SCIENCE AND MINING.

Otago Witness, Issue 1905, 25 May 1888, Page 12

SCIENCE AND MINING.

EULBS FOR FINDING LOST LODES.

By J. A. Miller, The rules ' dealt -with in last week's article hold good in most districts, and are the outcome of much and clos<» observation ; but every goldfield has its peculiarities which modify the rules and limit their application. Even the terms " kindly or contrary country " used by practical miners do not always mean the same thing in all gold-producing districts, and lodes have been known to carry gold only in their hardest and most solid parts. But these exceptions are few, and cannot affect or in any way refute the rules given In last week's article. SLIDING OF LODES. Lodes often show signs of having slid down upon their foot-walls, and this is sometimes the case to such an extent that the lode is broken off, a blank intervening between the several slipped portions. When this has been the case the signs are found upon the foot-wall, which is grooved by the inequalities of the harder quartz. This grooving is called sfcriations, and when there is any soft clayey substance between the lode and the enclosing rock it is named "flucan," or caseing, often very rich in gold. The walls of slipped lodes generally , have a polished appearance in spite of the striations, trad are often shattered upon both sides of the lode to such an extent as to have changed the dip of the layers of the rock, sometimes on both sides of the lode, but more frequently on the foot-wall side only. This disturbance of the rook does not extend far, but miners may be cautioned not to be deceived by such local displacement — a cross drive of a few feet will often reveal the true state of affairs. REGARDING THE RICHER PARTS OF LODES, it has been found that when a lode changes the angle of its dip, from a comparatively flat dip to one much steeper, the steeper portions are the richest parts. But it does not follow that a lode which throughout its whole dip is almost vertical is richer on that account than another which runs almost flat. In Otago the best lodes do go down vertically, as, for instance, the Cromwell lode at Bendigo, the Tipperary at Macetown, the Gallant Tip at Upper Shotover, the Invincible at the head of Lake Wakatipu, and a few others; while those which dip at a flat angle have been at best only patchy, and so far as worked are noted neither for their richness nor the continuity of their shoots of gold. It has also been established that those lodes which run parallel with the line of the upheaval are the most regular and richest, and that the best lodes are found at a little distance from the axis of the upheaval. They mostly occur upon the spurs of a main range, and are but seldom met with on its highest peaks. Advance Peak at Macetown forms an exception. A complete network of lodes and cross lodes interlace the peak, which approximates an elevation of 6000 ft above sea-level. All these lodes carry more or less gold, and while 6ome of them are very rich, an immense lode, approximating 30ft in thickness so far as it has been worked, is quite barren, and this lode runs parallel with the line of upheaval. But there are many other points to be considered in relation to the distribution of the rich parts of lodes. It will be remembered that lodes follow a sinuous line along the surface, or along any level in a mine, so that the rich parts will be unequally distributed along a horizontal line. This irregular distribution will be dependent upon the different rocks through which the lode passes along its course, and since it is generally found that the richer parts are those in which the direction of the lode corresponds most nearly to the line of elevation, this distribution of the rich parts in the upper levels of a mine will frequently serve as a guide by which to determine the " kindly country " of any particular district, and a geological examination of the district will afford data by which it may be inferred in what direction the shoots of ore will dip in the lode itself. As a rule the force which has produced one vein has also produced a series of parallel veins, and it is a no be worthy fact that where the behaviour of one of these has been determined the behaviour of) the others and the distribution of the rich parts can generally be predicted with precision from a consideration of the conditions I have enumerated. In the most known and better opened mining districts— -such as Cromwell and Freiberg—it has been shown that several successive series of lodes have been formed by tiltings of the strata from several different points, and the lodes following particular courses are generally characterised by special minerals; but the mineral districts of Australasia have not yet been sufficiently studied to state these authoritatively. < These tiltiugs of the strata along different lines have, however, not only opened a series of fresh reefs each time— the younger ones intersecting those- which had been previously formed — but ia not a few cases these later movements have reopened some of the old reefs in an irregular manner, and so have been formed some of the accessory deposits, which are frequently so difficult to account for, but which are met in lodes from time to time, apparently defying all attempts to explain their origin or to pre diet their extent. ' The rich deposits of lodes are also cut off abruptly at places by slides or faults, but my remarks on these and the means to be adopted for the recovery of lost lodes will form the subject matter for another article. SUMMARY OF THE ABOVE. 1. That mineral lodes are found almost exclusively in the older and changed (metamorphosed) rocks. 2. That lodes owe their origin to cracks in the rocks, and that these cracks are produced by internal action. 3. That water, charged with matters which make minerals and the substances of which lodes are built up, percolates through these cracks, depositing both under favourable conditions, thus forming mineral lodes. 4. That lodes change their courses both in strike and dip when they traverse different kinds of rock. 5. That the richness of lodes depends upon the changes in the angle of their dip — that is to say, when a lode changes from a flat angle to a steep one the steeper portion will be found to be the richest part. 6. That the relation between the dip and strike of the country and of the lodes disclose very important facts bearing upon the nature and richness of the lodes. 7. That lodes are faulted in their dip by slides. 8. That the distribution of the rich parts of lodes depends greatly upon local conditions.