Page:EB1911 - Volume 18.djvu/531

 chalcedonic silica, deposited around grains of sand. The most extensive of all ore-beds of this type and the mainstay of the German and Belgian smelting industry, are the Jurassic ores, locally called minette, of Luxemburg and the neighbouring territories. Three principal and several subordinate beds are distinguished, which furnish a product ranging from 30 to 40% of iron and between 1 and 2% of phosphoric oxide (P2O5). They are generally believed to have been deposited on the bottoms of embayments of the Jurassic sea. The iron was furnished by the drainage of the land and was precipitated, according to Van Werweke, as silicate, carbonate, sulphide and as several forms of oxide. More than two billions of tons are believed to be available. Very similar deposits occur in the Cleveland district, England, in the Middle Lias.

In the presence of much organic matter which creates reducing conditions, concretions and even beds of spathic ore or black-band may result and afford the ores of this type extensively utilized in the Scottish iron industry and formerly of some importance in the eastern United States.

The brown hematites often have more or less manganese, and manganese ores themselves may result by closely related reactions, since manganese is very similar to iron in its chemical properties. Aluminium is yielded by deposits of bauxite, the hydrated oxide, which in the states of Georgia and Alabama, of the United States, are the result of surface precipitations. In the depths it is believed that pyritous shales exist. The oxidation of the pyrite supplies sulphuric acid which takes into solution the alumina of the shales. Rising to the surface along a marked series of faults, the aluminium sulphate meets calcium carbonate in an overlying limestone, and the aluminium hydrate is precipitated as concretions at the vents of the springs.

Of scientific importance but as yet not of commercial value are the siliceous sinters deposited around the vents of hot springs which yield appreciable amounts of both the precious and the base metals. While surface precipitations in every particular, they are yet chiefly important in casting light on the processes of vein formation in the depths.

Non-metallic minerals which are deposited from solution on the surface of the earth are the salines, rock-salt, related potassium salts, gypsum and the rarer nitrates. The alkaline chlorides and gypsum are derived, in nearly all cases, from impounded bodies of sea-water, which, exposed to evaporation with or without constant renewal, finally yield beds of rock-salt and related minerals. Shallow estuaries cut off from the sea, it may be by the sudden rising of a bar during a heavy storm or brines impounded in deep bays with a shallow connexion as in the “bar theory” of Ochsenius, have given rise to the great stores of these minerals which are so extensively mined. The potassium compounds have only been found as yet in large quantities in the Stassfurt region of Germany, and seem to be due to the fact that in this locality the mother-liquors of the rock-salt deposits failed to escape, and were evaporated to dryness. The nitrates are chiefly obtained in northern Chile and are the result of the reaction of nitrogenous organic matter, upon alkaline minerals and under conditions where there is enough but not too much water.

Another very important mineral found in surface deposits formed from solution is asphalt. It has happened in various parts of the world, but especially in the island of Trinidad, in the Carribean Sea, that petroleum with an asphalt base has reached the surface, has evaporated, and has become oxidized so as to leave a residuum of asphalt suitable for street-paving or other purposes. So-called pitch-lakes are afforded which may be of great commercial value.

Again, if large sheets, crusts, stalactites and stalagmites are deposited from calcareous water by the escape of the solvent carbonic acid, beautiful ornamental stones are afforded, generally known as Mexican onyx.

B. Impregnations in Open-textured Rocks.—In a number of instances in various parts of the world naturally open-textured rocks have been discovered so richly impregnated with the metalliferous minerals as to be ores. The enriching minerals have been introduced in solution, and the solvent has found its way through the rock because of its natural character, and not because geological movements have opened it. Porous sandstones are one of the most common cases. Deposits of silver ores have been extensively mined at St George in southern Utah, consisting of films of argentite and cerargyrite, which have been precipitated upon fossil leaves, sticks, and in the sandstone itself. Over wide areas in the northern United States, copper in various minerals has been discovered in sandstones of Permian or Triassic age. At Silver Cliff, Colorado, silver ores have impregnated a volcanic tuff, while at the Boleo mines in Lower California tuffs yield copper ores. In at least two of the great copper mines on Lake Superior the native metal impregnates a conglomerate, and in a number of others it has enriched a cellular basalt, filling the blow-holes with shots and pellets. In the Commern district between Bonn and Aachen, sandstones of the Triassic Buntersandstein contain knots of galena, distributed over wide areas as impregnations. Organic matter is believed to have precipitated the galena by a reducing action upon percolating solutions of lead.

