which have later been enriched by the action of descending meteoric waters. They are more copiously yielded, so far as we may judge, by acidic magmas than by basic ones.
The natural waterways are furnished by the cavities in rocks. They vary in size from very minute pores, where movement is slow because of friction, but where solution takes place, through others of all dimensions up to great fault-zones. The smallest cavities are the natural pores of minerals; cleavage cracks; the voids along the contacts of different minerals; cracks from crushing during dislocation; cellular lavas; volcanic necks; voids among the grains, pebbles, or boulders of fragmental rocks; joints; caves, and faults. So far as waters have deposited ores and yielded ore-bodies by subterranean circulations the latter are guided by some such controlling influence as these in all cases, and they will be selected as the governing principle in a large part of the scheme of classification. The types will be reviewed in the following order:—
| I.—Of Igneous Origin. | |
| A. | Eruptive masses of non-metalliferous rock. |
| B. | Basic segregations from fused and cooling magmas. |
| C. | Deposits produced in contact metamorphism, most commonly by the action of intrusive masses on limestones.
|
| D. | Pegmatites. |
II.—Precipitated from Solution. | |
| A. | Surface deposits. |
| B. | Impregnations in naturally open-textured rocks. |
| C. | Impregnations and replacements of naturally soluble rocks. |
| D. | Deposits along broken anticlinal summits and in synclinal troughs.
|
| E. | Deposits in shear zones. |
| F. | Deposits in faults. |
| G. | Deposits in volcanic necks. |
| III.—Deposited from Suspension. | |
| A. | Placers. |
| B. | Residual deposits. |
| IV.—Carbonaceous Deposits from Vegetation. | |
I. Of Igneous Origin.—A. Eruptive Masses of Non-metalliferous Rock.—Among the non-metallic objects of mining and quarrying which are of igneous nature, building stone is the chief. Granites, syenites, and other light-coloured rocks are the most important. These rocks occur as intrusive masses called bosses when of limited extent and diameter, and bathyliths when of vast, irregular area. The main point of importance is the jointing and cleavage, which should in each case yield blocks as nearly rectangular as possible so as to save tool treatment. Dark, basic igneous rocks in dikes, sills and surface flows are employed for macadam, and are often of excellent quality for this purpose
B. Basic Segregations from Fused and Cooling Magmas.—A few ore-bodies, of which the best-known involve iron, are believed to result directly in the igneous processes by which molten rock cools and crystallizes. Thus magnetite, one of the common iron ores, is a widely distributed component in the eruptive rocks, rarely if ever failing in any variety. It is one of the first minerals to crystallize, and it possesses a much higher specific gravity than the other constituents. There is reason, therefore, to believe that, forming in some molten magmas in relatively large quantity, it sinks to or toward the bottom of the mass until the latter is at least greatly enriched with it, if not actually changed to iron ore. If the molten rock, after passing through a stage of partial crystallization, moves toward the surface of the earth, the body of ore may occupy almost any position in it other than the bottom. The flowing of the magma in original movements or from pressure sustained in subsequent metamorphic processes, or both, may give the ore the lenticular shape which is quite characteristic of magnetite bodies the world over. Almost all iron ores of recognized eruptive origin contain titanium oxide in amounts from a few units to over 40%. They are most frequently found in dark basic rocks. These ores are not at present of much commercial value because of the difficulties of treating titaniferous varieties in the modern blast furnace practice, but, there is little doubt that in the near future they will be extensively mined.
Non-titaniferous magnetites, which often form lenses in gneissoid rocks of more acidic character than those with which the titaniferous are associated, are likewise believed by some observers to be of igneous origin, but there are equally positive believers in sedimentary deposition followed by metamorphism.
Besides magnetite, chromite is a characteristically igneous mineral and is always found in the richly magnesian rocks. Whether the relatively large masses which appear in serpentine are direct crystallizations from fusion, or whether they have segregated from a finely disseminated condition during the change of the original eruptive to serpentine, is a matter of dispute, but the general trend of later opinion is toward an original igneous origin. Although not strictly an ore, corundum is another mineral which is the direct product of igneous action.
