Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

ORE-DEPOSITS.[207]

Ore-deposits are but a special phase of the rock-forming processes already discussed. They have peculiar interest because of their industrial value. An ore is simply a rock that contains a metal that can be profitably extracted, though for convenience the term is used more broadly to include unworkable lean ores and ore material. The metal need not preponderate or form any fixed percentage of the whole, for the criterion is solely economic and not petrologic. A gold ore rarely contains more than a very small fraction of one percent. of the precious metal, while high-grade iron ore yields sixty-odd percent. of the metal. In iron ore, the metallic oxide or carbonate makes up nearly the whole rock; in gold ore, the metal is the merest incidental constituent, from the petrologic point of view.

Concentration.—The essential fact in the formation of ores is the unusual concentration of the metal. There are vast quantities of all the metals disseminated through the rock substance of the earth and even throughout the hydrosphere, but they do not constitute ores because they have no economic value. They become ores when concentrated in accessible places to a workable richness. The degree of concentration required is measured by the value of the metal. The essential elements for consideration are, therefore, (1) the original distribution of the metallic materials through the rocks, (2) their solution by circulating waters (or, rarely, by other means), (3) their transportation in solution to the place of deposit, (4) their precipitation in concentrated form, and (5) perhaps their further concentration and purification by subsequent processes.

Exceptional and doubtful cases.—There are a few cases where ore-deposits are made by volcanic fumes or vapors, but these may be neglected here. Formerly, ores were often attributed to vapors supposed to arise from the hot interior, but this mode of origin seems incompatible with physical conditions. Ores have been attributed to water originally contained as steam in lavas, and to waters escaping from the interior of the earth, these waters being supposed to be especially mineralized. Direct evidence on this point is obviously beyond reach. Segregation in the molten state is recognized as a source of ores, but its function is probably confined chiefly to partial enrichment as stated below. There are other occasional methods, but the chief process of concentration, immeasurably surpassing all others, consists in the leaching out of ore materials disseminated through the country rock and their redeposition in segregated forms, as an incident of the recognized system of water circulation.

Original distribution.—The original distribution of ore material through the primitive rocks is beyond the ken of present science, for even the nature of the true primitive rocks is unknown. For present purposes it is sufficient to regard all rocks concerned in ore-deposition as either igneous or sedimentary, and to inquire, as a first step, how far ordinary igneous and sedimentary processes contribute to the segregation of ore material, leaving for a second stage of inquiry the subsequent processes of concentration.

Magmatic segregation.—In a few instances workable masses of ore seem to have arisen from lavas by direct segregation in the molten state, without the aid of subsequent concentration by water action, on which most ores are dependent. It is not improbable that the segregation of metallic iron and nickel, and perhaps other metals, in the deeper parts of the earth may be a prevalent process, giving rise to masses like the native iron found in basalt in Greenland. This iron closely resembles the nickel-irons of meteorites, which may be illustrations of similar action in small planetary bodies that have been disrupted. Metallic masses so segregated presumably gravitate toward the planetary center and hence, whatever their inherent interest, have little relation to a subject whose basal criterion is economic. It is not at all improbable, however, that in the magmatic differentiation of the lavas that come to the surface, there is some metallic segregation that may make the enriched parts effective ground for the concentrating processes of water circulation, and so determine the location of ore-deposits. Igneous rocks are not equally the seats of ore-deposits, even when the circulatory conditions seem to be equally favorable. These conditions may not really be equally favorable, but there is good ground to believe that some igneous masses constitute a richer field for concentration than others. No definite rule, however, for distinguishing rich varieties of rock from lean ones has been determined. The basic igneous rocks are, on the whole, perhaps somewhat richer in ores than the acidic class, but there is no established law. Many acidic rocks bear more and richer ores than many basic ones. The view here entertained is that both classes are subject to regional enrichment through conditions connected with their origin, as yet little known.

Marine segregation and dispersion.—In the formation of the sedimentary rocks from the primitive and igneous rocks there was notable metallic concentration in some cases, and even more notable depletion in others. The ground-waters of the land, after their subterranean circuits, carried into the water-basins various metallic substances in solution. These were either precipitated early in the marine or lacustrine drift of the waters, or became diffused throughout the oceanic body. In the main they appear to have been widely diffused, and either to have remained long in solution, or to have been very sparsely deposited through the marine or lacustrine sediments. As a rule, these sediments seem to contain less of valuable ore material than igneous rocks, and this is rational, for, as we shall see, the ground-water circulation of the land tends to concentrate and hold back a part of the metallic content of the land rocks so that only a residue reaches the sea. But there are important exceptions to this general rule of sedimentary leanness.

The iron-ore beds of Clinton age ranging from New York to Alabama, and appearing also in Wisconsin and Nova Scotia, form a stratum in the midst of the ordinary sediments, and contain marine fossils. The great ore beds of Lake Superior were originally of similar type, and so are most other important iron deposits. It cannot be said, in most cases, that these iron deposits are marine as distinguished from lacustrine or lodgment deposits, but they are at least sedimentary. The ferruginous material was originally disseminated widely through antecedent land rocks, but was concentrated in the course of the sedimentary processes.

Limestone appears to have been sometimes enriched locally in lead and zinc, and more rarely in copper, in the course of its sedimentation. The lead and zinc regions of the Mississippi basin have been regarded as dependent on such regional enrichment as a primary condition. This localized enrichment has been attributed to solutions brought into the sea from neighboring metal-bearing lands and precipitated by organic action in the sea-water,[208] this organic action being more effective in some areas than in others because of the unequal distribution of life and the concentration of its decaying products. It is assumed that such precipitates were at first too diffuse to be of value, and further concentration was required to bring them together into workable deposits; but the further processes appear to have been effective only where the preliminary enrichment had taken place. At any rate, the workable deposits are singularly localized, while the concentrative processes are very general.

Metallic material is sometimes partially concentrated in sandstones and shales in the process of sedimentation, though more rarely. The copper-bearing shale (Kupferschiefer) of the Zechstein group in Germany, so extensively worked along the flanks of the Harz Mountains, is a striking example.