Vein-mining for gold differs but little from working any other kind of metalliferous lode. When the vein-stuff has been raised it is reduced to a pulverulent condition, to liberate the gold from the gangue. In some cases roasting is first resorted to. This causes friability, and facilitates the subsequent comminution. When the gold is in a very fine state, too, it helps it to agglomerate. But if any pyrites are present the effect is most detrimental, the gold becoming coated with a film of sulphur or a glazing of iron oxide. The powdering of the vein-stuff is usually performed in stamp batteries, which consist of a number of falling hammers. While simple in principle, the apparatus is complicated in its working parts, and is probably destined to give way to the improved forms of crushing-rolls and centrifugal roller mills, which are less costly, simpler, more efficient, and do not flatten the gold particles so much. One of the most effective is that by Jordan. When the vein-stuff has been reduced to powder, it is akin to alluvial wash-dirt, and demands the same or similar contrivances for arresting the liberated gold and releasing the tailings--that is, mercury troughs, amalgamated plates, blanket strakes, &c.; but, in addition, provision is made for catching the other metalliferous constituents, such as pyrites, which almost always carry a valuable percentage of gold. These pyrites or 'sulphurets' are cleansed by concentration in various kinds of apparatus, all depending on the greater specific gravity of the portion sought to be saved.
Of the metals and minerals with which gold is found intimately associated in nature are the following: Antimony, arsenic, bismuth, cobalt, copper, iridium, iron, lead, manganese, nickel, osmium, palladium, platinum, selenium, silver, tellurium, tungsten, vanadium, and zinc, often as an alloy in the case of palladium, platinum, selenium, silver (always), and tellurium. The methods of separation vary with the nature of the ore and the conditions of the locality.
TREATMENT OF ORE AND GOLD IN THE TRANSVAAL.
The method of treatment of ore and gold in the Transvaal, the most perfect and effective known at the present time, has thus been described by Arthur Stenhouse: