THE CRYSTALLIZATION OF IGNEOUS ROCK 173 



Olivine and magnesia-rich pyroxenes are particularly susceptible 

 to magmatic end-stage products. Peridotites, olivine-rich pyroxe- 

 nites and dunites thus are converted to great masses of serpentine; 

 not by "weathering," but during cooling and final consolidation. 



This was recognized many years ago by De Launay^ during his 

 studies of ore deposits, and expressed as follows : 



II resterait, d'alleurs, a examiner si cette serpentinisation meme n'est pas 

 souvent contemporaine de la cristallisation (comme paraitrait le prouver sa 

 persistance a de tres grandes profondeurs) et si elle n'a pas ete produite par 

 un exces de vapeur d'eau, dont la presence, dans le magma fondu, aurait pu, 

 en meme temps, faciliter la concentration metaUique. 



Where basic magmas are involved, crystallization-differentiation 

 may concentrate sufficient quantities of the more acid end-stage 

 products to cause profound changes to take place and to produce 

 ore-bodies of magnitude and commercial importance. The 

 nickeliferous "norites" of the Sudbury region in Ontario, and the 

 Maskwa River norite in Manitoba are examples of such large-scale 

 effects. The "norites" of these areas have been so extensively 

 changed that in many instances they do not resemble true norites 

 either mineralogically or chemically. Their pyroxene has been 

 converted to actinolite and biotite, their feldspar have in some cases 

 been albitized, and in places they have been swamped in end-stage 

 quartz and heavily mineralized. Figures 5, 6, and 7 illustrate some 

 of these changes. 



One of the curious results of the conversion of pyroxene to 

 actinolitic hornblende is exhibited in the powerful penetrating capa- 

 city developed by the uraHtized pyroxene, since it is commonly foimd 

 in an aggregate of acicular crystals which frequently penetrate the 

 adjoining feldspars in every direction; soda seems to have been 

 added during the process, derived undoubtedly from the soda- 

 enriched liquid residuum responsible in part for the conversion. 

 In some cases sufficient soda has been acquired by the uralitized 

 pyroxene to cause the production of a blue amphibole of about the 

 quahty of glaucophane (norite from the Creighton Mine). 



All stages of flooding with the end-consolidation, more siKceous, 

 aqueo-igneous concentration-product may be seen, the maximum 



' H. L. De Launay, Contributions a UEtude des Giles Metalliferes, 1897, p. 25. 



