[Read before the Otago Institute, 8th October, 1907.]

Plate XVI.

(1.) Fissure-Veins.

Glenorchy Reef.

This outcrops on the steep left bank of the Bucklerburn, a mile and a half above its mouth at Glenorchy. The country rock is a slate, striking north and south, and dipping to the west at from 30° to 50°. The vein strikes east and west, and dips to the north at a mean angle of 15°. It has been followed on the surface for about half a mile, is well defined, with fairly smooth walls, and carries a strong continuous seam of quartz throughout. Its width between walls varies from 1ft. to 5ft.

In accordance with the varying width of the walls, the vein is lenticular in longitudinal section, a feature which evidently indicates some displacement of the walls of the original fissure (fig. 1).

Fig. 1.

Near the east end of the old workings the vein is thin and the seam of quartz insignificant. Followed west, it increases in width, and has a sinuous and irregular course, with numerous leaders coming in on both walls. The accompanying sketches (figs. 4 and 5) illustrate the characters of the vein.

Fig. 4.—Section Across Alta Reef.

Fig. 5.—Section Across Alta Reef.

Near the end of an adit driven close to the old battery-site scheelite is seen in conspicuous bunches on and near the south wall, which is here poorly defined (fig. 6).

Fig. 6.

The scheelite in this reef has never been exploited, but the reef was successfully worked for gold in the early days.

Veins on the Lammerlaw Range, Waipori.

Several of the small gold-bearing veins on the Lammerlaw and Burnt Ranges, near Waipori, carry scheelite, sometimes in considerable quantity, but they have never been developed to any extent.

A notable occurrence of the mineral is in the antimony-reef at Stony Creek, nine miles from Waipori Township. At one point in this reef scheelite and stibnite were found in close association, and accompanied by gypsum. This last is evidently a product of secondary origin, resulting from oxidation of the sulphide ore and interaction with the scheelite.

Among other occurrences, scheelite has been found in the Barewood reef, and in the Saddle Hill reef.

The reefs of Macrae's are bedded or segregated veins, and are of peculiar interest in that they embrace all the veins of this

It is significant that all the reefs in this district outcrop on a single plane in the schist. This indicates that the horizontal shearing movement which localised the reefs followed a particular zone in the rock, although it is quite likely that there may be one or more zones or levels of lode-formation beneath the one now exposed.

This mineral occurs, firstly, in segregated masses of varying size, typically seen at Macrae's. These generally cling to the foot-wall, and frequently pass right into the country rock, the foot-wall being then obscured. These comparatively pure masses grade off into highly quartzose ore scattered through the gangue. Secondly, it occurs in irregular veins in the quartz leaders and stringers, as well as in the larger quartz seams. It may constitute a whole vein, or it may have a broad or narrow selvage of quartz on either side.

The hand-specimen, which always contains some quartz, has a specific gravity of 5.12, that of the pure mineral being 5.9 to 6.1 (Dana). It is yellowish-white in colour, brittle and friable, with an irregular fracture. It shows interrupted cleavagesurfaces, and is massive in habit, no crystals being found, as far as my observations showed.

Microscopic Characters.

In section (Plate XVI, a, b), the mineral is dark-brown, with a high refractive index. In isotropic sections a faint positive uniaxial figure may be seen. The interference colours are more usually yellow and red of the first order. The individuals are large, with sharp boundaries and pointed or pyramidal terminations. Two interrupted sets of cleavage-traces crossing at 40° are seen in suitable sections, these being the characteristic cleavages, p (111) and e (101). The cleavage-lines are frequently crossed by irregular fractures, along which the mineral is dark and clouded. A faint lamellar structure is occasionally seen, resembling polysynthetic twinning. The lamellæ, however, are alternately broad and narrow, and can be distinguished, though with difficulty, in ordinary light. The appearance is probably a strain-effect.

Chemical Composition.

The following analysis indicates the average composition of Otago scheelite. Quartz is always present in intimate association, as shown in Plate XVI, a and b; in the analysis this constituent was eliminated, and the figures recalculated to 100 per cent.

Microscopic examination of the ore, and chemical analyses of the wall-rock of the veins, prove that the scheelite has been formed by metasomatic processes—namely, by combination of tungstic acid with lime-bearing minerals in the adjoining rock.

Microscopic Evidence.

The relations of scheelite and calcite, as seen under the microscope, are very striking. Plate XVI, c, shows scheelite in clear granules with fresh sharp boundaries enclosing a corroded core of calcite, and indicating the replacement process by which the ore has been formed. This phenomenon is best studied at Macrae's, where the country rock contains a considerable amount of calcite. In general, the scheelite is always fresh, the calcite where seen is much attacked and corroded. Several of the plates accompanying Mr. Lindgren's classic work on “Metasomatic Processes in Fissure-veins”^* show very similar processes to that illustrated in the above plate.

