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The Pamphlet Collection of Sir Robert Stout: Volume 75

Macetown, Otago, South Island of New Zealand

Macetown, Otago, South Island of New Zealand.

The country-rocks of the Otago gold-veins are metamorphic gradually changing from true phyllite in the east to much costorted and foliated mica-schists towards central Otago. In the N. Z. Geological Report of 1878-79 (p. 14) Sir James Hecton arranges these mica-schists in three divisions, in the middle one of which lie the most productive gold-fields of Otago.

These schists contain numerous quartz-folia, mostly not more page 19 than 1 or 2 inches thick, hut occasionally reaching 8 inches or a foot. In the latter case, they form lenses of quartz, not usually of great extent in strike or dip.

The quartz-lodes, which usually dip steeply, cut across the bedding of the schists.

The district of Macetown is in the heart of a mountain region, 20 miles N.E. of Queenstown, on Lake Wakatipu. Here well-defined lodes are worked by means of tunnels which reach a depth below the surface of at least 1500 feet. Samples to illustrate the deep underground circulation were taken from two mines named below, with the assistance of Mr. W. J. Stanford, the general manager of both.

The Tipperary Mine.—A specially good cross-section of the country at right angles to the strike of the reef is afforded by the prospecting deep adit of this company, which was started 1840 feet from the footwall of the Tipperary lode, and intended to strike the lode in a part which had proved rich in the workings above. At the time I took my last samples, the adit had been driven 1530 feet. At 164 feet from the mouth, the Dublin United reef, a gold-bearing lode 1 foot wide; and 876 feet further (or 940 feet from the mouth) another, the larney reef, 1 foot 6 inches wide, was crossed. The Tipperary reef was known to be about 300 feet further on. Over 70 samples were taken from this adit, the farther end of which was approximately 1000 feet below the surface. The samples taken between the mouth and the Dublin Limited reef represent the vadose circulation, and will be considered later under that head. The remainder form a very good typical series from the deep circulation.

Special interest attaches to the analysis of the quartz-folia already mentioned as abounding in the mica-schists of Otago. In this adit a number of such folia of fair width were crossed at various distances from the two lodes cut by the adit, and samples of these, as well as of mica-schist not containing quartz-folia of unusual size, were taken.

If gold be disseminated through the mica-schist country-rock, and the gold of the lodes be due to a leaching of this rock, there seems to be no reason why these quartz-folia—each of which constitutes, as it were, a miniature lode—should not have been similarly enriched from the rock on both sides, espe- page 20 cially as the solvent action of water percolating through sedimentary rocks during their metamorphosis would almost certainly be greater than that of the water which carried silica in solution and formed the quartz-reefs. The detection of gold in these quartz-folia, at a long distance from any auriferous lode, would therefore give strong support in this case to the lateral-secretion theory, and vice versa.

Table XII. contains the results of analyses of such quartz-folia from the Tipperary deep adit, together with a few similar samples from the Premier mine. It shows that the quartz-folia in this mica-schist are not auriferous when sulphides are not present, and that sulphides are present in appreciable quantity only when the quartz-folia are near an auriferous lode.

Table XIII. contains the results of analysis of the country-rock in the Tipperary deep adit, which are plotted in Diagram 6.

The Premier Mine.—This mine is situated a few miles from the Tipperary. In both this and the Tipperary mine a great deal of work has been done along the lode and at short distances from it. The lode-fissure is clearly traceable for the whole distance (in one case over 2000 feet) to which it has been drifted on; but the greater part of it is filled with "mullock," or broken mica-schist, with small quartz-veins, the whole being often fantastically contorted, and containing, as a rule, much carbonaceous matter. The quartz of the lode runs in shoots or varying width, only one of which has been worked in the Tipperary mine, while two have been worked in the Premier. For the purpose of studying the rock near the reef, samples were taken from short prospecting cross-cuts run from the lode in various directions, as well as from parts of the mine still nearer the lode-fissure. In the latter case care was taken to select sampies from parts of the mine opposite gold-bearing quartz, and also parts opposite places where the lode carried mullock, anil not auriferous quartz. The results are given in Table XIV, and plotted in Diagram 7.

Tables XIII. and XIV. illustrate again the striking difference already noted, in both the yield of gold and the percentage of sulphides, between samples taken near lodes and those taken at a distance.