REPORT OF THE DIRECTOR IQLI 1/3 



were entirely removed the residue would be much Igwer in iron 

 than was the original rock. AA^hile mica is the only mineral to which 

 the iron of pyrite can be definitely traced, it is quite probable that 

 other iron compounds, such as garnet, magnetite and amphibole 

 may haxt been sources of the metal which have undergone complete 

 decomposition. It is an interesting fact that no magnetite has been 

 noted in any thin sections, suggesting its complete conversion into 

 pyrite. 



This introduction of magmatic H2S is thought to have been a 

 potent agency in the production of the ores, but obviously the mere 

 conversion by this agent of original iron into pyrite could not pro- 

 duce the ore bodies unless the original rock was very rich in iron 

 compounds. That such may have been the case is entirely possible, 

 and this may account for the localization of the ore deposits. But 

 it is much more probable that the circulation that introduced the 

 HoS, with consequent formations of pyrite, also effected a con- 

 centration of this mineral with the consequent formation of the 

 existing ore deposits. 



In this connection there naturally arises the question as to why the 

 pyrite, if not a primary precipitate, should be confined to a certain 

 phase of the sedimentary formation, and this not the most soluble 

 or easily replaced phase. One reason for this doubtless is, as just 

 suggested, that magmatic sources supplied the sulfur of pyrite, 

 the iron being an original constituent of the sediments. On this 

 assumption the ore deposits would necessarily form in the gneisses 

 derived from the shales and sandstones, which would be the iron- 

 bearing members of the sedimentary formation, rather than the 

 limestones. All other conditions being the same, and H.,S circu- 

 lating through limestones to the same degree as through the meta- 

 morphosed shales and sandstones, there can be little doubt that any 

 pyrite thus formed would be chiefly confined to the latter rocks for 

 the simple reason that they alone would contain the iron necessary to 

 combine with the sulfur to form pyrite. Pyrite so formed together 

 with such original pyrite as may have been present, would tend to 

 cause precipitation and further concentration in these rocks of 

 pyritic materials of wholly extraneous origin, brought in by the 

 circulating solutions. In this way the magma might furnish not 

 only sulfur but iron as well. 



While the concentration of pyrite in the gneisses rather than in 

 the more soluble and chemically active limestones is thought to have 

 been eft'ected partly in this way, other factors have doubtless worked 

 in the fame direction. As already stated, the folding of the Gren- 



