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Report on the Geology & Gold Fields of Otago

Manipori Formation

page 27

Manipori Formation.

Distribution.—This formation is found only on the west coast, from Milford Sound to Preservation Inlet, extending inland to the Hollyford River, the To Anau and Manipori Lakes, and to the upper part of the Waiau.

Rocks.—It is composed chiefly of syenetic gneiss, made up of white and yellow felspars with some quartz, and imperfectly foliated with hornblende and yellow or black mica. Sometimes the hornblende preponderates and occurs in layers or bunches, in which case mica is absent and the felspar is always white. Sometimes it passes into a dark grey gneiss, finely foliated with black mica, and at the head of Bligh Sound red gneiss occurs. At Milford Sound syenetic gneiss, containing numerous large crystals of red garnet, is found; and at Resolution Island and Bligh Sound there is a true garnet schist, composed almost entirely of granular garnets with layers of quartz. At Breaksea Sound granulite occurs composed of fine-grained white quartz with some felspar, and small flakes of black mica scattered through it in parallel planes. Hornblende schists foliated with small quantities of quartz, and hornblende rock, in which the quartz is absent, are common at Anita Bay and Milford Sound, and the latter also occurs at Doubtful Inlet. Marmolite, or serpentine schist, is found at Anita Bay, and quartz schist at Deas Cove in Thompson Sound. There is also in the museum a specimen of mica schist, composed of black and yellow mica in large scales, foliated with quartz, which was brought by Dr. Hector from Seal Island. Cipollino, a white granular limestone, with an internal coarsely crystalline structure, and with numerous flakes of pink lithia-mica scattered irregularly through it, is found in Thompson’s Sound, and the same rock, but with little or no mica, occurs at Anita Bay.

Position of strata.—At the entrance to Wet Jacket Cove, between Dusky and Breaksea Sounds, the rocks are vertical, and strike north and south. In Shoal Cove, at the head of Bradshaw Sound, the dip is N.W., at a high angle. In Thompson’s Sound, the dip is about 45° N.W., and in the south fiord of Te Anau Lake, 67° N.W. In Milford Sound, opposite Harrison’s Cove, the dip is 80° W.N.W., and at Anita Bay it is 75° W. by S.

Thickness.—It will be thus seen that the dip is remarkably uni-page 28form throughout, and the formation must bo of enormous thich ness. The distance from the entrance into Thompson’s Sound to Lake Te Annu is about 40 miles, and I observed the dip to be N.W. at both ends and in the middle of the section, and never at a less angle than 45°. This would give a thickness to the formation of at least 160,000 feet, which appears to be incredible, but we can only escape from’ this conclusion by supposing either that I happened to make all my observations on similar sides of synclinal curves, which is quite possible, or that the plane of foliation does not correspond with the original plane of bedding for which at present there is no evidence.

Age.—Although these rocks are separated from the other metamorphic rocks in the centre of the Province by a band of slates and sandstones, their more highly metamorphic character is sufficient evidence that they are older than the mica schists of the Dunstan (Wanaka formation), for I shall presently shew* that this extreme metamorphism cannot be accounted for by the presence of the granite at Preservation Inlet, and consequently we cannot be very far wrong if, for the present, we consider them as belonging either to the Laurentian or to the Cambrian period.

Eruptive Rocks.—A considerable number of dykes and veins of eruptive rocks occur in this formation, but it is uncertain at what period they were erupted, and some of them may be due to segregation. Vein granite, generally fine grained with white felspar, quartz, and black mica occurs at Milford Sound, and Seal Island; and Titiroa, a high mountain behind Manipori Lake appears to be composed of a dyke of white granite. Pegmatite, consisting of coarsely grained white felspar and quartz, with silvery white nica, occurs at Milford Sound. Mica Trap, composed almost entirely of black mica in coarse flakes with some layers of felspar, was brought by Dr. Hector from Petrel Island, in Dusky Bay, in 1863; as also was a chlorite porphyry, consisting of a white felspathie matrix, with yellow mica and green fissile chlorite, from Seal Island. Eurite, composed of white felspar and quartz, occurs at Wet Jacket Cove and Milford Sound; and Porphyrite at Milford Sound.

Minerals.—The Serpentine which occurs plentifully at Anita Bay, is pale green, translucent and with a more or less schistose structure. It is tolerably hard, but can be scratched with a knife. It is the inferior greenstone, or Tangiwai, of the Maoris, and the Marmolite of mineralogists. The true greenstone, or Pounamou of the Maoris,—the Nephrite or Jade of mineralogists—appears to occur only in veins through the marmolite, and

* See Maitai formation, contemporaneous eruptive rocks.

Specimens brought by Dr. Hector.

For further remarks on this point, see Section V, Eozoic and Palœozoie Eras.

page 29does not exist in large separate dykes. Nephrite may be distinguished from Marmolite by its hardness, it being impossible to scratch the former with a knife. Chemically, nephrite differs from marmolite only by the absence of water in its composition, and any number of different varieties may be found differing much in hardness, even in the same block of marmolite.

This formation is remarkably barren of metalliferous veins. No gold in anything like a paying quantity has been found in any alluvia derived from it, but Dr. Hector has reported silicate of copper from Milford and Bligh Sounds, and cobalt bloom from some other locality not named.*

Nomenclature.—I have given the name of Manipori formation to these rocks, as the Manipori Lake is situated entirely in them. It is the same as the Gneiss-granite formation of Dr. Hector.

* N. Z. Exhibition Jurors’ Reports and Awards, p. 265.

Quar, Jour. Geol. Soc. 1865, p. 128, ix. (k).