Art. II.—Magmatic Segregation in its Relation to the Genesis of certain Ore-bodies.
[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.
Chromite in Peridotite.
In the South Island of New Zealand there are two mountain-masses of peridotite in which the magmatic segregation of chromite is exhibited on a scale of unusual magnitude.
A few miles from the City of Nelson, Dun Mountain rises to a height of over 4,000ft. above sea-level. It covers an area of about four square miles, and is entirely composed of massive olivine, in which chromite of iron is fairly uniformly disseminated in the form of fine grains, but is occasionally aggregated in large masses.† The adjacent rocks are slaty shales and limestone of Jurassic age, the limestone occurring at the base of the sedimentary formation. Between the limestone and the olivine, to which Hochstetter† gave the distinctive name “dunite,” there is a belt of serpentine, half a mile wide. The serpentine contains lenticular-shaped masses of chromite, native copper and copper-ores, principally chalcopyrite, with the usual products of oxidation. It also contains thin irregular veins of diallage, hypersthene, bronzite, enstatite, scapolite, wollastonite, and chrysolite. The larger deposits of chromite occur near the borders of the olivine and serpentine.
The second great mass of peridotite forms Red Mountain, situated twenty miles north of Milford Sound, in Otago.§ It
[Footnote] * J. Park, “On the Cause of Border Segregation in some Igneous Magmas,” Trans. N.Z. Inst., vol. xxxvii, 1905.
[Footnote] † S. H. Cox, “Chrome-deposits of Nelson,” New Zealand Geol. Reports and Explorations, 1881, p.8.
[Footnote] † Dr. F. von Hochstetter, Zeitschrift der Deutschen Geol. Gessellschaft, vol. xvi, p. 341.
[Footnote] § J. Park, N.Z. Geol. Reports and Explorations, 1886–87, p. 121.
rises to a height of over 6,000ft., and covers an area of about ten square miles. The mountain is composed of massive olivine containing disseminated chromite. The latter occurs in much greater proportion than at Dun Mountain. The peridotite is flanked on two sides by belts of serpentine, which separate it from the adjacent slates and sandstones of supposed Palæozoic age. Near the contact with the sedimentary rocks it is often so highly charged with chromite as to from compact bodies of ore. No deposits of chromite are known in the serpentine, but they may possibly exist, as the country is still practically unexplored.
The sands in the streams which drain the Red Mountain serpentine area yield small quantities of the rare nickel-iron alloy awaruite, discovered by Skey in 1885,* and afterwards found in situ in the serpentine.†
Since the discovery of awaruite nickel-ore alloys have been found in several places, most notably in gold-bearing sands associated with chromite in Elvo River, Biella, Piedmont, Italy; in sands derived from serpentine in Josephine County, Oregon; in the Fraser River, British Columbia, associated with chromite; and in Smith River, Del Norte County, California.
The association of copper and chromite in the serpentines at Dun Mountain has already been mentioned. Native copper is found in serpentine in Cornwall, New South Wales, New Caledonia, and other parts of the world.
Large masses of native copper associated with silver are found in amygdaloidal diabase at Lake Superior.
In 1879 Professor S. H. Cox‡ discovered in the Manukau district a number of dykes of andesite which near their borders were found to contain small scattered grains of native copper. The dykes are intruded in volcanic breccias of probably younger Miocene age.
Platinum has only been found in a few cases in the matrix in situ. In the Ural Mountains it occurs as grains in peridotite, serpentine, and olivine-gabbro. The bed-rock of the
[Footnote] * W. Skey, Trans. N.Z. Inst. vol. xxiii, 1885, p. 401.
[Footnote] † G. H. Ulrich, “On the Discovery, Mode of Occurrence, and Distribution of the Nickel-iron Alloy Awaruite on the West Coast of the South Island of New Zealand,” Quart. Jour. Geol. Soc. London, vol. xlvi, p.619
[Footnote] ‡ S. H. Cox, “Geology of Cape Rodney,” N.Z. Geol. Reports and Explorations, 1879–80, p. 27.
Vyzaj and Kaiva Rivers, on the western flanks of the Urals, consists of olivine-gabbro containing disseminated grains of platinum, but not apparently in payable quantities. An olivine rock was discovered in 1893 at Goroblago-datsk, on the western side of the Urals, containing chromite and platinum, the latter at the rate of 14 dwt. 9 gr. to the ton of rock.
Since the discovery of platinum in the nickel-copper sulphide ore at Sudbury, in Canada, careful analysis has disclosed the presence of the metal in minute quantity in many sulphide ores throughout the world. But in this and all cases where platinum occurs in sulphide-beds or in veins, its occurrence is probably not the result of direct magmatic segregation.
The importance of the role played by igneous rocks in the formation of ore-deposits has been specially urged in late years by Professor Vogt,* of Christiania; Professor Kemp,† of New York; Professor Suess,‡ of Vienna; and more recently by Waldemar Lindgren§ and W. H. Weed,¶ of the United States Geological Staff.
Vogt directs renewed attention to the close relationship existing between ore-deposits and eruptive processes. Ore-deposits which are generally connected with eruptive magmas are grouped by him into two principal classes, as under:—
Ore-deposits formed by magmatic segregation.
re-deposits formed by eruptive after-actions.
Ore-deposits belonging to the first group are infrequent, and therefore economically subordinate in importance to those of the second group. They include, according to Vogt,—
The occurrences of titanic-iron ores in basic and semi-basic eruptives;
Chromite in peridotite;
Sulphide deposits, including the nickeliferous pyrrhotite of Sudbury, in Canada;
Platinum metals in highly basic eruptive rocks;
Copper and metallic nickel-iron in serpentinised peridotite.
[Footnote] * Prof. J. H. L. Vogt, “Problems in the Origin of Ore-deposits,” “The Genesis of Ore-deposits,” 1901,p. 636.
[Footnote] † J. F. Kemp, “The Rôle of the Igneous Rocks in the Formation of Veins,” loc. cit., p. 681; also Trans. Amer. Inst. M.E., vol. xxxix, 1902, p. 681.
[Footnote] ‡ Prof. Edward Suess, Lecture, “Royal Geographical Journal,” vol. xx, 1902, p. 520.
[Footnote] § Waldemar Lindgren, “Character and Genesis of certain Contact Deposits,” “Genesis of Ore-deposits,” 1901, p. 716.
[Footnote] ¶ W. H. Weed, “Ore-deposits near Igneous Contacts,” Trans. Amer. Inst. M.E., vol. xxxiii, 1903.
That sulphides can be segregated from eruptive magmas in the first concentration has yet to be proved; and it is still doubtful how far Vogt's conclusions respecting the occurrence of sulphide ore as products of primary segregation from molten magmas are admissible.