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] † 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.