placer deposits

placer deposits A placer is a surficial mineral deposit formed by mechanical concentration of mineral particles from weathered rock debris. The concentrating medium can be either flowing air or water. Placer mining, which is the most ancient kind of mining, involves the extraction and concentration of minerals by the use of running water. Placer deposits supply a considerable fraction of the world's production of tin, titanium, gold, platinum, diamond, ruby, sapphire, and other mineral commodities. The economically most important minerals concentrated in placers are listed in Table 1.

For placer concentration to occur, a mineral must be resistant to chemical decomposition, and either tough or malleable so that it will not become fragmented during weathering and transport; the mineral being concentrated must also be more dense than the minerals with which it occurs. The common rock-forming minerals, quartz, feldspar, mica, amphibole, and pyroxene, all have densities equal or below 3.3 cm⁻³, which is less than all the minerals listed in Table 1.

Concentration of dense mineral particles happens because the less dense particles are removed by flowing air or water, leaving behind an enrichment of the more dense minerals. The environments in which concentration can happen are numerous, but can be classified in four categories:(1) Colluvial-slope deposits are those in which a blanket of weathered rock slowly creeps down-slope; less dense particles are washed down-slope more rapidly than dense particles, and so the dense particles become concentrated at the base of the weathering profile.(2) Alluvial deposits are those formed in stream gravels or in delta sands by the action of flowing water.(3) Marine deposits are formed along seashores by sea water flowing in longshore currents.(4) Aeolian deposits are those concentrated by flowing air, and are found either in shoreline dunes or in desert areas.Placer deposits can be of any geological age but most are geologically young. Placers of great antiquity are called paleoplacers, and one special group of paleoplacers, the gold deposits the Witwatersrand in South Africa, are about 2.7 billion years (2.7 × 10⁹ years) old. The Witwatersrand deposits are an enigma because they are much larger than any other placers of any age. In addition to size, there are many other features of the Witwatersrand that are puzzling: for example, the primary source of the gold is not known; details of how the gold was transported to its final resting-place are conjectural; and the fact that the gold is associated with pyrite (FeS₂), and uraninite (U₃O₈), two minerals which oxidize in today's surface waters and do not become concentrated in placers, suggests an environment at the time of formation that was very different from today's environment. For example, the presence of pyrite and uraninite can be explained if the atmosphere and surface waters 2.7 billion years ago contained little or no oxygen, a possibility that is supported by several lines of evidence.

Table 1 Minerals of economic importance recovered from placer deposits

Mineral	Composition	Density1
1Densities vary somewhat because of atomic substitution by different elements, e.g. silver for gold.
Gold	Au	15–19 g cm−3
Platinum	Pt	14–19
Cassiterite	SnO2	6.8–7.1
Monazite	CePO4	5
Ilmenite	FeTiO3	4.7
Zircon	ZrSiO4	4.7
Rutile	TiO2	4.2
Sapphire and ruby	Al2O3	4.0
Diamond	C	3.5

Brian J. Skinner