A “Deep-Lead” is a deposit of auriferous gravel, lying at a considerable depth beneath the surface, often covered by beds of lava or basalt, hundreds or even thousands of feet thick. This type of deposit is different from a shallow deposit in that it is the result of a drainage system which no longer exists. They should not be confused with channels that are open and remain dry during part of the year due to lack of water, or which have been abandoned by their streams for other channels.
A dry river bed is not a dead river. In most cases, these ancient deposits have been discovered by following a modern auriferous stream, and tracing the wash-dirt into deeper ground, where parts of it have been dislodged by recent stream activity. It is easy to determine whether or not the gold in a shallow placer came from a quartz reef or vein, or from a deep-lead. The gold from a quartz reef or vein will be coarse and heavy, and not much water-worn, whereas the gold from a deep-lead will be fine and rounded.
Erosion has destroyed the veins, depositing great deposits of mud and clay in the valleys, and in many cases the gold-bearing veins, the streams 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.
We can only guess at the age of the Tertiary rocks in which the deep leads are found. It is almost certain they are no older than the Pliocene age, as no evidence of marine relics or fossils have been found in auriferous drifts, nor has any gold-mining been carried on in drifts underlying marine fossiliferous strata.
To ascertain the course of the deep lead with any degree of accuracy is very difficult. Expensive subterranean boring explorations can be undertaken, but even then, when the general trend of the lead may be determined, which could extend a mile or so, the richest section of pay could very easily be overlooked.
The dead rivers of California lie on the western slopes of the Sierra Nevada, 500 to 7,000 feet above sea-level. They are all auriferous, the largest and richest of them being the “Big Blue” lead, which was traced for a length of 65 miles, and was believed to have extended 110 miles. This lead was discovered by the same process that has revealed the existence of most deep leads – surface washing. The miners found the streams were richly auriferous up to a certain point, increasing as it was approached, but disappearing when it was passed.
Entire volumes could be written on the subject of deep-leads and boring, shafting, tunneling, and other matters, all of which are expensive and unnecessary here. One method worth mentioning for the extraction of gold from deep leads however, is drifting. The manner in which pay-dirt has been reached by a shaft varies according to the circumstances, but the general plan is “drifting”. This consists of working out the auriferous gravels by a system of tunnels or galleries, bringing it to the surface to be run through a sluice or high-banker. Nothing but the gold-bearing gravel is removed, and the galleries are generally allowed to fall in as the work advances. Generally, the pay will be at a depth of 2 to 5 feet.
In places where the bed of modern streams is lower than the level of wash-dirt, horizontal drifts are the most economical. They are driven on an upward slope, so that any water accumulating will run down the slope. When the auriferous gravels contained in the ancient river beds are covered to any great depth due to past geological occurrences, or where the pay-gravel is concentrated in a layer 2 to 8 feet thick, either directly on bed-rock, or under a thick layer of poor gravel, it would be impractical to move the entire overburden. This is where drifting becomes the most economical solution. Of course, “hydraulicking” would be a simple solution as well, but that becomes a whole other ball of wax, not to mention the legal and environmental impacts.