LEAD AND ZINC ORES. 1147 



SECOND CONCENTRATION. 



It is highly probable that the regular vertical distribution of the 

 minerals is due to a second concentration mainly controlled by downward- 

 moving waters combined with denudation. 



OXIDIZED ORES. 



The oxidized ores include those above or near the ground-water level. 

 As already noted, the important ones comprise smithsonite, calamine, 

 cerussite, and anglesite. These ore bodies have been formed from the 

 sulphides by oxidation, carbonation, and hydration. The beginning of 

 these processes is coincident with the appearance of the sulphides above 

 the level of ground water in consequence of progressive denudation. Lead 

 sulphide (PbS) and zinc sulphide (ZnS), by simple oxidation are changed 

 to lead sulphate (PbS0 4 ) and zinc sulphate (ZnS0 4 ). The lead sulphate 

 constitutes the mineral anglesite. If this sulphate in solution reacts upon 

 calcium carbonate, lead carbonate and hydrated calcium sulphate are 

 produced according to the following equation: 



PbS0 4 +CaC0 3 +2H 2 0=PbC0 3 +CaS0 4 .2H 2 0. 



If zinc sulphate reacts on calcium carbonate, smithsonite is produced 

 according to the following equation: 



ZnS0 4 +CaC0 3 +2H.,0 = ZnC0 3 +CaS0 4 . 2H 2 0. 



In this connection it is interesting to note that in Wisconsin the smith- 

 sonite often occurs in beautiful pseudomorphs after the various forms of 

 calcite characteristic of the district. In many cases when these pseudo- 

 morphs are broken they are found to be partly hollow, showing not only 

 that the calcite was replaced, but that solution of the calcium carbonate 

 went on faster than the deposition of the smithsonite. In many other 

 instances the smithsonite is in plates of greater or less thickness over calcite 

 or limestone, the latter showing solution by unequal etching. 



Where the zinc sulphate finds abundant amorphous silica in chert the 

 reaction may be between this compound and the zinc salt in the presence 

 of water, the equation being as follows: 



Si0 2 +2ZnSO t +3H,0= ( ZnOH ) 2 Si0 3 +2H 2 S0 4 . 



