Читать книгу The Economic Aspect of Geology - C. K. Leith - Страница 52

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Mineral deposits are formed and modified by practically all known geologic processes, but looked at broadly the main values are produced in three principal ways:

(1) As after effects of igneous intrusion, through the agency of aqueous and gaseous solutions given off from the cooling magma.

(2) Through the sorting processes of sedimentation—the same processes which form sandstone, shale, and limestone. Organic agencies are important factors in these processes.

(3) Through weathering of the rock surface in place, which may develop values either by dissolving out the valuable minerals and redepositing them in concentrated form, or by dissolving out the non-valuable minerals and leaving the valuable minerals concentrated in place. The latter process is by far the more important.

The overwhelming preponderance of values of mineral deposits as a whole is found in the second of the classes named.

Under all these conditions it appears that the maximum results are obtained at and near the surface. On the scale of the earth even the so-called deep veins may be regarded as deposits from solutions reaching the more open and cooler outer portions of the earth. However, valuable mineral deposits are found in the deepest rocks which have been exposed by erosion, and the question of what would be found at still greater depths, closer to the center of the earth, is a matter of pure speculation.

Ultimately all minerals are derived from igneous sources within the earth. The direct contributions from these sources are only in small part of sufficient concentration to be of value; for the most part they need sorting and segregation under surface conditions.

We can only speculate as to causes of the occurrence of valuable minerals in certain igneous rocks and not in others. Many granites are intruded into the outer shell of the earth, but only a few carry "minerals"; also, of a series of intrusions in the same locality, only one may carry valuable minerals. It is clear that in some fashion these minerals are primarily segregated within the earth. Causes of this segregation are so involved with the problem of the origin of the earth as a whole that no adequate explanation can yet be offered. Our inductive reasoning from known facts is as yet limited to the segregation within a given mass of magma, and even here the conditions are only dimly perceived. A discussion of these ultimate problems is beyond the scope of this book.