Placer Placer, known also as alluvial, or alluvium, mining, is the operation of washing gold or other valuable minerals from gravel or sand, which is mostly alluvium deposited by a stream or in a beach, or as talus. It has been estimated by De Launay that be tween 1848 and 1875, 87 per cent was thus ob tained, but lode or quartz mining has increased so that in 1876 placer mining produced only 65 per cent; in 1890, 44 per cent ; in 1905, 15 per cent. In the future, placer mining may be come still less important. New deposits can be looked for only in sections which have been covered by ice, snow and marshy turfs in polar regions; in forests of the tropics; in deserts, high valleys and mountain chains. Further returns may also be obtained from gravels too poor to yield profit under present conditions. Placer deposits have been classified as shallow, deep, creek, hillside, bench, river bar, gravel plain (tundra), sea beach, lake bed and dry placers. Such deposits have been the easiest to work for nature has done the major part of the mining and reduction work, and man has the concentrates for further treat ment. Nature's process was erosion, and her tools, clouds, rain, ice, snow, glaciers, rills, torrents and rivers. These have carved out valleys, built up flats and by attrition milled or ground up the materials, carrying away the softest and lightest. Gold, being the heaviest of known minerals, excepting platinum and a few other rare minerals, naturally works to the bottom of the streams add the coarser particles cling to the bed rock. The finer the gold dust the further it is carried down the river systems.
Most placer is done in the open and largely in connection with modern river systems. Some of the auriferous deposits, however, are covered by lava or other non-producting cap ping, which necessitates underground working tunnels, shafts, drives, slopes, etc. Such min ing is known as drift or deep-lead mining and it has been done extensively in California and Australia. In most cases the pay dirt is in a few feet of gravel next to the bedrock, but in some such deposits yields high returns. After the gravel is mined and brought to the surface it is washed on floors connected with suitable sluice system. Auriferous gravel de posits with thickness of only a few feet or in banks of several hundred feet in thickness have been worked successfully. These gravels may contain boulders several feet in diameter, or pebbles graduating in size from a few inches in diameter to fine sands. The gold occurs in scales, grains or nuggets, and all particles are smooth and rounded, thus differing from vein gold which is sharp and angular, and often the particles, or "colors," of gold dust are so fine and scaly as to defy ordinary devices of catch ment. The nuggets have varied in size from small grains to the Australian "Welcome stranger," weighing 2,520 ounces, valued• at about $42,000. The largest nugget found in California weighed 280 ounces. In Russia the largest, weighing 96 ounces, was found near Miask The Klondike has produced an 85-ounce nugget. The greatest proportion of gold re covered is medium and fine gold dust.
Prospecting The miner's pan, made of stiff sheet iroik and flat-bottomed; the bateau, made of wood vAth conical bottom; and the horn spoon, cut out of ox horns, are used for testing the gravel and working the rich spots. These, with pick and shovel, and beans and bacon, were the old-time prospector's out fit. For small but rich -deposits the cradle or rocker and Long Tom were long employed, but now they are rarely used.
Sluice and Sluice The sluice is the mainstay of shallow placer mining. It is a long open box, made of rough boards usually in sections 12 feet long, with varying width and depth. The sides are protected from wear
ing by wooden liners renewed frequently. The bottom is protected and made effective for gold saving by a series of transverse or longitudinal riffles, or as in hydraulic sluices or flumes, with wood pavement of rounded blocks. The length varies with grade and other natural conditions favorable for disposal of tailings. The grades used vary from 2 inches to 20 inches per box (12 feet). Mer cury is sprinkled in at the head of the sluice after washing has been in progress a short time. The charge of this metal varies with the rich ness of gravel and the magnitude of the work. A stream of water is turned in at the head of the sluice where the gravel is shoveled or dumped in. The amount of gravel mined and shoveled per man varies from 3 to 12 cubic yards per day. In sluice mining the sluices can be shifted to suit the mining or the gravel conveyed to the sluice by different appliances. At many workings long stationary sluices are constructed with drops to facilitate disintegra tion of the gravel and with undercurrents to classify the gravel and provide special treat ment for the finer and heavier material. These devices are used mostly in hydraulic mining. The riffles are taken out periodically, the gold washed from the concentrates by rocker and pan and the mercury separated from the gold by retorting.
Hydraulic Mining.— Hydraulic mining was introduced in California in 1852, to deal with huge deposits of low-grade gold bearing gravel favorably situated for obtaining the water necessary for washing, and also for the dis posal of tailings or debris. It is necessary that abundant water supply should be available. Hydraulic mining differs from large scale sluicing only in that instead of using pick and shovel for mining and dumping the gravel into the gold saving sluices, the water excavates the gravel and debris and conveys them into large and strongly constructed sluices. Water is brought to the gravel deposits in ditches and flumes from reservoirs or streams many miles distant and at such a height that it can be used under a head of several hundred feet. It is conducted in pipes to "monitors" which throw great jets of water on the gravel banks, some of which are from 300 to 400 feet high. The nozzles of the larger monitors are from 8 to 10 inches in diameter and throw a stream of from 3,000 to 5,000 cubic feet per minute. The gravel banks are prepared for the washing by a loos ening by explosives. These are used in a T sys tem of small tunnels driven into the banks, the entering one being used for tamping, and the cross drifts for the reception of a heavy charge of black powder or low-grade dynamite sufficient to shake up but not scatter the gravel. After the jets of water haveplayed upon the banks for a while, large boulders and lumps of pipe clay remain which have to be drilled and shattered by explosives to allow the water to convey them to the sluices. To wash a cubic yard of gravel, from 500 to 1,000 cubic feet of water is required. Under most favorable natural conditions and suitable equipment, operating costs have been as low as three cents per yard, but for most deposits, large outlays for the purchase of rights, storage and con veyance of the water, and for sluices and tun nels, etc., are necessary to yield satisfactory results. In 1882 all hydraulic mining in Caa fornia dumping tailings into the Sacramento River was enjoined as detrimental to the navi gation of the river and the °slikens" or fine sediment assumed injurious to the farm lands in the valley. This has been a serious set back to hydraulic mining.