ZINC ORES OF THE EDWARDS DISTRICT 35 



fixed by calcium, while zinc chloride and hydrogen sulphide would 

 remain and react according to the equation 



ZnCL + H 2 S ^ ZnS + 2HC1. 



In the absence of calcium carbonate (calcite,) the reaction 

 would take place chiefly from right to left. With calcium 

 carbonate present, however, hydrochloric acid would be fixed as 

 calcium chloride so that the reaction would proceed from left to 

 right, till the supply of either zinc chloride or calcium carbonate 

 was exhausted. Thus, the limestone is replaced by blende, the latter 

 being less volatile than zinc chloride and remaining when formed. 



This explanation of the genesis of the Christiania ores indicates 

 why the zinc ores occur in limestone, since the calcium carbonate 

 of the latter is an essential factor in the precipitation of the zinc 

 sulphide. 



While it is not maintained that this explanation of the chemistry 

 of ore deposition in the Christiania region can be transferred bodily 

 and without modification to the Edwards deposits, there can be 

 little doubt that it indicates, in a general way, the nature of the 

 process by which the ores of this district were formed. 



As already stated, there is reason for believing that the materials 

 of these ores were transported in solution rather than in gaseous 

 compounds, as was the case with the Christiania ores. This con- 

 clusion is based on field relations and on the absence of pneu- 

 matolytic minerals in the deposits. The evidence on this question is, 

 however, far from satisfactory and it is quite possible that future 

 study may show the constituents of the sulphides to have been 

 transported in a volatile state. 



The importance of chlorine in ore concentration, Goldschmidt 

 bases upon the abundance of scapolite (a chlorine-bearing silicate) 

 among the contact minerals. No scapolite has been noted in the 

 Edwards ores, but it is a frequent and abundant mineral of the 

 contact zones throughout the region and may, very probably, occur 

 in the immediate vicinity of the ores. Thus, the presence of suf- 

 ficient chlorine to play a vital role in the process of ore formation 

 is entirely possible ; indeed, the entire absence of scapolite would 

 by no means negative the possibility of the chlorine. But even if 

 the zinc were actually transported as some compound other than 

 chloride, the process might still have much in common with that 

 outlined by Goldschmidt. 



For blende deposited at a much lower temperature than that 

 assigned to the Christiania ores, approximating that of the formation 

 of serpentine and talc, the zinc may have been transported as 

 carbonate or bicarbonate rather than as chloride. 



