0·0011–0·008; zinc, 0·0048–0·009; silver, 0·00007–0·00016; gold, 0·00002–0·00004. Iron and aluminium seldom fail, and vary from 1 to 2% as a minimum, up to 25% as a maximum.
In order that the several metals may constitute ores, their percentages must be the following—the percentages of each vary with favourable or unfavourable conditions at the mine, and can therefore be expressed only in a general way; ores favourable to milling and concentration may go below these limits, and the mingling of two metals of which one facilitates the extraction of the other may also reduce the percentages:—
| Aluminium | 30 | Nickel | 2–5 |
| Copper | 2–10 | Platinum | 0·00005 |
| Gold | 0·003–·00016 | Silver | 0·03–0·16 |
| Iron | 35–65 | Tin | 1·5–3 |
| Lead | 2–25 | Zinc | 5–25 |
| Manganese | 40–50 |
Cobalt is a by-product in the metallurgy of nickel and is usually in much inferior amount to the latter.
When we compare the first and second tabulations with the third it is at once apparent that with the possible although only occasional exception of iron the production of an ore-body from the normal rocks which constitute the outer mass of the earth requires the local concentration of each of the metals by one or several geological processes, and to a degree that is only occasionally developed in the ordinary course of nature. It is, therefore, an instance of somewhat exceptionally good fortune when one is discovered, and it is only the part of ordinary prudence to develop and utilize it as one would treat a resource which is limited and subject to exhaustion.
The minerals which constitute ore-bodies are divided into two great classes: the ores proper, which contain the metals; and the barren minerals or gangue, which reduce the yield.Classes of Mineral.
The ores are generally and naturally subdivided into two groups: first, the sulphides and related compounds containing arsenic, antimony, tellurium and selenium; and, second, the oxidized compounds embracing oxides, carbonates, sulphates, silicates, phosphates, arsenates, chromates, &c. With the oxides are placed, because of related geological occurrence, a few rare compounds with chlorine, bromine and iodine into which silver more than any other metal enters, and to the same group we may add a few metals which occur in the native state. Iron, manganese, aluminium and tin differ from the rest of the metals in their original occurrence in the oxidized form, whereas the others with the exception of gold, platinum, and possibly copper, in their first precipitation in ore-bodies are in the form of sulphides or related compounds. Only by subsequent changes, characteristic of the upper parts of the deposits, do they pass by oxidation into the minerals of the second group.
With regard to the nature and source of the water which serves to gather up the widely disseminated metals and concentrate them in ore-bodies two contrasted views are now current, not necessarily antagonistic but applied in different degrees by different observers. The older view attributes the water primarily to the rainfall, and therefore it is called meteoric water. After falling upon the surface the meteoric water divides into three parts. The first, and smallest, evaporates; the second, the largest portion, joins the surface drainage and is called the run-off; while the third, intermediate in amount, sinks into the ground and mingles with the ground-waters. The ground-waters rise in springs, usually fed from no great depth, and themselves pass into the surface drainage after a small subterranean journey. While as a rule the ground-water level is fairly definite, yet it sometimes displays even in the same mining district great irregularity.
The section of active circulation and work of the descending meteoric waters between the surface and the ground-water level was called by Franz Posepny (1836–1895) the vadose or shallow region (“Genesis of Ore-deposits,” Trans. Amer. Inst. Min. Eng., xxiii., xxiv., 1893; reprinted as a book, 2nd ed., 1902). It has been long recognized by miners as the home of the oxidized ores, and the place of the work of the descending waters. The deep-waters are relatively motionless and their movements as far as visible are comparatively slow. But the really important feature of the ground-water as regards the filling of veins is the depth to which it extends. This remained a somewhat indefinite matter until L. M. Hoskins showed mathematically that cavities in the firmest rocks became impossibilities at about 10,000 metres. Down to some such limiting depth as an extreme the ground-water was believed by many to descend; to migrate laterally; to experience the normal increase of temperature with depth; the effect of pressure; the increased efficiency as a solvent peculiar to the conditions; and finally with a burden of dissolved gangue and ore to rise again, urged on by the “head” of the descending column. In its ascent it was supposed to fill the veins. Mining experience has, however, indicated that the known ground-waters are comparatively shallow and seldom extend lower than 500–600 metres. It is conceivable that during faulting and the formation of great dislocations this upper reservoir might be tapped into greater depths and set in limited circulations through deeper-seated rocks. But so far as these objections have weight they have greatly restricted the vertical range of the meteoric ground-waters as they were formerly believed to exist.
In contrast with the meteoric waters outlined above, other waters are believed by many geologists to be given off by the deep-seated intrusive rocks, and are generally called magmatic. We are led to this conclusion by observing the vast quantities of steam and minor associated vapours which are emitted by volcanoes; by the difficulty of accounting in any other way for the amount and composition of certain hot springs; and by the marked and characteristic association of almost all ore-deposits in the form of veins with eruptive rocks. That igneous masses have been connected with the formation of veins is further brought out by the following general consideration, which has hitherto received too little attention. Aside from pegmatites, veins rich enough to be mined and even large veins of the barren gangue-minerals are exceptional phenomena when we compare the regions containing them with the vast areas of the earth which have been carefully searched for them and which have failed to reveal them. As components of the earth's crust the useful metals except iron and aluminium are extremely rare. Some sharply localized, exceptional, and briefly operative cause must have brought the veins into being. The universal circulation of the ground-water of meteoric origin fails to meet this test, since if it is effective we ought at least to find veins of quartz and calcite fairly universal in older rocks. In North America, moreover, by far the greater number of veins which have been studied date from the Mesozoic and Tertiary times. The ore deposits of older date are chiefly of iron and manganese and can be satisfactorily explained in many cases by the reactions of the vadose region, or by crystallization from molten masses.
In summary it may be stated that the meteoric waters are of great importance and of unquestioned efficiency in the shallow vadose region, or, as named by C. R. van Hise, “the zone of weathering.” In it the disintegration of rocks exposes them to the searching action of solutions, and the portions of ore-bodies already deposited undergo great modifications. The deeper and far more immovable ground-water probably extends to but moderate depth and is chiefly affected as regards movement by the head of waters entering heights of land and by local intrusions of igneous rocks. It is very doubtful if the normal increase of temperature with depth produces much effect. The meteoric waters are of altogether predominant importance in all surface concentrations of a mechanical character. The magmatic waters, on the other hand, seem to be of paramount importance and of great efficiency in producing the deposits of ores in the contact zones next eruptives, and in the formation of veins which are reasonably to be attributed to uprising heated waters in regions of expiring vulcanism. They start with their burden of dissolved metals and minerals under great heat and pressure, amid conditions favouring solution, and migrate to the upper world into cooling and greatly contrasted conditions which favour precipitation. Undoubtedly they are responsible for many low-grade deposits
