228 University of California Publications in Geology [Vol. 13 



condition also obtained in the massive ore. While the polydymite ( ? ) 

 and ehalcopyrite of the ore body show some tendency to occur in vein- 

 like forms, such veins are not nearly so well marked as are those in 

 nickeliferous pyrrhotite from other districts. The occurrence of these 

 veins has been used by several investigators as proof that the nickel 

 and copper minerals were introduced from without after the solidi- 

 fication of the pyrrhotite. That such is not the case in the Friday 

 Mine deposit is shown by the following observations : 



( 1 ) The polydymite ( ? ) and ehalcopyrite are found only in pyr- 

 rhotite. When a "vein" of one of the former minerals reaches a 

 silicate mineral it stops abruptly. This is well shown in plate 9, 

 figure 3. 



(2) Certain rocks immediately adjacent to the ore body carry no 

 trace of nickel, although they do carry considerable pyrrhotite. A 

 dike of hornblende norite that cuts the ore body carries nearly 1 

 per cent of pyrrhotite, but shows not a trace of nickel with the most 

 delicate tests. 



THEORIES OF ORIGIN OF NICKELIFEROUS PYRRHOTITE 



Deposits of nickeliferous pyrrhotite occurring in intimate associ- 

 ation with basic igneous rocks are of world-wide distribution. The 

 marked similarity both mineralogical and geological between the dif- 

 ferent deposits suggests that all have been formed by essentially 

 similar processes. 



The geologic literature descriptive of these deposits is voluminous 

 and the theories advanced as to their origin are varied. The writer 

 will not attempt to summarize the literature on this subject, as ex- 

 cellent summaries have already appeared in the works of Tolman and 

 Rogers 24 and of the Royal Ontario Nickel Commission. 25 A brief 

 statement will be made, however, of the various theories advanced for 

 the origin of these ores. 



Since 1891 a controversy has been waged as to whether the nickel- 

 iferous pyrrhotite deposits are of a magmatic or non-magmatic origin. 

 Up to a recent date the term magmatic has been applied to those 

 deposits in which the ore minerals are conceived to have been essential 

 constituents of the igneous magmas which consolidated to form the 

 country rock of the deposits, the segregation of the ore minerals, into 

 masses more or less free from silicate minerals, taking place before 



2i Tolman, C. F., Jr., and Rogers, A. F., A study of the magmatic sullid ores, 

 Leland Stanford Junior University Publications, 1916, pp. 23-55. 



25 Royal Ontario Nickel Commission, Toronto, Report, 1917, pp. 95-286. 



