Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/769

Rh GOLD Since 1851 the yield has been very largely increased by the discovery of the Australian and Californian sources, the annual averages being—- In 1851--1855 ........................... ..6,350,180 ounces ,, 1856 -1860 ........................... ..6,62-1,850 ,, ,, 1861-1866 ......................... ..5,951,770 ,, ,, 1866-1870 ........................... ..6,169,660 ,, ,, 1s71—1s75....... . ......5,487,400 ,, Proportion of Gold in Deposits.—A rich gold-bearing deposit is quantitatively very different from one to which the same term is applied when containing ores of other metals. In the latter the useful material must as a rule form a considerable proportion—one or more parts in a hundred-—-of the mass ; while in the former, owing to the superior value of the product, it rarely attains as much as l per cent., and is generally very much less, the amount of gold contained in easily worked alluvial deposits being often extremely small. For example, the yield of the Siberian gold washings ranges from 12 grains to l dwt. 12 grains per ton ,‘ while in the lodes, which are more difﬁcult and expensive to work, the proportion is about 8 dwts. per ton. In the alluvial washings of California it is estimated at about two shillings worth, equal to about 3,16-tlt of an ounce, per ton of gravel. In Australia the alluvial ground worked in the colony of Victoria in 1878 is returned as averaging 25 grains (1 dwt. 1 gr.) per ten, or about double the above quantity. In vein mining, which is more diflicult and costly, a ltrger yield is necessary, but probably 5 dwts., or about .9]. in value per ton, will in most places represent paying quantities from quartz containing free gold, 2'.e., not asso- ciated with pyrites. The proportional yield and quantities of the different kinds of auriferous materials treated in the colony of Victoria during the last three months of 1878 were— T0ns_ Yield per ton. oz. dwt. gr. Alluvial sand “ washdirt” ............. .. 173379 1 1 59'6 (jcment (gravel) requiring crushing....5871 4 ‘21'4 Quartz ..................................... .. 2'2’) 775 9 21 Quartz tailings. . . . . . . . . . . . . . . . . . . . . . . .. 11'139 . 1 18 l’yrites and blanketing (ore col- 1,599 2 6 13,7 lo.-ctcd on blanket tables) ........ .. In the less tractable minerals, such as arscnical pyrites occurring in the lower portions of the veins, as much as to 3 oz. may be required for proﬁtable working. When associated with the ores of other metals, such as silver, lead, and copper, the extraction of the gold is in most cases an incidental and ﬁnal operation in their metal- lurgical treatment, and may therefore be best considered in the articles on these metals. .l[inin_q.—Tl1e various deposits of gold may be divided into two classes——“ veins ” and “ placers.” The vein min- ing of gold does not greatly differ from that of similar deposits of metals. It will only be necessary to refer here to certain details of the extraction of gold in such cases. In the placer or alluvial deposits, the precious metal is found usually in a water-worn condition imbedded in earthy matter, and the method of working all such deposits is based on the disintegration of the earthy matter by the action of a stream of water, which washes away the lighter portions and leaves the denser gold. In alluvial deposits the richest ground is usually found in contact with the “ bed rock"; and, when the overlying cover of gravel is very thick, or, as sometimes happens, when the older gravel is covered with a ﬂow of basalt, regular mining by shafts and levels, as in what are known as tunnel—claims_. may be required to reach the auriferous ground. In the early days of gold washing in California and Australia, when rich 1 1 dwt. per ton corresponds to 1 part in 653,333 by weight, and about 1 in 5 or 6 millions by volume. 7-15 alluvial deposits were common at the surface, the most simple appliances sufﬁced ; the most characteristic being the “ pan,” a circular dish of sheet-iron with sloping sides about 13 or 14 inches in diameter. The pan, about two- thirds ﬁlled with the “pay dirt” to be washed, is held in the stream or in a hole ﬁlled with water. The miner, after separating the larger stones by hand, imparts a gyratory motion to the pan by a combination of shaking and twisting movements which it is impossible to describe exactly, so as to keep its contents suspended in the stream of water, which carries away the bulk of the lighter material, leaving a black residue consisting of magnetic iron ore and other heavy minerals, together with any gold which may originally have been present in the mass. The washing is repeated until enough of the enriched sand is collected, when the gold is ﬁnally recovered by careful washing or “panning out ” in a smaller pan. In Mexico and South America, instead of the pan, a wooden dish or trough, variously shaped in different districts, and known as “ batea,” is used. The “ cradle,” a simple appliance for treating somewhat larger quantities, varies in length from 3 feet 6 inches to 7 feet, but the shorter length is that usually adopted. Its nature will be evident from ﬁg. 1, in which a is a movable hopper with a perforated bottom of Hi sheet-iron in which the ’* “ pay dirt” is placed. Water is poured on the dirt, and the rocking motion imparted to c the cradle causes the U ﬁner particles to pass through the holes in F1“ 1"Cr"-Cue‘ the hopper on to the screen (7, which is of canvas, and thence to the base of the cradle, where the auriferous par- ticles accumulate on the transverse bars of wood c, called “riffles.” Washing by the cradle, which is now but little used except in preliminary workings, is tedious and expensive. The “tom” is a sort of cradle with an extended sluice placed on an incline of about 1 foot in 12. The upper end contains a perforated riddle plate which is placed directly over the riflle box, and under certain circumstances mercury may be placed behind the riffles. Copper plates amal- gamated with mercury are also used when the gold is very ﬁne, and even in some instances amalgamated silver coins have been used for the same purpose. Sometimes the stud is disintegrated with water in a “ puddling machine,” which is used, especially in Australia, when the earthy matters are tenacious and water scarce. The machine frequently resembles a brickmaker’s wash-mill, and is worked by horse or steam power. In workings on alarger scale, where the supply of water is abundant, as in California, sluices are generally employed. They are shallow troughs about l2 feet long, about 16 to 20 inches wide, and 1 foot in depth. The troughs taper / 4/g/,,:::7 ;/g//,’¢'.-,’//é/.///’,.//,-’_/;:/// '-, 5 ///,/.., ..,,»:,//A ////// ///17///V ’ FIG. 2. —Sluice. slightly so that they can be joined in series, the total length often reaching several hundred feet. The incline of the sluice varies with the conformation of the ground and the tenacity of the stuff to be washed, from 1 in 16 to 1_1I1 8- Fig. 2 represents one of the simplest fxornis of sluice as 4. — 94