[Read before the Otago Institute, 13th September, 1904]
Ore-Deposits are of diverse form and composition. They are found as true veins, as detached masses, and as members of a sedimentary formation. It is now known that their mode of occurrence, and, to some extent, their composition and form, are determined by the prevailing geological conditions.
In the past decade a vast mass of facts has been added to the literature of the subject, particularly in America, where the magnitude of the operations connected with mining has afforded great facilities for observation and research.
The genesis of ore-deposits presents many difficult problems, and naturally the literature of the subject is rich in theoretical deductions. The introduction of petrographical methods of investigation, and the demonstration of the principle of metasomatic replacement, marked a new point of departure, and led to a truer conception of the formation of ore-deposits than had formerly existed.
In this investigation we must remember that existing conditions are but a reflection of the past. The agencies that built up the crust of the earth in its present form are still in operation, and still governed by the same natural laws. We are living on the edge of a geologic epoch, and if we would rightly understand the past we must study the present. The occurrence of ore-deposits is merely a geologic happening—an incident in the tectonic arrangement of the materials forming the outer shell of the globe. Recent petrographical investigation has shown that ore-deposits are always more or less intimately connected with igneous rocks. This constant association naturally leads to the broad generalisation that
mineral-deposits are genetically connected with the intrusion or eruption of igneous magmas.
It has been shown by Professor Sandberger and others that igneous rocks contain all the constituents of mineral veins. Professor Vogt, of Christiania, maintains that the belief in a deep-seated inaccessible repository of the heavy metals can no longer be sustained.* Modern geologists, he points out, have abandoned the old conception which supposed that the interior of the earth was an enormously compressed liquid molten mass of high specific gravity charged with heavy metals. The composition of the molten magmas that have issued at the surface in successive geological ages does not favour any hypothesis which assumes the existence of a greater proportion of the heavy metals in the barysphere than in the upper crust, or lithosphere. Referring to the distribution of the elements in the earth's crust, Vogt states that of the entire earth-crust—including the rocks, sea, and atmosphere—oxygen constitutes by weight about one-half, and silicon about one-quarter.† The proportion of the other elements are, he says, as follows:—
|Alumina, iron, calcium, magnesium,||Per Cent.|
|sodium, and potassium||10 to 1|
|Hydrogen, titanium, carbon, and chlorine||1 to 0·1|
|Phosphorus, manganese, sulphur, barium, fluorine, nitrogen, zirocnium, and strontium||0·1 to 0·01|
|Nickel, lithium, vanadium, bromine, and perhaps beryllium and boron||0·01 to 0·001|
|Cobalt, argon, iodine, rubidium, tin, cerium, yttrium, possibly arsenic and others||0·001 to 0·0001|
In igneous magmas deficient in acid-forming constituents the heavy metals will segregate as oxides during the process of cooling, assuming the form of individual crystals, grains, or irregular aggregates in small and great masses.
The petrographical researches of Vogt and Brogger disclosed in basic dykes a tendency of the heavy minerals to segregate near the borders. The occurrence of massive mineral aggregates near their borders is a marked characteristic of peridotites and serpentines in all parts of the globe.
The most typical examples of magmatic border segregation are found in peridotite and its serpentinised forms. At pre-
[Footnote] * Professor J. H. L. Vogt, “Problems in the Origin of Ore-deposits,” “Genesis of Ore-deposits,” 1901, p. 637. (Published by American Institute of Mining Engineers.)
[Footnote] † Loc. cit., p. 639.
sent the laws governing magmatic differentiation are but imperfectly understood. By some border segregation is ascribed to molecular flow due to differences of temperature in the magma; by others to convection currents, which it is believed would tend to carry the first crop of minerals, such as magnetite, olivine, &c., to the borders of the igneous magma.
The writer* is inclined to ascribe border segregation to the difference of osmotic pressure that must exist in a finite mass of magma cooling more rapidly in the borders than in the central portion.
The valuable ores that may be considered primary constituents of eruptive rocks, resulting from direct differentiation in the cooling magma, are as follows:—
Chromite in peridotite and serpentine.
Copper and nickel-iron in serpentine.
Platinum metals in highly basic eruptives.
Magnetite and titanite in basic and semibasic eruptives.
[Footnote] * J. Park, “On the Cause of Border Segregation in some Igneous Magmas,” Trans. N.Z. Inst., vol. xxxvii, 1905.