1 144 A TREATISE ON METAMORPHISM. 



ASSOCIATION OF LEAD, ZINC, AND IRON COMPOUNDS. 



In order to understand the relations of the lead, zinc, and iron com- 

 pounds where they occur together in ore deposits, it seems advisable to 

 consider an individual region rather than to make a general statement. 

 An excellent illustration of the association of these metals is furnished 

 by the deposits of the Mississippi Valley, and this region will therefore be 

 considered. 



FACTS OF OCCURRENCE. 



In the Mississippi Valley, as is well known, in openings in limestones, 

 lead and zinc minerals are associated with marcasite, pyrite, and some 

 chalcopyrite." Calcite and dolomite are abundant gangue minerals, as 

 would be expected. Barite is also a common gangue mineral and locally 

 is very plentiful. There are other gangue minerals which will not be 

 taken into account. Since chalcopyrite is very subordinate, it will not again 

 be alluded to. 



The vertical order of occurrence in the region is commonly as follows : 

 Above the level of ground water in the belt of weathering the dominant 

 valuable minerals are galena and the oxidized ores — smithsonite, calamine, 

 cerussite, and anglesite. Other oxidized ores occur in subordinate amount, 

 but they will be ignored. The cerussite and anglesite frequently incrust 

 the galena or are in crystals upon it. With the smithsonite and calamine 

 there is some sphalerite. The smithsonite and calamine may extend 5 or 

 10 meters below the level of ground water, but deeper the oxidized products 

 almost wholly disappear. The presence of oxidized products below the 

 level of ground water is due in some cases to the occurrence of the material 

 along a main channel of downward-percolating waters or to the well-known 

 general downward movement of oxidizing water somewhat below the level 

 of ground water; in other cases probably is due to a present higher level of 

 ground water than in pre-Grlacial time, as a result of depression and valley 

 filling at the close of the Glacial epoch; and in still other cases is probably 

 due to reactions between oxidized lead salts and sphalerite. (See p. 1150.) 

 Below the galena and the oxidized ores are sphalerite and galena together, 

 the former being dominant except in the Southeastern district of Missouri. 6 



«Chamberlin, T. C, The ore deposits of southwestern Wisconsin: Geol. of Wisconsin, vol. 4, 

 1882, pp. 380-393. 



6 Chamberlin emphasizes the inferior position of the zinc as compared with the lead and the asso- 

 ciation of the zinc and iron, but he does not consider the positions of these compounds with Reference 

 to the level of ground water. Loc. cit . , pp. 488-491. 



