A deep lead is a gold deposit lying at a considerable depth beneath the surface and often covered by beds of lava or basalt, hundreds and even thousands of feet thick. Basically, this deposit is characterized by the fact it has been the work of a drainage system which no longer exists. Deep leads are generally, though not always riverine. Also, they are known as dead rivers, a term which is applied only to channels which were occupied by running streams in old times and now are filled with earthy or rock compounds.
They are not to be confounded with channels that are open and remain dry during a part of the year from lack of water, or which have been abandoned by their streams for other channels. A dry river-bed is not a dead river. In almost all cases, these ancient deposits have been discovered by following up an auriferous stream and tracing the wash dirt onwards into the deeper ground, whence part of it had been dislodged by the recent stream. It is often easy to judge whether the gold in a modern placer has been derived directly from a quartz reef or from a deep lead, as that from the reefs is usually coarse, heavy and not much water worn, while that from the leads is usually fine and rounded.
The erosion due to rivers has destroyed the veins and formed a great deposit of mud and clay in the valleys; while in many cases the gold-bearing veins the stream which wore them away, and the deposit formed by the streams, have all disappeared, and the gold has to be sought in more recent accumulations, which have undergone a variety of changes and perturbations. It is scarcely possible to do more than guess at the age of the tertiary rocks in which the deep leads are found, and the evidence on this head has to be sought rather in relations borne by these deposits to the adjacent formations than in any fossil which they may yield. Still the limits are often sufficiently well marked to meet practical demands.
The deep leads were long supposed to be in no case older than Pliocene, no marine relic has apparently been found in auriferous drifts, and no gold mining is known to be carried on in drifts underlying marine fossiliferous strata. Rarely is it possible to trace the course of a deep lead within such limits as will serve the turn of the gold prospector, merely by the indications furnished by the natural surface of the land overlying them. Basically, the course of a channel originating in arrange of Paleozoic rocks may for some distance be well marked by strata of gravel, sand and water worn quartz pebbles filling up the lower part of the valley; but when it is followed out into the low plains, through certainly will, increase in thickness so that it becomes impossible any longer to follow the trend of the ancient river bed.
In some places, some little guidance to the possible deposit may be derived from the fact that it has hitherto been found that almost all the leads run parallel to the modern rivers. Nevertheless, in other sites they are more commonly at right angles to the latter; and even under the best conditions, the sinuosities of the ancient channels cannot be taken into calculation, neither it is possible to foretell where the bed may widen and where it may become narrow, points which have a vital bearing upon the potential richness of the deposit.