It is in every way reasonable to suppose that land-waters, on reaching the margins of the water-basins, must occasionally find conditions favorable for the precipitation of their metallic contents, and that the ratio of these precipitates to other material might be relatively high in the more favorable situations, and that this enrichment of the country rock may be a condition precedent to a sufficient subsequent concentration to yield workable accumulations.
It is, therefore, inferred that while the processes of sedimentation tended on the whole to leanness, they gave rise to (1) some very important ore-deposits, notably the chief iron ores, the greatest of all ores in quantity and in real industrial value, and (2) a diffuse enrichment of certain other areas which made them productive under subsequent concentrative processes, while the sedimentary formations in general were left barren.
Origin of ore regions.—From these considerations it appears that for the fundamental explanation of “mining regions” we must look mainly (1) to magmatic differentiation, so far as the country rock is igneous, and (2) to sedimentary enrichment, so far as the rock is secondary. The determining conditions in both cases are obscure and unpredictable, but the recognition of such regions, and of the function of preliminary diffuse regional enrichment, contributes to a comprehensive view of the complex processes of ore concentration. The subsequent processes consist in the further concentration of the ore material into sheets, lodes, veins, and similar aggregations by ground-water circulation, or else in the purification of the ores by the removal of useless or deleterious material, or in both combined.
Surface residual concentration.—The simplest of all modes of concentration takes place in the formation of mantle-rock. An insoluble or slightly soluble metallic substance sparsely distributed through a rock may be concentrated to working value by the decay and removal of the main rock material, leaving the metallic material in the residuary mantle. The tin ores of the Malay peninsula[209] are especially good examples. The crystals of tin oxide were originally scattered sparsely through granite and limestone, but by their decay and partial removal it has accumulated in workable quantities. Certain gold fields and certain iron ores have acquired higher values in the same way. Such residuary material may be further concentrated by wash into gulches or alluvial flats, in the course of which the lighter parts of the mantle-rock are largely carried away, and the heavier, including the metal or its compounds, are mainly left behind. Gold placers are the best example. The mining of placers by hydraulic processes is but a further extension of the natural process of concentration.
Such concentrates in past ages have in some cases been buried by later deposits, and hence certain ancient sandstones, conglomerates, and mantle-rocks have become ore-bearing horizons. The Rand of South Africa appears to be of this type.
Purification and concentration.—A somewhat different mode of concentration and purification has affected certain of the great iron deposits. As already explained, the iron compounds were originally dissolved from the iron-bearing constituents of the primitive or of igneous rocks, or their derivatives, and were deposited in beds as chemical stratiform deposits. In some cases they were sufficiently pure, as first precipitated, to be worked profitably, but in most cases they were seriously affected by undesirable mineral associates. When, however, such impure deposits are subjected for long periods to the percolation of waters from the surface under favorable conditions, the impurities are often dissolved and the ores concentrated. The great Bessemer ore-deposits of Lake Superior are examples. Originally impure carbonates or silicates, they have been converted into rich and phenomenally pure ferric oxides along certain lines of ground-water circulation, and in certain areas of free leaching. Van Hise has shown the definite relation between the water circulation and the production of the high-grade ores.[210] Vast quantities of unconcentrated lean ores lie in the tracts not thus purified and enriched by circulating waters. This does not appear to be simply residual concentration. The waters seem to have added ferric oxide brought from above, while they carried away the “impurities,” silica, carbon dioxide, etc. Perhaps this is an instance of mass action in which the ore present aided in causing additions to itself.
Concentration by solution and reprecipitation.—By a process almost the opposite of residual concentration, ore material is often leached out of the surface-rock by water circulating slowly through its pores, cleavage planes, and minute crevices, and is carried on with the circulation until it reaches some substance which causes a reaction that precipitates the ore material. This substance may be a constituent of some rock which the circulating water encounters, such as organic matter. More commonly, the precipitation seems to be due to the mingling of waters charged with different mineral substances, the mingling inducing reaction and the precipitation of the ore. Precipitation, however, does not necessarily follow such commingling. The junctions of underground waterways are sometimes characterized by barrenness instead of richness. In the expressive phraseology of the miners, a tributary current sometimes “makes” and sometimes “cuts out.” In chemical phrase, when the mingling waters reduce the solubility of the appropriate substance sufficiently, an ore-deposit is formed; when they increase its solubility, they promote barrenness. Changes of pressure and temperature may enter into the process, and mass action may lend its aid when once a deposit is started.
More concretely stated, the general process of underground ore formation appears to be this: the permeating waters dissolve the ore material disseminated through the rock and carry it thence into the main channels of circulation, usually the fissures, broken tracts, porous belts, or cavernous spaces. If precipitating conditions are found there, deposition takes place. The precipitating conditions may be merely changes of physical state, such as cooling or relief of pressure, but probably much more generally they consist in the commingling and mutual reaction of waters that have pursued different courses and become differently mineralized, as implied above. In these cases the metal-bearing current may be scarcely more important than the precipitating current.
Since the solvent action is a condition precedent to deposition, the location of the greatest solvent action first invites attention. At present it must be treated in general terms, for it is not known what solutions must be formed beyond the fact that they must include the ore material. Probably they must include much besides. Furthermore, it is not known that deep-seated rocks carry more ore material than similar rocks at or near the surface or at any other horizon. Fantastic conceptions of deep-seated metallic richness are to be shunned as quite beyond practical consideration. The water circulation is probably very slight below a depth of two or three miles at most, and above that depth there is little ground to suppose that the rocks of one horizon are inherently more metalliferous than others of their kind. There is no assignable reason why the igneous rocks at the surface are not as rich in ore material as the igneous rocks two or three miles below, since all are probably eruptive and of much the same nature on the whole, being in many cases parts of the same eruptions.
Location of greatest solvent action.—Solvent action is probably most intense where the temperature and pressure are highest, that is, in the deeper reaches of water circulation; but the amount of water passing in and out of the deeper zone is but a small fraction of that which courses through the upper horizons, and the total solvent action is quite certainly much greater in the upper zone than in the lower. At the same time the solutions in the upper zone are quite certainly more dilute than those below. The horizon of greatest solution lies between the surface and a level slightly below the ground-water surface, or, in other words, in the zone where atmosphere and hydrosphere coöperate. Surface-waters are charged with atmospheric and organic acids and other solvents, and their general effect upon the rocks is markedly solvent down to or often below the permanent water-level. In this zone concentration by residual accumulation may take place, as already noted, if the metallic compounds resist solution; otherwise this zone is depleted of its ore material by solution, and preparation is made for deposition elsewhere.