Analyses of Vadose Country-Rock, etc., at Different Distances from Auriferous Lodes.

At the beginning of this chapter I have pointed out the importance of separate rock-assays in the vadose region. It was relatively difficult to obtain good samples (other than surface-samples) of this class, because most of the mines are now deep, and the former long cross-cuts run on upper levels into the country-rock are abandoned and closed. The samples therefore comprise chiefly oxidized rock from pretty near the lodes, and 10 to 100 feet below the surface, and surface-samples taken at all distances from the lodes. Particular interest attaches to samples of (1) oxide of iron and manganese deposited along bedding-planes or fractures; (2) solid rock as little altered as possible by the action of percolating water; and (3) secondary^* sulphides, abundant in the vadose region of many mines.

page 47

The remarks on p. 2 covering the methods of concentration and assay pursued with samples from the deep levels, apply here also. But concentration of an oxidized rock is much more difficult, because oxidation destroys the heavy sulphides, and also liberates very finely divided gold, which there is danger of losing. Hence my results with vadose country-rock are not quantitatively correct. To minimize the probable error, the samples were not concentrated nearly as far as those from deep levels had been.

Walhalla.—A good cross-section of the gold-bearing rocks of Walhalla is extended along both sides of the Walhalla creek, where the cliffs rise steeply from 300 to 600 feet. Analyses of specimens are given in Table XX.

Ballarat.—A deep cutting, about ¼-mile long, running E., across the strike of the rocks, from the summit of Sovereign: hill, in this district, presented a good cross-section of the country-rock near the surface. Table XXI. gives the result of analyses.

Otago.—Examinations of country-rock from the deep region in I Otago were confined to one district, Macetown; but quartz-reefs have been worked at various depths in different parts of the province; and vadose samples were taken from three other Otago districts besides Macetown, namely, Waipori, Nenthorn and Saddle Hill. In every case the country-rock is either phyllite or mica-schist. Table XXII. gives the analyses of 13 sampies.

Otago.—Samples from Districts Remote from Auriferous Lodes.—Vadose samples were also taken in Otago, far from any auriferous lodes. Previous assays having shown that the mate rials most likely to contain gold were the broken rock, iron oxide, etc., filling fault-fissures, the samples were taken of such materials only. If all the mica-schists of the Otago gold-field contained gold, some of it would be carried by percolating water into such crevices, and lodge there with the ferric oxide. The analysis of samples at a long distance from any auriferous reef is specially interesting, since, as will be seen in the last three page 48 tables, deposits of ferric oxide in the vadose region, even at a considerable distance from a reef, were nearly always auriferous

A good section of the favorable rock (the middle division of the foliated schists already mentioned), in which, however, for several miles, no gold-bearing reef has yet been discovered, is exposed by the "Skipper's" road from Queenstown to Skipper's creek. This road is cut round almost vertical cliffs for several miles, on the north side of the Skipper's range. Nine samples of broken rock and ferric oxide from joints, cracks and faulted country in these cuttings were analyzed with the results shown in Table XXIII.

Lake Wakatipu.—Fourteen vadose samples of little-altered Upper Devonian and Carboniferous rocks were taken from the western shore of Lake Wakatipu. No gold was found in them. The examination is reported later on, under the head of "Gold in Marine Sediments."

Ohinernuri District, Thames.—The samples collected for me from the vadose region in the Ohinernuri district, in the southern part of the Thames gold-field, differ somewhat from the oxidized samples of other gold-fields. All those analyzed are highly altered andesites; and, in many cases, even when most oxidized, they contain much pyrite. This sometimes doubtless represents the pyrite found in the propylites of the deep region in this district; but, from the mode of its occurrence, I am inclined to think the greater part of it is due to the oxidation of the older pyrite to ferrous and ferric sulphate, and the subsequent reduction of such sulphates by organic matter. I have therefore called it "secondary" pyrite. (The bullion associated with it does not carry the abnormally high percentage of silver which was noticed in the bullion from the sulphides of the deep region—a fact which should repay further investigatioa, and might throw important light upon the solution and re-precipitation of gold and silver by natural agents.) Table XXIV. gives the results of the analyses of these samples.

Remarks.—These examinations show a striking difference in gold-contents between the vadose and the deep region of the same district. Diagrams 8 and 9 show this difference graphically, in curves plotted for the vadose samples and for samples from the deep region (900-foot and 1422-foot level) of the Walhalla Long Tunnel mine.

page 49

In the deep region, as has been shown, gold was obtained only when pyrite was present in the rock; and when such pyrite occurred at a considerable distance from the reef, it was seldom gold-bearing. In the vadose region, on the contrary, the country-rock was found to be impregnated with gold to a much greater distance from the reef, and to a much greater degree.

It is of course possible that some of the gold found in the vadose country-rock was carried into it mechanically by percolating surface-water, and its presence may therefore be no proof of the solution and re-precipitation of gold. But in view of the positions from which most of the samples were taken, I think the results indicate that such solution and re-precipitation have gone on to a considerable extent in the vadose region—the gold being in all probability derived from higher parts of the lode, which have long since disappeared through surface-detrition.