A similar association of scheelite and calcite is occasionally seen in sections cut from Glenorchy ore. A characteristic feature of the Glenorchy mineral is the manner in which strings of pyrite occur along the border between scheelite and gangue (Plate XVI, d). The pyrite thus appears to have segregated along the line of most intense metasomatism. The process of osmosis, regarded by many authorities as the central factor in ore-deposition,†^† would evidently be equally favourable to the formation both of scheelite and of pyrite, the latter being, like the former, essentially a replacement product.

In this case the loss of half the total mass of the rock, including 40 per cent. of the original rock, is in accordance with the fact that the Macrae's veins are segregated veins. In other words, wall-rock + lode (gangue) = original rock. This approximate equation will hold good for volumes as well as for masses—that is to say, the wall-rock has suffered a corresponding diminution of 50 per cent. in volume. This explains why the specific gravity of the altered rock (2.69) is equal to that of the unaltered rock.

Further, there is to be observed in the wall-rock a depletion of lime and addition of alkalies, as at Glenorchy, and likewise due to the processes by which the scheelite was deposited. The notable loss of carbon-dioxide is no doubt due to the destruction of calcite during the metasomatic action.

The bunchy tendency of the ore, particularly at Macrae's, is evidence of the segregation of the mineral during the formation of the lodes.

Rise and Progress.

The history of scheelite-mining in Otago dates from about 1888, when the first mine was opened up on the Glenorchy reef by the Lake Wakatipu Scheelite Company, and an expensive ore-dressing plant was installed. Some 27 tons of dressed ore was shipped to Hamburg, but the price was low—£20 to £29 per ton; and after two years the demand ceásed, and the company liquidated, after an outlay of £3,000.

About two years ago the mine was taken over by a new company, and a crushing and dressing plant installed. With a good market and improved methods of concentration, this company is making rapid strides.

On the Macrae's field scheelite was first exploited in 1893, by Messrs. A. B. Kitchener and William Donaldson, who sent 6 ½ tons of 40-per-cent. ore from the Golden Point Mine to London. The returns did not leave much profit, but the work was perserved with, and a later shipment realised £58 per ton. The market was subsequently transferred to Hamburg, and the demand and price steadily increased. Improved plant was installed, and considerable prospecting for scheelite was carried on, in consequence of the success attending Messrs. Donaldson's efforts. Up to date the Golden Point Mine has produced scheelite to the value of £24,000, the price having risen progressively in the last fifteen years from £20 to £160 per ton. During this period 400 tons of ore has been shipped from Macrae's while the Glenorchy Mine during the last eighteen months has dressed 60 tons.

Present Minning Methods.

There are at present three working mines—Messrs. Reid and Lee's Glenorchy Mine, and Messrs. W. and G. Donaldson's two mines at Macrae's. The method in vogue of concentrating the ore is to pass the pulp from the battery where it is crushed over shaking-tables or vanners, where it is dressed to an average value of 65 per cent. of tungstic acid (WO₃), the impurities being quartz and pyrites. The ore thus concentrated is dried, bagged, and shipped.

For crushing the ore there are five stamps in operation at Glenorchy, ten at Golden Point, and a 5 ft. Huntington mill at

Mount Highlay. The concentrating-tables used are a Wilfley at Glenorchy and Mount Highlay, a Woodbury and Frue vanner at Golden Point. of these, the Wilfley appears to find most favour. The Glenorchy company have lately installed a Wilfley slime-table, with the object of recovering the slight loss in the tailings.

The pulp is dried over small wood or coke furnaces, a method that would scarcely be suitable for a large output. Further, a more efficient method of drying—or, rather, roasting—would burn off the sulphur of the pyrites, and thus indirectly raise the percentage value of the ore, which is a desideratum in view of the fact that the price per unit or per cent. varies with the percentage.

Prospects.

The success of the industry in Otago has been due to the steadily improving market at Hamburg, to which the ore is now shipped, and to greater attention on the part of present firms to the securing of clean and high-grade concentrates. The problem of concentration is a very important matter, as a poorly dressed ore will soon cause buyers to fight shy of the mine which ships it. The market, also, requires to be studied. In 1900, Messrs. G. P. Blackwell and Sons, metal-merchants, of Liverpool, reported thus: “The indiscriminate shipping of tungsten-ore from Australia and New Zealand is unwise, and has depressed the market, which is a peculiar one, and requires careful handling. Shippers should send their ore through one channel to a firm which understands the business, and can keep the market firm.”^*

In view of the steadily increasing demand for tungsten, the prospects of the scheelite industry in Otago must be considered bright. Unfortunately, the fluctuations which have hitherto occurred in the market affect the production of small mines. This can only be guarded against by insuring that the mines shall be backed by sufficient capital, which would render them secure against closing down in the face of a slightly lowered quotation, an event which has happened more than once in New South Wales and Queensland.

Considering the success of the present producing mines, it is highly desirable that the other scheelite-veins, both in Otago and Marlborough, should be taken up, and there can be little doubt, provided the market remains firm, that they would prove successful ventures.