considerable ability who have studied the deposits in the field and 

 that such geologists may put an entirely different interpretation on 

 their work.' 1 



Reference has been made in the introduction to this chapter to 

 the work of Tolman and Rogers. Their paper 05 presents the results of 

 study of "magmatic sulphide" ores from most of the noteworthy de- 

 posits of the world. Their summary of geologic literature of these 

 deposits is good and their photographs of thin sections and polished 

 surfaces of the ores and associated rocks are the best that have been 

 published. 



Tolman and Rogers' conclusions are: 



The ore minerals are the final magmatic product, and are formed later than 

 the magmatic hornblende, which we believe to be produced by magmatic 

 alteration. 



The ores replace the silicates and, in general, the later-formed ore minerals 

 replace the earlier ore-minerals. 



There is a regular order of formation of the magmatic minerals, which shows 

 no variation in the deposits studied. For the nickel-copper group of sulfid 

 ores it is as follows: (1) silicates, (2) magnetite and ilmenite, (3) pyrrhotite, 

 (4) pentlandite, and (5) chalcopyrite All alteration minerals except horn- 

 blende are later than the above mentioned magmatic ores. 66 



They state that in the Sudbury ores uralitization (tremolitization) occurred 

 after the introduction of the sulphides. 67 



"i Weinsehenk, E., Zeit. f. Pratt. Geol., vol. 15 (1907), Jahrg. Heft 3, pp. 

 82-84. 



62 Ibid., p. 86. 



ea Campbell and Knight, Econ. Geol., vol. 2 (1907), pp. 353-365. 

 Gilbid., pp. 365-366. 



65 Tolman and Bogers, A study of the magmatic sulfid ores (Stanford Uni- 

 versity, 1916). 



wlbid., p. 14. 67 Ibid., p. 31. 



