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SUPERFICIAL ALTERATION OF ORE DEPOSITS.
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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 immediately overlies the unoxidized mixture of copper and iron pyrites, which averages only from 2 per cent. to 4 or 5 per cent. in copper. 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 sulphate (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 silver, 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 (cerargyrite), bromide of silver (bromyrite), iodide of silver (iodyrite),

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