Gold Gold dredging originated in New Zealand and Australia as early as 1865, but the endless bucket chain system was not started there until 1882. In Montana this sys tem was started in 1894, and in California in 1896. The dredges were used first in active running water, as in river beds, and later to work ground in benches or below drainage not profitable or possible by the sluice system.
A gold dredge is a flat-bottomed boat, with the forward part of the hull so divided as to i form a well in which the ladder and bucket chain may be raised or lowered. Upon the boat is mounted a digging ladder and chain of buckets ; a disintegrating and screening appa ratus; a system of sluice and other gold saving devices; pumps, anchoring and moving arrange ments; a stacker for the disposal of the coarser gravel, and motors, winches, gearing, etc. The early dredges were small and driven by steam power, but the best ones now use electric power and have been so enlarged and perfected that they have been able to dig gravel from 50 to 80 feet below water level at the rate of from 300,000 to 400,000 cubic yards a month, with an operating cost of trom three to six cents per yard. The outlay for such dredges is from $300,000 to $500,000. The main dredging fields now are California, Alaska, Colombia, New Zealand and Australia. The output from all dredging operations in the United States, from the start to the end of 1916, amounted to $120,103,117. In Alaska, sluice dredging, as also drift mining, is impeded and made costly by the necessity of thawing frozen ground by jets of dry steam which is applied through pipes, also called points, driven in the gravel.
Before starting dredging, the ground is prospected by shallow pits or drilled by Key stone drills, a light self-contained machine capable of boring a six-inch hole to moderate depths and of saving samples of the material passing through. In placer mining the most valuable minerals associated withgold are plati num, found chiefly in Russia, Colombia and California, and tin (cassiterite), found princi pally in Australia. The most abundant mineral association, however, is magnetite (black sand), garnet, zircon (white sand) and monazite (yellow sand).
Lode, Vein or Quartz Mining.— The prin ciples and methods of mining lode gold ores apply as well to those of silver, copper, lead, tin and zinc. The first requirement is accurate sampling and assaying, valuation of the deposit and the determination of the scale of working; second, prospecting, including diamond drill ing, temporary shafts, winzes and raises; and third, developing and exploiting the de posit underground. This brings in its train
the consideration of the size and nature of opening into the mine, whether by tunnel or entry, or incline; rectangular, round or octag onal shafts; also whether cars, skips, cages, kibbles or buckets should be used in the hoist ing ways. The system of stations involves loading devices, levels, cross-cuts and drifts from the main openings. For exploitation it is necessary to consider the winzes, raises, and advisability of overhand, underhand, long wall, rill or shrinkage stropes; sub-level, top slicing or other caving systems; and whether the support of the ground should be by pillars, square set timber or stalls, waste rock, cribs of timber and waste and tailings.
In the matter of mechanical equipment, the type of hoisting or winding engines and the probable output and depth of their working has to be taken into account, as also the advisability of the use of steam, electricity, water or gas, as source of power. It is also necessary to con sider whether direct steam pumps, rod driven pumps (Cornish), hydraulic, compressed air or electrically driven pumps, are preferable. Bail ing by tanks attached to the winding engines is often expedient and in some cases gives ex cellent results. In underground transportation or tramming, the problems of the use of man power, compressed-air locomotives, electric storage locomotives and electrical haulage pre sent themselves. Drilling and explosives are vexed subjects. Hand drilling until recently was the method most in use; now most of the boring is done by air drills, the two main types being, first, the use of the drill as the piston extension, and second, the action of the piston as a hammer striking the head of the drill. The sizes of the piston and weight of the drills are matters of much import. Air compressors are of divers makes and are driven either by steam, water, electric or gas power. Great diversity of opinion exists as to grades of explosives to be used in different deposits. The advisability of the systems are greatly dependent on the nature and class of the deposit and the economic conditions prevalent in the region. These and many other details connected with the technique of mining are more fully treated and explained under mining (q.v.).