SUPERFICIAL ALTERATION OF ORE DEPOSITS. 307 



sulphide deposits of Arizona, Chile and elsewhere. In Arizona 

 the upper parts of the deposits are composed of brown or black 

 ferruginous masses, with brilliantly colored oxidized copper min- 

 erals, as cuprite, malachite, azurite, chrysocolla, etc.; while below, 

 at depths varying from a few feet to several hundred feet, the 

 deposits usually pass into a mixture of copper pyrites and iron 

 pyrites, the latter usually being far in excess. Sometimes other 

 copper sulphides occur, either mixed with copper pyrites or free 

 from it, and they may or may not have been derived from it. 

 Here the carbonates and some of the other alteration minerals 

 contain not only more copper than the unaltered copper pyrites, 

 but they are also in a much more concentrated condition than 

 the sulphide which is disseminated through iron pyrites. The 

 total amount of copper has not been increased, in fact it may 

 be decreased by leaching, but it is in a more concentrated 

 form, and therefore the ore obtained from these concentrations 

 averages from eight to thirty per cent, or more in copper, while 

 the mixture of unoxidized copper pyrites and iron pyrites below 

 averages only about five per cent, in copper. Moreover, the 

 altered ores are much more cheaply treated than the unaltered 

 ones, and are therefore still more desirable. It will thus be seen 

 that the economic value of the deposits as a whole has been 

 greatly increased. 



In the surface alteration of these deposits, the copper sul- 

 phides have first been converted to copper sulphate and then, by 

 the action of surface waters and the materials contained in solu- 

 tion in them, they pass into the forms of copper carbonates, 

 oxides, silicates, and occasionally to the chlorides and bromides, 

 and sometimes to native copper. The iron sulphide is first con- 

 verted to sulphate and then this, through other stages, is 

 converted into the hydrous sesquioxide (limonite), though 

 the iron sometimes now occurs in the form of the anhydrous 

 sesquioxide (hematite). This may have been derived from 

 the limonite by dehydration, or, under certain conditions, 

 may have been formed directly by the oxidation of iron 

 pyrites. The oxidized copper minerals in the upper part of 



