ZINC ORES OF THE EDWARDS DISTRICT 33 



because of the absence of calcium carbonate to act as a precipitant 

 of the latter. Thus, in the schists, there would be formed deposits 

 of pyrite free from blende. In the limestone, on the other hand, zinc 

 would also be precipitated as blende, together with the pyrite, form- 

 ing the pyritiferous blende deposits. 



This explanation leaves out of account the graphite of the pyrite 

 ores, but this mineral constitutes a problem much too large to be 

 considered here and has, moreover, no direct bearing upon the 

 genesis of the zinc ores. 



An obvious practical conclusion to be drawn from the foregoing 

 is that, in seeking new zinc deposits in the district, the prospector 

 should confine himself to limestone areas while, if his search is for 

 pyrite, he should examine regions of Grenville schist and, particu- 

 larly, the so-called '* rusty gneisses." It seems also to be true that 

 pyrite is more apt to appear in limestone than is blende in schists. 



From all that has been said, it is evident that the zinc ores are 

 intimately related to the pyrite deposits. Indeed, it may be inferred 

 that the zinc, pyrite, talc and, possibly, graphite deposits of the 

 region are, from the genetic point of view, closely interrelated. 



As to the distribution of the zinc deposits, it might be expected 

 that, if they are derived from the granite magma, they would occur 

 near the contact between granite and limestone ; and such is usually 

 the case. But, in view of the magnitude of the granite intrusions 

 as compared with that of the limestone belts, and the fact that the 

 zinc compounds have, probably, been transported in solution rather 

 than as vapors and, thus, were capable of traveling long distances 

 before being deposited, there is no reason why, mechanical con- 

 ditions being favorable, ore deposits should not occur anywhere in 

 the limestone. It is considered highly probable that no part of 

 the limestone has escaped the influence of the granite intrusion. 



The actual localization of the ores is, unquestionably, due, as their 

 form shows, to fractures or planes of weakness in the limestone, 

 through which the ore-bearing solution circulated. These fractures 

 were not wide, open fissures but were mere cracks or narrow sheer 

 zones admitting the solutions, which were thus enabled to attack 

 and replace the wall rock, with the resultant formation of the ore 

 deposits. 



CHEMISTRY OF ORE DEPOSITION 



Any attempt to determine details as to the chemistry of ore 

 deposition must, of necessity, be highly speculative, and yet an 

 effort in this direction seems justifiable. 



