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Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

Chapter V

page 256

Chapter V.

Westland Formation.

Before treating of the next, or Waihao formation, overlying the crystalline metamorphic (gneiss granite) formation, I wish to refer to that assemblage of older sedimentary and metamorphic beds lying west of the latter, and being generally in close proximity to the granite axis previously described. In order to avoid confusion, I have given it a distinct tint in the Geological Map, and placed it next to the gneiss granite formation in the table of reference. I am well aware, as already pointed out, that it consists of beds of more than one age, but owing to a want of more intimate knowledge of them, and having only had an opportunity of examining them in very few spots, distant from each other, it has been impossible for me, at present, to separate them.

Extent.

The boundaries in the Geological Map are, in many instances, only approximate, as I was unable for the greatest part to trace them myself. However, having had the advantage of obtaining a great deal of valuable information from Mr. Gerh. Mueller, the Chief Surveyor of Westland, and from several other friends in that province, upon whom I could rely, I believe that in a general way, the extent of this formation which promises to become, at no distant date, a large field for mining operations, will be found pretty correct. The Westland formation begins between the lower course of the Hokitika and Mikonui rivers, and stretches as far south as the Waikukupa, skirting the western foot of the Southern Alps. Small outliers occur between the latter river and the Mahitahi, after which it again attains large dimensions, page 257covering a great portion of the country between the Paringa and Waits rivers. From here to the northern banks of the Arawata river, it appears to exist only as isolated hills. I have coloured the slate formation on the south bank of the last-mentioned river as belonging to another formation, next to be considered. It consists of an alternation of silky clay slate and bluish felstones striking south-west and north-east, and having a dip of 70 to 80 degrees to the north-west. It is very possible that these beds may also belong to the Westland formation, the more so as I have since been informed that rocks belonging to the gneiss granite formation have been discovered south of the Arawata river.

Texture of Rocks and Position of Strata.

The strata under consideration, judging from their lithological character, can however be divided into two main divisions, of which the first consists of gneiss and mica schists, and other beds of a similar highly metamorphic structure; and the other, of silky and mostly light-coloured slates, alternating with fine-grained sandstones and felstones. The former is well developed near Lake Hall, south of the Paringa river; and the latter in Mount Greenland on the northern banks of the Mikonui. The rocks near Lake Hall consist of finegrained gneiss and mica schists with veins and dykes of granite, the latter often coarse grained—dipping at a high angle to the west.

Phyllites, altering to gneissic schists, also occur in this district. The principal locality in the other zone—a zone which I examined carefully, and which seems to cover the greatest extent of ground, but, by many gradations, appears to pass into the first—is situated on the northern banks of the Mikonui river. Bluish clay slates, with occasional beds of fine-grained sandstone, are here exposed in many spots. In Redman's Grully, they dip 60 degrees to the north-by-east. In some other places higher up the river, they are more inclined, and dip'to the north-east. They are full of quartz veins, mostly of small dimensions; but none of them, as far as I am aware, have hitherto been found of sufficiently auriferous nature to be worth working. Some of the most interesting localities in that neighbourhood, as, for instance, Mount Rangitoto on the southern banks of the Mikonui, are unknown to me, but the Canterbury Museum possesses a large series of specimens, collected by many intelligent friends of that Institution, page 258since my last journey in 1868 to the West Coast, from which it appears that the silky clay slates are altered in many localities by the intrusion of granites and syenites into micaceous and gneissic schist, the change from the one to the other being gradual and very instructive, as may be seen from a series of specimens.

It thus appears that there are at least two series of beds included in my Westland formation. Owing, however, to the short time at my disposal when at the West Coast, I was unable to separate these. One is probably of the same age as my gneiss granite formation, forming the western base of the Southern Alps, and the other of much later origin, the slates and sandstones belonging to it having, in their turn, become changed into metamorphic schists in many localities, by plutonic action.

Igneous Rocks.

The Canterbury Museum possesses a series of igneous rocks collected in this zone, which are very varied, ranging from a coarsegrained granite to a very fine-grained petrosilex. In the Geological Map, I have indicated the occurrence of granite in the isolated Mosquito Hill on the northern banks of the Haast river, but Mr. G. Mueller informs me that, according to his observations, the whole hill consists of blue clay slates with a capping of alluvium on the very summit. In 1863, when passing along the northern banks of this river, which, for some distance from the western base of the Southern Alps, had consisted of alluvium, I came, opposite the Mosquito Hill where I camped, upon a number of very large blocks of granite, mixed with some smaller fragments of clay-slate, showing convincingly, from their sharp edges and the manner of deposition, that they had not travelled any distance. Some of the blocks consisted of a very fine white hornblende granite with black and silvery mica, the latter in large concretions, and the hornblende crystals, also large and needle-shaped, which, from the contrast of colour, gave a very fine appearance to this rock. Another good-sized block with large crystals of felspar (orthoclose) and plates of mica, was traversed by veins of a fine-grained granite, the whole again crossed by veins of quartz. The clay slates were of the usual silky nature. The whole appeared to have either been brought by a slip from Mosquito Hill, or to be perhaps the remnant of a ridge which had once united that hill with a similar isolated hill lying opposite to it on the southern bank of the Haast.

page 259

Mineral Veins.

I am not aware that any auriferous reefs are worked in Westland in this formation, although I have no doubt that in years to come, important discoveries of such lodes will be made, the more so as the rich auriferous reefs at Reefton and its neighbourhood, situated more to the north, in the Province of Nelson, appear, judging from specimens collected in the mines, to occur in the same slates as we find them in Mount Greenland and many other localities. Lately the argentiferous galena lode discovered in Mount Rangitoto, south of the Mikonui, has attracted a great deal of attention. This lode is said to be, on an average, 12 inches thick, dipping at an angle of 35 degrees to the north-west. Judging from the specimens in the Canterbury Museum, received from that mine, this lode is formed of iron pyrites intersected by small strings of galena. Professor Biekerton, of Canterbury College, at my request, has made an assay of it, and found as follows:— "An assay of this ore gave 11.4 per cent. of lead. The lead, on cupellation, gave 82.5 ozs. of silver per ton of lead. Thus the ore gave 9.4 ozs. of silver per ton. This silver contained an appreciable amount of gold." Subsequently, another assay of a different specimen was made by this gentleman for commercial purposes, with, practically, the same result. These assays agree closely with the analysis made by the Government Analyst in Wellington, who obtained 10 ozs. 17 dwts. per ton. They are, however, very different from that made at the Melbourne University, which, according to the newspapers, reached as much as 735 ozs. per ton. I am unable to explain the great discrepancy in the results of the New Zealand and Australian analysis, unless the ore should contain sulphide of silver, or other rich silver ore diffused irregularly through it.