The vein lies in a zone of soft crushed slate, surrounded by harder sandstone. Down to No. 6 level it has a dip of 60° to the south; below that it flattens out, and from No. 8 to No. 11 level—a distance of 400 ft. vertical —it made 1,500 ft. of base. It is worked by two shafts—one, the eastern, opens up 5 and 6 levels; the other, the western, opens up the deeper levels. The shaft-head is 1,600 ft. above sea-level, and No. 11 is 1,460. ft. deep.
Except in the harder sandstone some distance from the reef, the country is soft, and stands badly. The estimated annual cost of timber in this mine during the last ten years has been £3,000. The walls are generally poorly defined, and rarely do a foot-wall and hanging-wall occur together; they are often obscured, while quartz merges gradually into slate.
The quartz occurs in lenses of varying size and value, frequently much disturbed by small faults. The following varieties of ore are met with:—
(1.) Seamy: The most characteristic form. It is seamed with strings of pyrite and mullock, and is nearly always gold-bearing.
(2.) Brecciated: This is only occasionally found.
(3.) White glassy or “dog's-tooth” quartz: This varies in its gold-content. In blocks of this quartz which have been regarded as poor or barren, gold is frequently found in good payable patches. Miners claim that it is possible to distinguish good and poor ore in this mine by means of its appearance to the naked eye; but this is very doubtful. Frequently quartz set down as poor has been found to give good returns.
The difference between the seamy and white glassy varieties seems to be largely due to processes of replacement in the former case, and to simple deposition without replacement in the latter.
The quartz blocks are in the main low-grade throughout, and poorer in the lower than in the upper levels. The gold is patchy in its distribution, does not run in shoots, and is very fine. This vein carries the finest gold on the field. It occurs both free and involved with pyrite, which is the common sulphide.
Stibnite occurs, frequently and patchy. In places it has been found in seams up to 30 ft. thick, but of low-grade (30 per cent.) ore. In this mine antimony-ore is found generally to accompany poor values in gold.
The blocks or lenses of quartz follow in the main three roughly distinct shoots, which all show a certain pitch to the south-west. They are all fairly continuous, though with frequent pinches and makes.
The main shoot has been worked right down to No. 11 level. Between 8 and 11 it flattened considerably, as shown in figure, and in No. 11 it lies practically horizontal. This is the most peculiar feature in this mine.
The vein thus appears to occupy the limb of a syncline; but further development is required to test whether the other limb exists, and whether it carries ore. Owing to the great confusion of strike and dip of the slate I was unable to find evidence of a synclinal arrangement from surface observations. The flattening observed may, indeed, be only a local phenomenon, or an irregularity in the course of the vein. At any rate, the evidence in favour of a syncline is not yet conclusive.
In the development of this mine the diamond drill has played a considerable part, sometimes with much success. Much care has, however, been found necessary in interpreting its indications. Thus, in veins of this interrupted type a bore may be put in and may miss a block altogether. Again, it may pass through a block at such an angle as to give a very exaggerated idea of its thickness. A bore was put down from a point near the shaft-chamber in No. 11 level for a depth of 1,000 ft., or 2,500 ft. below the surface: no promising indications were found, but the bore is interesting as being the greatest depth reached in a New Zealand mine. The results of other bores made have given no indication of the existence of a synclinal vein.
The deeper levels of the mine, as of all others in the district, are conspicuously dry and dusty, except for surface-drainage in the vicinity of the shaft.