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Report on the Geology & Gold Fields of Otago

Modes of Opening and Exploitation of the Reefs

Modes of Opening and Exploitation of the Reefs.

Opening of the Reefs.—As the steeply mountainous, rugged, and broken character of the country where most of the reefs are situated affords special facilities for their being opened and worked by adits, or tunnels, as they are generally, though incorrectly, called,* this method has extensively been made use of, and there are but a few reefs which had by necessity to be opened by shaft (Saddle Hill Reef, near Dunedin; Criterion Reef, Arrow—lying in low flats), or for which economic reasons, obtaining of quick returns, &c., rendered shaft sinking the most advisable at the start. Touching the facilities afforded by the adits in working the reefs, they are in all cases, no doubt, more or less considerable, according to circumstances; but with regard to positive advantages in a money point of view—considering in comparison the cost of working by shaft to the depths the adits come in—they appear in some instances very doubtful, or are quite on the side of the latter method. Want of water power, and great expense connected with employing steam power for hoisting the stuff and pumping the water, as also the procuring of easy transport of the stuff to the crushing machines, formed in these cases the main reasons for driving the adits; but then some of the latter might, at a comparatively but small increase in expense, have been at once put in much lower, or more careful calculation and scrutiny of circumstances at the outset would, in another case, have clearly shown that the length, respectively to the cost of the adit, was out of all proportion to the small height of reef overhead available for working, and that the water difficulty might at that depth have easily been overcome by horse-whim. It is not, of course, possible to lay down special rules, applicable in all cases, touching the advisability of opening page 169reefs by adit instead of shaft, for local circumstances differ, and monetary reasons come into play; however, I may draw attention to several general or, as it were, starting points that ought never to be left out of sight in the consideration of cases in point:—

1.If adits can be driven direct in the lines of strike of reefs in which the gold occurs in such a way that they (the adits) prove productive workings from the commencement, they offer the greatest advantages as compared to shafts.
2.Where the direction of an adit would have to be at right angles, or obliquely against a reef, representing dead work the whole capital required in its construction must be considered as lost, or calculated as part working expenses of the portion of the reef available overhead, if, when this is worked out, a new lower adit can be driven for working the reef under foot; for it (the upper adit) is thereby generally rendered quite useless. It maintains, of course, a certain value, if shaft sinking from it, and erection of hoisting and pumping machinery in a chamber constructed inside, have to be adopted for deeper working. But in such cases it is generally questionable, considering the discomfort and greater expense of this method, whether the working by shaft from the surface might not in reality have been the cheapest at the start.
3.Whilst every lower adit will generally be much longer, and consequently more costly than the preceding higher, useless one, and take a longer time in construction, a good shaft remains permanently useful for direct deeper working and in the generality of cases a new block of reef will thereby be opened far sooner and cheaper than by a long adit coming in at the same depth. Where adits are considered the most advantageous, and the prospects of a reef underfoot of an upper adit warrant it, an intended lower one should by rights always be commenced at the same time as the exploitation of the reef above the upper adit, in order to render the interval between the productive periods of working as short as possible.
4.In case of the abandonment of a mine, the machinery attached to a shaft has always a certain value.

These several considerations on the question of "shaft v. adit" are only intended, however, to bear on the real opening and working of single mines. Shafts are out of the question in the case of large main-adits, intended for working several adjoining mined or of draining them of water, or of such to be driven for prospecting purposes at considerable depth across the country. For both these kind of adits, the Carrick, Bendigo, and Skipper’s Creek districts offer special inducements and facilities; and whilst at Bendigo a deep Tunnelling Company is already in operation, another is contemplated to be formed at the Carrick Range. Particulars about these are given in the respective appendices.

page 170

Exploitation of the Reefs.—The method of exploitation, or of working the reefs, practised in the progressive mines of the different reefing districts, is one single case excepted, "over-hand stoping," and I must say that, unless the broken nature of the ground prevented it (Nugget and Cornish, Skipper’s Creek), wherever the mines were far enough advanced (most of the mines on the Carrick reefs), I found this mode of working carried out systematically and with due regard to rendering levels and winzes safe, by timbering and filling worked-out places with mullock. The exceptio al case alluded to is Logan’s Reef mine, which, considering the small width of the reef, is, contrary to the general mining rule, worked by "underhand stoping," and very economically, I must admit, yet, in a certain measure, at the expense of safety to the workmen, by not properly filling the worked-out spaces with waste. Speaking generally, this method, which is only practised in very wide lodes (2 to 3 fathoms)—for instance, in Cornwall—has, no doubt, many advantages over stoping overhand; but also very serious disadvantages. As the principal advantages may be mentioned, that working, by being carried on downwards, is easier, allows the use of heavier tools, and the men stand secure and convenient, whilst in over-hand stopping, it is more inconvenient and tiring, though the weight of the rock, in acting downwards, facilitates it; it is also more dangerous, especially in jointed, fissured, and loose ground, as the men have to work underneath. The disadvantages of the underhand stoping consist chiefly in the great expense entailed by the necessary timbering and mode of disposing of the waste in filling up of the worked-out places, it (the waste) having to be piled on platforms in front of the stopes. Next comes, that more manual labour is required in raising the ore from stope to stope, that the miner has to contend with water, and is often troubled with bad air. These drawBacks are so serious that, according to mining report, the method comes more and more out of use, and is likely to be soon entirely superseded by overhand stopping. As working is at present being carried on in the progressive mines I examined, hardly anything is done in prospecting the country in the hanging and foot walls of the reefs; but as this is of great importance and often very profitable, touching the discovery of rich, small companion reefs, branches and leaders, the driving of occasional small crosscuts ought not to be neglected. Before leaving this subject I have to draw special attention to one great defect I found in the management of most of the mines, namely, the non-provision of correct and detailed plans of the workings. I need not enter upon detailing the advantages such plans have in the systematic management of a mine, and the understanding and tracing of occurrences of unforeseen mining features (faults, turns, breaks of the reefs, etc). Every experienced mining manager knows these well enough. But I must say, that before [unclear: enering] upon the driving of adits, preliminary mining surveys of the page 171ground, and the preparations of working plans, are quite indispensable, in order to avoid false conclusions as to distances, errors in direction, etc. (See Lucknow Reef in appendix 6). The reason of these deficiences hitherto consists, I was informed, in the want of qualified mining surveyors, and considering the importance of the subject, I would therefore take the liberty to recommend the Government to appoint several surveyors of this class for the principal mining centres Touching the duties of these gentlemen, they might, as is the case in Victoria, consist in the surveying of leases and claims, and in the execution of over and underground surveys, all which, as private work, to be paid for by the parties requiring it, whilst, during intervals between such work, the preparation for the Government of detailed topographical plans of the mining districts, to aid geological examination, might be carried on at certain fixed rates.

* To be tunnels, in the strict acceptation of the term, they ought to run right through the mountains, each connecting two points on the surface, which none of those under notice do.