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Page:Popular Science Monthly Volume 72.djvu/543

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539
THE GENESIS OF ORES

solutions came can often be detected; and peculiarities of shape and position observed which can be explained with difficulty on any other theory.

Practised miners often point to the richness of ore shoots near the junction or crossing of veins. Indeed such pockets and shoots are usually sought and frequently found where two veins come together. This fact alone may not signify the instrumentality of downward moving waters. But when in connection with it we discover that rich ore shoots are also frequently found at the intersection of veins by faults, and zones of movement so recent or of such shallow depth or limited extent that the faults themselves are not veins, and have not been mineralized except near the intersected veins, and when the ore shoots thus formed occur on that side of the fault plane where they could have been formed most naturally by descending waters, and are wanting entirely in the corresponding place on the other side, then, indeed, we recognize beyond a doubt the agency of meteoric waters in both situations.

It is often possible where sulphide ores have been deposited in soluble rocks to distinguish between the products of ascension and descension, and here too the latter are frequently of much the highest grade.

This theory of secondary enrichment which is so frequently referred to in recent mining literature; and is still so little understood, depends, of course, on the existence of a body of primary ore, probably formed by ascending solutions. If there are no ores to be oxidized the downward moving waters will have no metalliferous burden to deposit. But wherever the rocks contain disseminated ore, no matter how small the percentage, there is a possibility of the formation of richer ores through the action of surface waters. And where the primary mineralization was itself comparatively rich, even though not a minable product, there the downward-moving waters may the more readily bring about concentrations of high-grade bonanza ore.

Bearing in mind this conception of the meaning of "secondary enrichment," and admitting that it is frequently accomplished through the agency of descending meteoric waters, let us briefly consider the conditions under which they are most active and efficient:

It is a proposition requiring no argument that if by the aid of mineral bearing solutions the ores occurring in veins are to be enriched, these solutions must enter the veins. And if all the meteoric waters which fall upon the outcrop of a vein or upon rocks containing disseminated ore run off rapidly down the mountainside without remaining to oxidize, dissolve and penetrate the vein with their load of mineral, there can not be any enrichment caused thereby. Furthermore, if the work of the surface waters is chiefly destructive mechan-