ZINC ORES OF THE EDWARDS DISTRICT II 



The ore bodies sometimes are sharply defined, with well-marked 

 walls, but in other cases are irregularly shaped portions of the 

 limestone more or less permeated with blende and pyrite, which 

 gradually decrease in quantity as the ore body passes into normal 

 limestone. 



APPEARANCE AND CHARACTERISTICS OF THE ORES 

 The ore is a compact, granular aggregate of the sulphides, pyrite, 

 blende and, sometimes, a little galena, with varying amounts of 

 calcite, diopside, tremolite, phlogopite, serpentine, talc, a little barite 

 and occasional films of greenockite of secondary origin, and, while 

 differing from the ordinary crystalline limestone of the region in 

 containing the sulphides, does not dififer materially from it in 

 structure, so far as is apparent to the naked eye. In consequence of 

 this, a casual inspection of the ore gives the impression that it has 

 passed through the entire process of metamorphism which has given 

 to the limestone its present character. As will be shown later, this 

 is not the case, but, instead, the minute texture of the ore, the relation 

 of the sulphides to the normal constituents of the limestone and 

 many details observable only with the microscope all point to^ the 

 conclusion that the ore deposits were formed at considerable 

 depth — probably some thousands of feet — during the metamorphism 

 which has so profoundly modified the sedimentary rocks of the 

 region. If this conclusion is correct, it fixes the age of the ore 

 deposits as Precambrian and as comparatively late in the series of 

 events recorded in the Precambrian rocks of this region. Indeed, it 

 appears that since the ores were formed they, and the surrounding 

 country rocks, have suffered no radical modification of any kind. 

 On the contrary, during the vast period of time that has elapsed, 

 the region has been so free from rock folding and igneous intrusion 

 that the ore deposits have retained their original character, excepting 

 slight modifications of a superficial nature which have not affected 

 the deposits as a whole. 



If the foregoing view is correct, the ores are of extreme antiquity 

 and, roughly speaking, comparable in age with the noted zinc 

 deposits of Franklin Furnace, N J., though differing greatly from 

 these in mineralogical composition. 



As the above conclusions are based not only upon the larger 

 relations already discussed but also upon the more minute details 

 to be ascertained only by microscopic study, the results of the 

 latter need to be considered. 



