1160 A TREATISE ON METAMORPHISM. 



extent are more favorable to the development of the oxidized products, denu- 

 dation is frequently so rapid that time is not given for the transformation of 

 the sulphides to such products, for a given part of the belt of weathering 

 is above the level of ground water only a short time before the material 

 of that portion is removed by erosion. Also the belt above the level of 

 ground water is narrow, and thus there is comparatively little room for the 

 oxidized deposits. 



While under favorable conditions oxidized products are very common 

 above the level of ground water, it is well known that below it the oxidized 

 and carbonated products occur in greatly diminished quantity, and there 

 are frequently present rich sulphurets, such as chalcocite (Cu 2 S), bornite 

 (Cu 3 FeS 3 ), chalcopyrite (CuFeS 2 ), and sometimes covellite (CuS). Some- 

 what deeper below the level of ground water the oxides and carbonates are 

 not found. Furthermore, the chalcocite, covellite, and bornite are very 

 generally restricted to the upper part of the belt of cementation; deeper, 

 the places of these minerals are largely occupied by chalcopyrite. Not 

 only is this true, but still deeper in many instances the chalcopyrite is less 

 prominent, and the iron sulphides more so. In the lower workings of many 

 of the deeper mines the only metalliferous product found is cupriferous iron 

 sulphide, the chalcopyrite having wholly disappeared. But in some places 

 where the dominant sulphide of the deeper levels is pyrite or pyrrhotite 

 fracture openings within these sulphides are filled with the rich copper 

 sulphides. 



Whether or not this general statement is correct for a particular area, 

 each mining engineer can judge from his own knowledge. There may be 

 exceptions to it, due to various causes, one of which has been alluded to in 

 explaining bonanzas. Thus below cupriferous pyrites there may again be 

 found richer copper sulphides. Indeed, as before stated, ore deposits vary 

 greatly in their richness both horizontally and vertically, and the above 

 statement can only be considered as a general average." 



The above order is believed to be explained by the work of downward- 

 moving waters. The combinations of lead, zinc, and iron were followed from 

 above downward. The reactions which occur in the case of the copper- 

 iron deposits may perhaps be followed, to vary the treatment, from the base 



«See Raymond, R. W., Discussion of "Genesis of ore deposits," by F. Posepny: Trans. Am. Inst 

 Min. Eng., vol. 24, 1895, pp. 991-992. 



