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Volume 37, 1904
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Art. LIII—Magnesian Rocks at Milford Sound.

By Dr. P. Marshall.

[Read before the Otago Institute, 13th September, 1904.]

Plate XLVII.

The remarkably wide occurrence of magnesian rocks in New Zealand has often been remarked upon. The well-known Dun Mountain region was described by Hochstetter in 1865. In 1887 the reports of the Geological Survey of New Zealand contained a paper by Professor Park on the Big Bay district, where a very large mass of these rocks constitutes the Red Hill Range and some neighbouring portions of the coastal regions. This district was more fully described by the late Professor Ulrich in the Quarterly Journal of the Geological Society of 1890; when the occurrence of the nickel-iron alloy awaruite in these rocks was first mentioned.

A small occurrence of these rocks, hitherto only briefly referred to in Hutton's “Geology of Otago,” has long been known to exist at Milford Sound, but beyond the fact that bowenite was found there little more has been known. A recent visit to Anita Bay, at the entrance to Milford Sound, enabled the author to make a cursory examination of the district, and bring away specimens for microscopical and chemical investigation.

At the east end of the bay there is a large outcrop of gneissic granulite, much contorted. A small beach separates this from a mass of dunite about 150 ft. wide, striking apparently S. 80° W., and dipping 70° S. The dunite is perfectly

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fresh beyond a certain amount of reddish weathered crust. It contains grains of chromite and occasionally small crystals of diopside.

To the west of the dunite no rocks are to be seen in situ, but huge boulders of granulite and hornblende schist form the beach, and probably indicate that these rocks succeed the dunite in this direction. The granulite extends about a quarter of a mile, and thereafter the beach consists for 400 yards of a light greenish-grey serpentine, with occasional unaltered grains of olivine and some small veins of bowenite* running through it. The serpentine gives place further on to a beach of small pebbles, which is about 300 yards long. Another mass of boulders is then found, and then another small beach followed by granulite, much contorted, in situ.

Among the boulders between the two beaches just mentioned were several boulders of hartzbergite, in which the enstatite crystals stand out from the weathered surface. They are sometimes 6 in. wide by as much as 2 in. broad.

Another large boulder, weighing about 100 tons, was found still further west than the granulite outcrop just described.

So far as these geological indications afford evidence, it seems probable that there is a large intrusion of dunite, some part of which is changed to serpentine with veins of bowenite. The hartzbergite appears to form another intrusion of smaller size further west.

These rocks were examined, with the following results:—

Bowenite.

Hand-specimen clear transparent green, with a splintery fracture and small opaque inclusions. Microscopically a dense mesh of minute needles of colourless transparent serpentine, occasional small unaltered cores of olivine, and some grains of chromite.

Dunite. (Plate XLVII., fig. 1.)

Hand-specimen pale olive-green rock, very dense, but showing occasionally small crystals of chromite, some bright cleavage-surfaces of diopside, and rounded larger grains of olivine.

Section.—The large olivine grains have undulose extinction, and are imbedded in a mass of olivine grains with irregular-fractured boundaries evidently produced by crushing. The whole structure is distinctly cataclastic. The diopside is perfectly transparent, with good cleavage, and extinction angle over 40°. Chromite barely transparent, with a brown colour. No indication of serpentinisation of the olivine.

[Footnote] * This mineral is mined from an adit 1,500ft. above sea-level.

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Hartzbergite. (Plate XLVII., fig. 2.)

Hand-specimen pale-green rock with fine large bright cleavage surfaces of enstatite, and between them granular olivine. Scattered dark grains of iron-ore.

Section.—Enstatite in large perfectly colourless transparent plates showing good cleavage-traces. The plates include pœcilitic grains of olivine and iron-ore, and in some sections a great deal of a carbonate showing rhombohedral cleavage. As the analytical results give no calcium in the rock, this carbonate must be magnesite. Its birefringence is similar to that of calcite, giving even in thin sections colours and even white of the higher order. The iron-ore is metallic in reflected light, and absolutely opaque even in the thinnest section. No cleavage discernible. Attacked by HCl, so apparently magnetite. The alteration of enstatite into magnesite appears rather peculiar. All the rocks found as boulders have evidently rolled down in stream-channels and landslips from the hillsides that are thickly covered with forest. The change may be due to the fact that percolating waters are more than usually charged with organic acids derived from the mass of decaying vegetable matter that forms the surface soil.

A partial chemical analysis was made of the hartzbergite, with the following result:—

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

A. B. C.
SiO2 30.00 39.99 41.43
Al2O3 3.55 0.04
Fe2O3 5.76 2.52
Cr2O 2.30 0.76
FeO3 2.40 8.56 6.25
CrO 0.48
MgO 45.48 41.26 43.73
CaO 4.19 0.55
Loss 13.36 2.07 4.51
Total 99.79
A.

Hartzbergite, Milford Sound; anal., P. Marshall. B. Hartzbergite, Olivine Range, N.Z., Q.J.G.S., 1890. C. Hartzbergite, Oregon County, U.S.A.

In this analysis the high percentage of loss is evidently due to the presence of magnesite. The iron-oxides and chromium-oxides are at present only approximately estimated, for ordinary methods of separating these substances fail. The iron-ore has been separated by magnetic means, and the total quantity of iron-oxides and chromium-oxides estimated. The ore completely dissolves in HCl. This must indicate that

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some chromous oxide at least is present, but I have not yet satisfactorily estimated what quantity of it is present.

The analysis of the other hartzbergites given above shows that this rock, except for the magnesite, is a very pure sample. Its composition seems to be,—

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Magnesite 28.12 13.36 CO2
14.76 MgO
Olivine 36.42 16.37 SiO2
20.05 MgO
Enstatite 24.30 14.85 SiO2
9.45 MgO
Magnetite (chromous) 10.09
—
Total 98.83

The present occurrence is evidently closely connected with that at Red Hill, whence the rock (analysis B) came, and dunite has also been reported from the same district, as well as serpentine and bowenite. The relations of these rocks to surrounding deposits, as well as their age, still remain a problem, though the Geological Survey has referred them with some confidence to the Devonian period. The high specific gravity—3.2 in this case, and 3.35 in some of the Red Hill rocks—shows that these rocks are properly resident at an enormous depth from the surface, so some specially potent tectonic disturbance, whose nature will, we hope, be afterwards unravelled by New Zealand geologists, must have been necessary to bring these rocks to the earth's surface. It is perhaps of interest to add that no trace of awaruite was found in the rocks or sands of the Milford Sound magnesian rock district, and only a faint trace of Ni could be obtained chemically.

Picture icon

Section one mile and a half long of Anita Bay, south side of Milford Sound, near entrance.

Explanation of Plate XLVII.
Fig. 1.

Dunite. Shows cataclastic structure of rock. A large crystal of chromite is in the centre of the irregularly shaped broken fragment of olivine crystals.
Fig. 2.

Hartzbergite. A large plate of enstatite encloses pieces of iron-ore and rounded grains of olivine showing a distinct pœcilitic structure. No magnesite is shown in the photograph.
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