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.