THE' ORIGIN OF MINERALS 253 



The surface sulphides of many deposits have been completely 

 oxi<li/e<l. Most of the products of oxidation are carried off in solu- 

 tion ; only ;i small proportion remaining to indicate the nature of the 

 original minerals. The red oxide of iron, hematite, is especially 

 characteristic of such conditions and often covers the lower un- 

 oxuli/.ed sulphides like a cap; from its position it has been termed 

 the "iron cap." The upper oxidized surface areas of ore deposits 

 are also termed the gossan. Minerals characteristic of the gossan 

 are the carbonates, hydrates, sulphates, arsenates, and oxides. 

 The depth of these oxidized minerals, or the gossan, will depend 

 upon climatic conditions, the nature of the formation, and the depth 

 of the ground water. The oxidized areas gradually give way to 

 the region, in ore deposits, termed the areas of secondary enrich- 

 ment, where the sulphides have not only not been oxidized, but 

 have been increased in some of their constituents. The metallic 

 sulphides carried down in solution from the gossan above are pre- 

 cipitated and added to the original metallic content of sulphides be- 

 neath. That the waters descending from the oxidized areas above 

 do carry sulphates in solution is abundantly proven by large num- 

 bers of analyses of mine waters, and also by the fact that in many 

 mining regions the mine waters carry copper sulphate in such quan- 

 tities that it pays to precipitate it from the solutions with scrap 

 iron. Of the metallic sulphides, pyrite, marcasite, and pyrrhotite 

 are the most readily oxidized, and when in contact with solutions 

 containing copper sulphate they precipitate the copper as sulphide, 

 forming chalcopyrite. At the Copper Queen Mine, at Bisbee, 

 Arizona, pyrite too poor in copper to pay for smelting was used to 

 fill old stopes, is now, after some ten or twelve years, remined and 

 smelted, having collected nearly 10 per cent, of copper by precipita- 

 tion from the mine waters through replacement of the iron. Pyrite 

 not only serves as a precipitant for copper, but also for lead, silver, 

 zinc, and other elements less basic. 



By this process of oxidation and precipitation the area of second- 

 ary enrichment, or the unoxidized sulphides directly underneath 

 the gossan, contains not only the valuable metals which were their 

 original content, but added to this by replacement is a large propor- 

 tion of that formerly contained in the gossan above. The ore in 

 many mines has been found too poor to work when the regions be- 

 low the areas of secondary enrichment have been reached. This 

 process is therefore one of prime importance, and must be consid- 

 ered in the valuation of ore deposits. Surface waters and spring 



