1/2 NEW YORK STATE MUSEUM 



perhaps indeed the chief source. But, as ah-eady stated, microscopic 

 study shows that the pyrite of these rocks replaces the other min- 

 erals, particularly the feldspar, and thus is secondary. 



While it is evident therefore that the pegmatites are not the 

 direct source of the pyrite, there is much reason for believing that 

 their association with the ore deposits and their presence throughout 

 the Grenville in such abundance, as well as the other evidences of 

 extreme igneous activity point to magmatic sources for ore-forming 

 solutions. Magmatic emanations permeating the gneisses would 

 certainly appear to be the most probable agents to bring about the 

 deposition of pyrite, replacing the older and usually very stable 

 minerals. 



The absence of contact and fumarole minerals approximately 

 contemporaneous with the pyrite indicates that the latter was formed 

 late in the history of igneous activity, when instead of the very 

 potent fluorine, boron and chlorine compounds, hydrogen sulfid 

 which marks a declining stage of igneous activity, was the dominant 

 agent. Thus the pyrite probably was deposited after active intru- 

 sion of the magmas, from which it was indirectly derived, had 

 ceased, and during a prolonged period of relatively gentle circula- 

 tion of magmatic waters. To these agencies may be ascribed not 

 only the deposition of pyrite but the closely, associated formation 

 of the peculiar chloritic alteration product, which is so characteristic 

 of the ore deposits and of the less abundant sericite. 



It is evident that magmatic agencies might operate in two ways 

 to bring about the observed results, namely by concentrating pyrite 

 already present in a disseminated condition, and by introducing 

 entirely new pyrite of magmatic origin. These two processes would 

 work together and combine to build the ore deposits. But it is 

 probable that an intermediate operation may also have been effec- 

 tive, one constituent of the pyrite, iron, being already present as an 

 original constituent in the rock, and the other, sulfur, supplied by 

 tlie magma. Bleaching of the mica is a characteristic feature of 

 the conversion of gneisses into ores, indicating the removal of iron 

 from this constituent. Some of this iron reappears in the chlorite, 

 but the latter is itself nearly always of a conspicuously pale tint, 

 indicating low iron. 



While the elaborate series of analyses needed definitely to estab- 

 lish the fact is lacking, it appears to be true that most of the iron 

 originally present in the essential minerals of the gneiss, is now 

 combined with sulfur to form pyrite in the ores, and if the pyrite 



