Gold Ores and Placers

natural mineral formations in which the content and total quantity of gold are sufficient for economically profitable extraction of the metal. In ores, gold occurs mostly in the free (native) form. It is not chemically pure; rather, it is a solid solution with other metals (primarily silver). In addition, gold in ores is present in the form of tellurides (for example, calaverite AuTe2), although these are not of great industrial importance.

Gold deposits may be primary- or placer-type. Primary deposits are represented by veins, vein systems, beds, and vein-impregnated ore zones of various sizes and shapes. Ore bodies are located in fissures, shatter zones, and schist-forming zones. They range in thickness from 0.05 m to dozens of meters and may be dozens or even thousands of meters in length. The largest zones are the Kolar vein zone in India (length, 20 km; mined to a depth of 3.2 km; average vein thickness, 1.2 m), the Mother Lode vein system in the USA (length, 200 km), several vein zones of the Yenisei Ridge, and the mineralized zones of the Muruntau deposit in Middle Asia. Gold deposits exist in folding zones, platforms, and areas of tectonic activity.

Gold ore deposits were formed during various geological epochs (from the Precambrian to the Cenozoic) at various depths (from dozens of meters to 4–5 km below the earth’s surface). The process usually occurred in connection with large fractures in the earth’s crust, forming so-called auriferous (gold-bearing) zones. The origin of these zones is associated basically with the activities of thermal springs.

The mineral composition of gold ores is diverse: quartz and iron sulfides (pyrite, marcasite) are predominant, while arsenic (arsenopyrite) is less common; sulfides and sulfosalts of copper, lead, zinc, bismuth, antimony, silver, oxides, and carbonates are also present. The following formations are distinguished according to composition and the conditions of formation: (1) gold-sulfide-quartz, represented by quartz veins and streaks with 0.5 to 30 percent sulfide content; the gold is fine and coarse (nuggets are also found), not uniformly distributed, its content varying from 10–50 to 1,000 g per ton (USSR, Canada, USA, Brazil, India, Australia, Ghana, Southern Rhodesia); (2) substantially sulfide, represented by beds, streak zones, and impregnated mineralization; the gold is finely dispersed, its content rarely exceeding 1–2 g per ton; it is extracted by side recovery with other metals, such as copper, zinc, lead, and nickel (Tanzania, Namibia, Canada, Mexico, and Australia).

Ore oxidation zones (gossans) of some copper pyrite and polymetallic deposits are enriched with gold redeposited from primary ores and are independent objects for the extraction of the redeposited gold. Most of these deposits are found in the USSR (in the Urals and Kazakhstan).

Gold placers are loose (seldom cemented) gold-bearing deposits of detrital material that were formed as a result of the destruction of primary deposits and auriferous rocks. The following basic types of placers are distinguished, according to conditions of formation: eluvial (residual), diluvial, alluvial, littoral, submarine, and lacustrine. Alluvial placers, including recent channel-fill, valley, and bench placers and ancient buried placers, have the greatest industrial value. The length of placers ranges from 1–3 to 25 km (sometimes to 50–100 km), the width, from 1 m to 200–300 m (occasionally to 1 km and more); and the thickness, from 1 to 3 m. The gold content varies from tenths of a gram per cu m to tens of kg per cu m. Gold placers were formed in various geologic epochs. Ancient placers are often buried under younger sediments and lie at depths of 100–150 m below the surface of the earth.

The richest gold-bearing placers known abroad are in Canada (the Yukon and Klondike river basins), the USA (Alaska and California), Colombia, Australia, New Zealand, New Guinea, and the Philippines; in the USSR, gold placers occur in the Yenisei, Lena, Bodaibo, Vitim, Aldan, Kolyma, Yana, and Indigirka river basins.

Metamorphosed placers (auriferous conglomerates and, less commonly, gritstones) constitute a special type of gold deposit. Witwatersrand (in the Republic of South Africa), the largest gold deposit in the world, is of this type.