SUPERFICIAL ALTERATION OF ORE DEPOSITS. 29 1 



various stages of oxidation of materials from organic matter, but 

 they all eventually, if allowed to become completely oxidized, 

 pass into carbonic acid; while if they are in combination with 

 different bases, these salts are eventually converted to car- 

 bonates. 



Method a?id chemical effects of alteratio7i. — Surface waters thus 

 charged with various chemical ingredients percolate down into 

 ore deposits, and there meet various materials which are even 

 less stable under their influence than most of the common rocks. 

 The alteration, therefore, is comparatively rapid, and, though 

 only superficial, generally extends to much greater depths than 

 in the surrounding country rock. From a chemical standpoint, 

 the first effect of this superficial influence is usually the oxida- 

 tion or hydration, or both, of certain ingredients, followed gen- 

 erall}' by the formation of other chemical combinations and by the 

 leaching of certain materials. In the formation of these other 

 chemical combinations, however, the base usually remains the 

 same, and the alteration consists generally in a change of the 

 materials associated with the ba^e, that is, in the acidic portion 

 of the mineral or the part that represents the acidic portion. 

 Thus, iron sulphides are oxidized to iron sulphate, and then this is 

 converted by further oxidation and by hydration to the hydrous 

 sesquioxide. Copper sulphides may be oxidized to copper sul- 

 phate ; and from the sulphate, by the agency of materials in sur- 

 face waters, may be formed copper carbonates, haloid com- 

 pounds, silicates, oxides, and even metallic copper ; while from 

 some of these, still other compounds may be produced. Similar 

 reactions occur in many lead, zinc, silver, gold, and other deposits. 



Occasionally, chemical changes may occur without previous 

 oxidation, and sometimes, though rarely, surface influences 

 under peculiar conditions may have a reducing effect, as in the 

 formation of iron pyrites and copper pyrites from the sulphates 

 of iron and copper, or in the formation of native copper by the 

 action of a ferrous salt on certain copper salts, or in the forma- 

 tion of native silver in surface outcrops. In many of such cases, 

 however, the chemical action is primarily one of partial oxida- 



