SUPERFICIAL ALTERATION OF ORE DEPOSITS. 309 



hydrous sesquioxide of iron. Below this a part of the copper, 

 which has been leached from above, has been carried down and 

 deposited as a dark material, probably composed largely of 

 oxides and sulphides of copper, and averaging sometimes 20 to 

 25 per cent, or more in metallic copper. This material immedi- 

 ately overlies the unoxidized mixture of copper and iron pyrites, 

 which averages only from 2 per cent, to 4 or 5 per cent, in cop- 

 per. The commercial copper mined in this region came from 

 the part of the deposit below the iron capping and above the 

 unoxidized sulphides. When this was exhausted, the mines had 

 to be closed, for the unaltered sulphides were too poor to be 

 utilized. 



In Chile, Peru, and elsewhere in South America, changes in 

 copper deposits, somewhat similar to those described in the 

 United States, frequently occur. In fact, the great reputation 

 which Chile once had as a copper producer, was largely due to 

 this surface alteration, for the oxidized ore once supplied a rich 

 and easily treated source of copper, but when the mines reached 

 the unoxidized sulphides, the ores became poor in copper and 

 more difficult to treat, so that the copper industry of Chile began 

 to decline. In that region, however, the oxidation has in some 

 places extended down as far as 1,500 feet. 



Alteration in lead deposits. — In the case of lead deposits, the 

 mineral galena, which is the commonest ore, is frequently more 

 or less altered on its surface outcrops and converted to the sul- 

 phate (anglesite) and the carbonate (cerussite). The first 

 product of oxidation is anglesite, but this is a soluble compound 

 and readily unites with carbonic acid or soluble carbonates in 

 surface waters, forming the carbonate of lead, or cerussite. In 

 rarer cases, other lead minerals, like phosphates, may also be 

 formed. 



Alteration in silver deposits. — Galena deposits often contain sil- 

 ver, possibly sometimes in the same condition of sulphide as the 

 galena, and this material is altered at the same time as the lead, 

 with the formation of native silver, chloride of silver (cerargyr- 

 ite), bromide of silver (bromyrite), iodide of silver (iodyrite), 



