HYDRAULIC MINING AT HUMPHREYS GULLY.

Kumara Times, Issue 2615, 23 January 1885, Page 2

HYDRAULIC MINING AT HUMPHREYS GULLY.

Dr. Hector in his report as director of Geological Explorations in tbe Colony during the year 1883-84, says:— Hydraulic mining will be the great industry of the future in Westland. In former reports I have pointed out the distribution of the gold drifts and the manner in which the enormous discharge of water from the Alpine sources had rearranged and concentrated the auriferous drifts that had given the enormous yield of gold in proportion to the number of mines which has so characterised the "Westland goldfield. But this very violent action has, by cutting deep river channels through the first formed gold drifts, left undisturbed large areas of these drifts at a level inaccessible to the ordinary drainage system. By the construction of water-races, and by the system of hydraulic mining, the re-sorting can be effected artificially and with profit. I had an opportunity of inspecting one of the most promising of these adventures about twelve miles from Hokitika, where at Humphrey's Gully all the small claims are now amalgamated, so that about 200 acres is held by a company in one block. It forms a triangle in the junction of two tribubutary valleys to the Arahura River, so that it lies favourably for getting rid of the tailings, which is the most important element in the success of a sluicing claim. The thickness of the gravel is from 150 ft. to 500 ft., as the bottom on which it rests dips into a hill, which also rises rapidly on the surface. The gravel is of two kinds, b and c, which rests on A, a silt clay with lignite seams, which passes downwards into the blue reef or marine fossiliferous clay marls of miocene tertiary age that are so largely developed in the flat country on the West Coast. This silt deposit marks the transition from marine to fluviatile deposits in the district, or in other words the passa«« from miocene to pliocene formation. The gravels (b) which belong to these passnge beds were the first found rivor beds on the Coast and are generally tho richest, though the gold is hardly, if at ali, sorted out in Ui»sh "ravels in the form of dcliaii« lc,ut.v. This gravel

is very easily sluiced away, as the pebbles are of moderate and about equal size, and are, moreover, rotten and decomposed, so that their surface washes away and all the gold is set free. In ordinary gravels, as all miners are well aware, the pebbles are hard, and the pebbles are set in a dense feruginous cement that adheres with great strength to the surface of the stones. On the top of the terrace is (c) the newer gravel of very heterogeneous materials, being, in fact, a morainic deposit formed by glaciers that, in late pleistocene times descended from the Alps and spread over the plains and river-beds of earlier periods. This deposit (c) contains huge blocks of stone mixed with sand and fine gravel. These two deposits, b and c, are the chief repositories of gold on the West Const, but c is not so uniformily auriferous as B. Dis the ordinary flanking deposit of the existing valleys, and is auriferous or not, just according to whether the river is traversing the older gravels in the upper part of its course.

Now as to the richness of these gravels b and c. They have been tested in various ways—(l) by surface workings at various points ; (2) by tunnels driven through the hill in various disdistances, and (3) by sluicing the faces of the terraces. I made about twenty trials in the first way from various points and never failed to get a few specks of gold to each dish. The average from the middle of the deposit b gave ten specks to the dish of stuff about 15K>s weight. These specks averaged '095 of a grain each ; and as the stuff averages a specific gravity of 2.7 this would give the value per load (equal to one cubic yard) of ldwt 4gr. for each speck present, or for my trials lldwt. 20gr.—equal to £2 6s per load. The layer near the junction of the deposits B and c in like manner gave £1 Is 6d per load on the average, and the deposit c gave an average of £2 14s per load. These results are remarkable, but I do not see how they can be erroneous: but still experience shows that, when the sluicing on a large scale is commenced, while much coarser gold is obtained than by prospecting, the total average yield falls short of what was anticipated. The tunnel works were purposely so put as to follow the richest layers, so that there is no wonder that they make the stuff appear even richer than the pan prospecting. The third source of information is the best, and I understand that sluicing 500 loads of the gravel b gave about 62Jozs. of gold, or about 10s per load, which is probably about the real value of the stuff. Keducing the area to 100 acres and thickness to 100yds. on the average, the claim will contain 60,000,000 loads of stuff, and it is only the question of the cost of getting that remains to be considered. On this point I do not think any hitch is likely to occur. When the dam is finished it will have an area of 11 acres, and afford 55 heads for 24 hours, with a possible inflow in dry weather of 100 heads. A short tunnel (the undertaking of which was a mistake, as it caused delay) hns only to be completed now, when 20 heads should sluice about 16,000 loads per day; when takiug one speck of the smallest size (viz., -02 of a grain, valued at one-fifth of a farthing) to each load would give £l2 10s a day, or £3750 per year of 300 working days, which would be about half tho interest at 10 per cent, on the capital already invested, so that two specks per load would pay the interest; and if it were an average sized speck to the dish—which is a very moderate estimate from the trial I have seen and described above, the yield would be £BOO to £ISOO per day and taking the lowest of these figures, the claim will contsin at present£3,ooo,ooo, and at the above rate be worked out in twelve yearß.