A form of ore-body which marks a connecting and transitional member between those just treated and those of the next group is furnished by the sulphides of iron, nickel and copper which are found in the outer borders of basic igneous intrusions. Observers differ somewhat as to the relative importance to be attributed to reactions purely of the nature of crystallization from fusion or those brought about by the agency of gases or other highly heated solvents in the cooling stages. The most important example is afforded by the mingled ores of nickel and copper which are developed in their largest form in the region of Sudbury, Ontario, Canada, and are now the principal source of nickel for the world.[1] The ores are chalcopyrite and pyrrhotite, the latter containing throughout its mass at Sudbury the mineral pentlandite, a rich nickel-iron sulphide and the real source of the nickel. With the base metal there are also found minute traces of the metals of the platinum group. Wherever these ore-bodies have been observed they invariably occur in the borders of intrusive masses. The sulphides constitute an integral part of the rocky mass, which shows almost no signs of alteration or vein production in the ordinary sense. Only some slight rearrangements have subsequently taken place through the agency of water, but all this is a small factor in the total.
C. Ore-Bodies produced by Contact Metamorphism.—Great bodies of igneous rock have often been forced in a molten and highly heated condition through other rocks when at a distance below the surface of the earth. After coming to rest they have remained during the cooling stages for long periods in contact with the surrounding walls. All molten igneous magmas are more or less richly charged with aqueous vapour, doubtless in a dissociated state; with carbonic acid and probably with other gases, especially those involving sulphur. During the cooling stages the gases are emitted and carry with them silica, iron, alumina and metallic elements in less amount, of which copper is the commonest, but among which are also numbered lead, zinc, gold and silver. If the rock standing next the intrusive mass is limestone, the silica and iron, and to a less degree the alumina, combine with the lime to the elimination of the carbonic acid and produce extensive zones of lime silicates, of which garnet is the most abundant. Disseminated throughout these garnet-zones are large and small masses of pyrite and chalcopyrite, oftentimes in amounts sufficient to yield large ore-bodies. Again in the limestone outside the garnet-zones, but none the less closely associated with them, are bodies of sulphides containing copper. The copper ores of Bisbee and Morenci, Arizona, of Aranzazu near Concepcion del Oro, Mexico, and of many other parts of the world not yet studied in detail are of this type. The eruptive which most frequently produces contact zones is of a marked acidic or siliceous character, since among eruptives these are the ones most richly charged with gases. When the copper ores are of low-grade in their original deposition it often happens that processes of secondary enrichment, which are later described, are required to bring them up to a richness which warrants mining. Less often than copper appear lead, zinc or gold ores in the same relations.
D. Pegmatites.—One other phase of eruptive activity needs also to be briefly mentioned before passing to the discussion of the ore-bodies, which have hitherto chiefly occupied students of the subject. In the regions surrounding intrusive masses of granite we almost always see dikes or veins of coarsely crystalline quartz, felspar and mica radiating outward, it may be, for very long distances. They are believed to be produced by emissions from the eruptive similar to those which yield the garnet-zones just mentioned. The veins are technically called pegmatites. They are characteristic carriers of tin and of minerals containing the rare earths, and less commonly are known to yield gold or copper.
II. Precipitated from Solution.—A. Surface Deposits.—The chief ore-body under this type is furnished by iron. The peculiar chemical property possessed by this metal of having two oxides, a ferrous, which is relatively soluble, and a ferric, which is insoluble, leads to its frequent precipitation from bodies of standing or comparatively quiet waters. Ferruginous minerals of all sorts, but more particularly pyrite and siderite, pass into solution in the descending oxidizing or carbonated surface waters, either as ferrous sulphate, or as salts of organic acids, or ferrous carbonate, the last-named dissolved in an excess of carbonic acid. On being exposed to the atmosphere when the solutions come to rest, or to the breaking up of organic acids, or to alkaline reagents, or sometimes to fresh-water algae, the hydrated sesquioxide 2Fe2O3, 3H2O is precipitated as the familiar beds of bog ore. The ore usually forms earthy aggregates or crusts and cakes, but may also, as in the interesting case of the Swedish lake deposits, yield small concretions. Bog ores are not very rich in iron and are apt to have much sand and clay intermingled. If subsequently buried under later sediments they may become dehydrated and changed to red hematite, as in the case of some of the Clinton iron ores of the eastern United States. These widely extended beds in the lower strata of the Upper Silurian are often oolitic red hematites, consisting of concentric shells of iron oxide and
- ↑ A. H. Barlow, “On the Sudbury Deposits,” Geol. Survey of Canada Ann. Rept., vol. xiv., part H; A. P. Coleman, Ann. Report of the Ontario Bureau of Mines, vol. xiv., part iii. (1905).
