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Report on the Geology & Gold Fields of Otago

Gold

Gold.

Gold mining in the Province is the subject of a separate report by Mr. Ulrich, and I shall therefore confine myself to making a few remarks on those points that belong more to a geological than to a mining report.

It has been already pointed out that we have in the sandstone of the Shag River (ante, p. 47), in the conglomerates over the coal at Wangaloa (ante, p. 48), and probably also in the Blue Spur (ante, p. 93), evidence of gold drifts having been formed in the eocene period; consequently the age of any of our gravel beds will not affect its auriferous character, the oldest being quite as likely to contain gold as the newest.

It has also been pointed out that during the later oscillations of the land the rivers have filled up their beds during subsidence, and that some of them have excavated fresh ones during the subsequent elevation. Consequently, if deep leads are defined as auriferous gravels formed in old river beds which have afterwards been filled up, many must certainly exist in Otago, and in the case of the Shotover, Lammerlaw Creek, &c., these deep leads have already been found. But owing to the mountainous character of Otago, and to the fact that the principal river valleys were in existence long before the oldest gravels and cements now remaining were page 116formed, the deep leads of Otago are very different in character from these of Victoria. Our deep leads are confined to the neighbourhood of the present rivers and streams, and must be always more or less parallel to them, as both the old river channel and the present one are but different cuts in the same great valley. Owing also to the last elevation of the land having been longer continued than the one preceding it, the rivers now run at lower levels than they did in their old channels and therefore most of our deep leads will be at higher elevations than the surface workings in the neighbouring streams. This is, of course, very advantageous in working them.

The terrace formation, which was formed by the rivers during subsidence of the land, is intermediate in age between the old gutters and the drifts at present forming, and the workings in it are also intermediate between deep leads and shallow workings, but I do not know that they call for any special remarks.

The previous observations apply only to those deposits that have been formed by rivers, and not to the old lake basins, the chances of finding deep leads in which must be discussed separately.

If we suppose a valley through which a river was running to have its slope gradually reversed by unequal elevation or subsidence of the land, it is evident that the velocity of the river would gradually diminish until the water was stationary, and a lake was formed, and that this lake would then gradually increase in depth. The streams on the sides and at the head of the lake would form deposits of gravel, probably auriferous, at their mouths, while the finer material, freed from gold, would form a bed of clay on the bottom, covering up the old river gravel. The gravel deposits at the mouths of streams continuing to increase at each flood, would constantly advance further into the lake and cover up the clay, (Fig. 28), until ultimately the lake would be filled up, and the river would once more run uninterruptedly through the valley, but on the top of the lacrustine beds.

Fig. 28.—Section through River fan forming in a Lake: a, Gravels and sand; b, clay.

Fig. 28.—Section through River fan forming in a Lake: a, Gravels and sand; b, clay.

The extent to which these gravels would be advanced towards the centre of the lake would depend upon the relative proportions of gravel and finer material poured into it. If the amount of fine material was small, the gravel deposits on either side might advance across the lake until they met one another. If, on the contrary, the fine material was in excess, the lake might be filled up by clay before the gravels had advanced far from the margin.

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If on the other hand the lake, instead of having been formed by unequal movements of the land, had been scooped out by a glacier, the hollow left when the glacier melted away would be at once filled by still water forming a lake. The filling up process would then commence as in the last case, but as no river had ever run along the bottom of the lake there would be no river gravel for the clay to cover up, only the angular unsorted fragments of rock that might have dropped off the glacier as it melted away. But this would not be the only difference, for the glacier formed lake would for a long time after its formation be fed by streams flowing from glaciers, which bring down a far larger proportion of fine mud than ordinary streams. Consequently in a glacier formed lake not only would there be no deep lead along the bottom, but the lateral gravels would have less chance of pushing far into the lake than in the other supposed case.

I have already given—(ante p. 87)—the reaso is that have convinced me that the old lake b[gap — reason: illegible]sins in this Province have been formed by glaciers, consequently I am of opinion that no deep leads exist along the bottom of them, and that the auriferous gravels are confined to those parts of the margin of the old lakes immediately adjoining the mouths of the streams that ran into them. It must, however, be remembered that in many cases these streams did not fall into the lake at right angles to it, but often very obliquely, and the gravels, therefore, would be carried in the direction of the streams and not straight across the lake.

We must also remember that during the course of the gravels down the river the gold gets separated from its matrix, and falls to the bottom, where it gets collected together, but it is very gradually pushed down the stream during floods by the violent movement of the gravel over it. But when these gravels and gold get pushed into the lake, they get scattered over the fan shaped deposit forming at the mouth of the stream—some being pushed one way and some another, and being now in still water there is no further movement to bring the gold together. Consequently the gold in these deposits is not collected in such definite leads as in the beds of rivers, and it is seldom that these deposits would be rich enough to pay except by hydraulic sluicing; consequently all these deposits when situated below the surface drainage of the country are useless.

The difference between these ideas on the distribution of gold, and those of Dr. Hector, published in the Otago Provincial Government Gazette, for September 3rd, 1862, are due partly to Dr. Hector considering the old lakes to have been caused principally by the subsidence of the mountains, which, he says, "appear to have sunk more rapidly where most lofty and massive,"* and partly to his thinking that rivers play a very minor part in liberating gold from its matrix. He says, "bearing in mind the now well established fact that afterpage 118the original wearing down of the rocky matrix, the mechanical liberation of its contained gold has been effected for the most part by long continued wave and current action, either in the sea or large inland lakes, and that the comparatively feeble and limited action of streams has merely exerted an ever-recurring sifting and sorting influence on the materials thus prepared, we can at once perceive that in this rock-bound basin the conditions have been most favourable for retaining and assorting the debris formed by the degregation of the surrounding schists during the submergence of the lands in tertiary times."

These views are of course quite different from mine, and I think different also from those of most geologists, but as Dr. Hector has re-asserted the correctness of his opinion, and stated that it has "now been well established by the results of the workings on the different gold fields," I think it desirable to reproduce it here in order that the reader may have both views before him to choose from,

It has been already said that the gold in the Province has been chiefly derived from the schist rocks, and the only important exception to this is at Orepuki. Here the country consists of slates and sandstones pierced by dykes of eruptive rocks. Exactly the same geological conditions extend to Stewart Island, and to Preservation Inlet, and it is quite possible that a gold field may exist west of Waiau, in the neighbourhood of Howloko and Monowai Lakes.

But it must be remembered that in the schist rocks the conditions are similar over large districts, and the gold is more or less universally distributed and easily found. But in the slate rocks the favourable conditions are local, and the gold consequently will be local also. This is probably the reason why gold has not been found in the north part of Stewart Island, for the geological conditions appear to be very favourable.

* Progress report of Geological Survey, 1868-9, p. vi.

Progress report of Geological Survey, 1868-9, p. vi.