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SUPERFICIAL ALTERATION OF ORE DEPOSITS.
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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 intermediate 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 condition 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 generally 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 particles 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

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