Page:Popular Science Monthly Volume 72.djvu/539

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535
THE GENESIS OF ORES

Copper, lead, zinc, tin, silver and gold, although metals of great importance to man, constitute so small a part that their percentages are expressed by four to eight decimals, that is, between hundred thousandths and billionths of a per cent.

In some eruptive rocks, however, the percentage is much higher, and has been determined to be in the thousandths of a per cent. in the case of copper, lead and zinc, and one tenth to one hundredth as much of silver and gold.

The amount of metallic content found to occur as a primary constituent in unaltered rock is thus seen to be far too small to constitute workable ores, and indeed is often so insignificant as to be determined with difficulty. You all know that several per cent. of iron, manganese, zinc, lead and copper are required to make an ore valuable, the percentage varying, of course, with the locality, complexity of the ore and other familiar factors.

It is therefore apparent that a process of natural concentration is essential for the production of ore deposits, bringing into limited space the material formerly disseminated through ten thousand or a hundred thousand times that extent of ground, or accomplishing the same result by the removal of the admingled rock impurities.

Wherever this concentration is brought about by assembling of solid particles under conditions that admit of freedom of movement, we have placer deposits as of gold and platinum, of tin, iron and chromium ores, and sometimes of precious stones, such as diamonds, sapphires, rubies, garnets and others.

The ores found in veins, in disseminations throughout the rocks and in irregular shaped deposits in soluble rocks can not have been collected in any such manner. Their mode of occurrence and relation to the enclosing rocks make it evident that they have been slowly deposited from solution. And the only solvent of general distribution is water, with its varying content of acids and alkalies under changing conditions as to temperature and pressure.

Water is the magic instrument by which all the copper in Butte's vast mines, all the gold and silver of the Comstock and of Goldfield, were assembled; more potent than the Philosopher's Stone, more universal than the air we breathe; constantly at work, dissolving, transporting and redepositing. With indefatigable zeal and never-flagging industry it searches through the innermost recesses and penetrates the most closely locked chambers of the rocks, removing treasures through their very walls, and often repairing breaches made in the attack so skilfully as to defy detection, or to make the masonry stronger than when first laid. Small wonder that the ancients regarded it as one of the four prime elements!

But, although for several years water has been recognized as the