SUPERFICIAL ALTERATION OF ORE DEPOSITS. 311 



iron pyrites, and which has remained stable during the oxidation 

 and leaching of that mineral. Such quartz is usually porous and 

 spongy, and is filled with cavities which represent the shapes of 

 the original crystals of iron pyrites, and which, during an inter- 

 mediate stage, have been partly filled with hydrous sesquioxide. 

 This leaching, however, is rarely complete, and the quartz is 

 usually stained brown on the surface. 



In gold deposits of this kind, other minerals, such as copper 

 pyrites; galena, blende, etc., frequently occur, and when the 

 deposit is affected by surface influences, these minerals act in 

 the manner already described under copper, lead, and zinc. It 

 is not uncommon to see gold-bearing quartz stained green by 

 oxidized copper minerals, or black by manganese minerals. 

 Sometimes, especially in the Rocky Mountain region, gold 

 occurs in the form of a telluride instead of in iron sulphide, 

 and in such cases, the telluride is oxidized and the gold set free 

 from its combined state. The gold, in being freed from pyrites 

 or other minerals, is not only concentrated by the removal of 

 certain ingredients of the deposits, but it is brought into a condi- 

 tion in which it is much easier to treat than the unaltered part of 

 the deposit, and, therefore, the upper parts of most gold- 

 bearing veins are greatly enhanced in value. The ore from 

 these parts is known as " free milling " ore, because it can gen- 

 erally be ground and the gold extracted by direct amalgamation 

 with mercury ; while the ore in the unaltered parts of the deposit 

 cannot usually be thus easily extracted, but must be smelted or 

 treated by chlorination or some other more or less expensive 

 process. 



When such deposits as those described are eroded, the parti- 

 cles of gold separate from the quartz and are concentrated in the 

 streams as placer gold. These detrital deposits are the source 

 of a large part of the gold of commerce, and, in fact, were once 

 the source of most of it. Now, however, many of the richest 

 placer deposits known have been exhausted ; and besides, the 

 methods of treating the ores in the original deposits are better 

 understood, so that the latter are supplying yearly a larger and 



