The prospection activity of gold dredging deposits must consider some fundamental aspects in order to don’t draw wrong ideas of the potential gold zone. It may seem a simple thing to say that a careful study of all the conditions should be made before a dredge is ordered, no matter how large the values in gold may he found to be. The values may in many cases be of almost minor importance, or of no importance, where, as is the case in some of the mountain districts, the hardness of the bedrock, with much of the gold on the bedrock, has prevented dredging very rich ground. There have been many failures in California, and all because proper examination of the ground was not made before the dredges were built.
For example, in one case, a dredge was put in and failed and has been dismantled because the fact that the ground was valueless was not proved. There is another case where a dredge was built and operated for a time on very heavy ground, but it was so weak in construction that it failed and was dismantled. It has been said in regard to sampling, that which will apply to any other part of the examination of a dredging property, and occasional sampling and assaying have to be studied carefully. Wrong interpretation may lead to great losses of valuable capital, frequently to the total abandonment of good properties, and, worse than all, to a false sense of satisfaction that discourages improvement by denying its necessity. The following are some of the conditions to be determined by drilling and other examination prior to determining whether the ground may be dredged, prior to estimating costs, and prior also to determining the kind of equipment to be used: value in gold per cubic meter, and its distribution, bedrock level, water amount of supply, and cost; hardness of gravel; cost of power; total depth of gravel to bedrock; wages; depth of gravel from water level to bedrock; cost of transportation; whether level or rough on surface; cost of supplies; Size of bowlders; amount of clay in gravel; cost of land; hardness of bedrock; climatic conditions.
Generally speaking, any gravel that may be picked may be dug with the dredge, without the use of powder. If it is so hard as to require blasting, the cost per cubic meter will be increased. The most satisfactory depth to dredge with some of the newer dredges now in use is not to exceed 20 m, but they will operate down to 30 m. In order to work to a greater depth than 30 m, some changes in the digger, stacker, and power would be required. With an uneven surface the use of headlines is not so satisfactory as spuds. Very large bowlders cannot be handled with the ordinary dredge, and much gravel may be left on the ground. The size of the bowlders must be considered also in deciding whether to use an open or close connection of buckets in the digger, the latter rendering it impossible to handle the larger bowlders. Climatic Conditions, of course, freezing weather will prevent washing the gravel, and cause a stoppage of work. The clay will not permit of clean dumping from the buckets, and not only is much gold carried back to the bottom and lost, but the capacity of the dredge is decreased. Besides this, the clay carries off gold and fine amalgam from the sluice-boxes. If the bedrock is too hard to dig, gold in the crevices or lying near the bedrock is lost. The drill will locate the position of the pay which may not be too close to the bedrock. The bedrock should be near enough level to permit the boats to float over all the ground to be worked. The presence of arsenic or anything else that will prevent the good performance of amalgamation tests to determine the presence of free gold, also, riffles of little avail, and matting, plush, or other such means must be adopted to save the gold, especially where it is in fine particles.