All these porous rocks have been fed by solutions which have entered along waterways, clearly due to faults or some extensive

breaks which have provided introductory conduits. The solutions have then been tapped off from the main passages by the porous rock. They are, therefore, closely connected with faults.

Non-metallic minerals in the form of petroleum and asphalt may also impregnate sedimentary beds or other rocks of open texture. Many oil wells derive their supplies from lenticular beds of sandstone in the midst of impervious shales, and others, as those in the Mexican fields near Tampico, from volcanic tuffs. Asphalt may saturate both sandstones and limestones in such richness as to furnish a natural paving material when crushed, heated and laid. Brines are also yielded by porous strata and supply much of the salt of the world.

C. Impregnations and Replacements of Naturally Soluble Rocks.—Ore-deposits of great importance appear in different regions which can only be interpreted as having been formed by the replacement of some or all of a rock with the metallic minerals. The most common rock to yield in this way is limestone, because of its soluble nature, but important cases occur of others composed of silicates. Replacement implies the precipitation of the ore and gangue, molecule by molecule, in the position of the original minerals but without, as in pseudomorphs, the necessary reproduction of crystalline forms. Some waterway must of course introduce the ore-bearing solutions, but it may be slight compared with the great size of the resulting ore-bodies. Lead and zinc ores, often carrying some silver, are those most widely distributed, as they were also the earliest recognized in deposits of this character. More than any other metals their association with limestone is pronounced. The replacements may be found near the supply fissure as in the great zinc deposits near Aachen, or the supply fissures may be obscure as at Leadville, Colorado. While ores occur in the limestone, they are often close along its contact with some relatively impervious stratum, which seems partly to have directed the circulations, partly to have checked or stagnated them, while precipitation took place. With the lead and zinc sulphides, pyrites and chalcopyrite are commonly associated in greater or less degree, the copper increasing locally. All the sulphides are exposed to oxidation above the ground-waters and mining in the upper levels has been often directed against the carbonate and sulphate of lead, or the mingled carbonate and hydrated silicate of zinc.

A non-metallic deposit formed by replacement and of much scientific interest is furnished by sulphur when derived from gypsum, as in the Sicilian and other localities of Europe.

D. Deposits along Anticlinal Summits and in Synclinal Troughs.—When strata experience folding they are violently strained at the bends, and, if stiff or brittle like limestone, often crack in limited fissures, which in anticlines open upward and in synclines downward. They thus yield joints in relatively great numbers. Softer rocks, such as shales, are moulded by the strains without fracturing. Very gentle folds seem to have yielded such abundance of cracks in the lead and zinc district of the Upper Mississippi Valley as to cause the so-called “gash veins” which have been worked for many years. The crevices are not all vertical, but often run horizontally and are due to the parting and buckling of individual beds. The resulting ore-bodies are chiefly limited to a single great stratum, and are believed to have been formed by the infiltration of galena, blende and pyrite from overlying formations.

When strata are stiff enough to buckle under violent folding and part so as to produce openings of a crescentic cross-section which afterwards become filled, there result the “saddle-reefs” so remarkably illustrated in the gold veins of Victoria, Australia, and in pitching anticlines of a much larger character in Nova Scotia.

Of far the greatest importance of all the ore-bodies in troughs are the iron ores of the Lake Superior region, now the most productive of all the iron-mining districts. In a series of sedimentary formations, generally of Huronian age, and with associated eruptives, there occur strata consisting of a cherty iron carbonate, which were probably originally marine deposits akin to glauconite. They rest upon relatively impervious rocks, and are often penetrated by basaltic dikes. The entire series has been folded, so that the cherty carbonates, shattered by the strains, have come to rest in troughs of relatively tight, impervious rocks. The descending surface waters have next altered them, have taken the iron into solution, and have redeposited it in the troughs as a slightly hydrated red hematite. The silica has usually been precipitated elsewhere.

The most important of the non-metallic which occur along anticlinal summits are petroleum and natural gas, but it is true only in a very limited sense that they are introduced in solution. The general cause of the accumulation is, however, the same as that of the metallic minerals, i.e. that storage cavities are afforded. In the most productive oil-fields it is the general experience to find the oil and gas impounded in porous rocks, either sandstones or limestones, at the crests of anticlines and beneath impervious shales which do not shatter or crack with gentle folding.

E. Deposits in Shear-Zones.—It sometimes happens both in massive rocks and in sediments that strains of compression have been eased by local crushing along comparatively narrow belts without appreciable or measurable displacement of the sides such as would be required by a pronounced fault. The word shear-zone has become quite widely used in recent years as a descriptive term applicable to these cases.