Gold Placers & their formations

Placers gold ores contain alluvial, eluvial or colluvial material in active ore deposit-forming systems and have been classified as deposits where diagenetic processes have occurred to only a limit extent. Ore crushing and grinding is unlikely to be required in the treatment of such ores. Hard rock palaleo-placers, e.g. Witwatersrand ores have been classed as free milling ores.

Placer deposits are formed by the liberation of gold and hydraulic transport of gold particles away from primary gold deposit. This can be possible because gold is chemically inert and dense; resulting in accumulations of gold relatively close to exposed primary deposits. The prerequisites for the formation of placers include:

• · A primary source of gold, e.g. gold-quartz veins, auriferous sulphide deposits or former placers.
• · A long period of chemical and physical weathering to release gold grains from the host rock.
• · Concentration of gold particles by gravity, almost certainly involving moving water as the transportation medium.
• · Stable bedrock and surface conditions over a long period, e.g. no glaciation or folding, to allow significant concentrations of gold to accumulate.

There are several classes of placer ores which relate to the means of gold concentration and the distance from the primary gold deposit. These are listed as follows:

Eluvial (or residual) placers, which usually overly or are located at or very near the parent deposit (Fig 2.1). Eluvial placer consists of weathered rock from which some of the finer and lighter minerals have been washed away, leaving gold at higher concentration. Prolonged mechanical erosion by water has not occurred and therefore the gold grade is lower than other placer types. Eluvial gold deposits in tropical regions are commonly lateritizied (with the host rock weathered to form hydrated iron and aluminium oxides with silica).

Colluvial (or deluvial) placers. In which gold has been transported some distance from the parent deposit, but where the gold is not located in an established stream system, e.g. on the slopes surrounding outcropping source rocks.

Fluvial (or alluvial) placers are those in which gold occurs in stream or river systems. The gold tends to concentrate upstream of obstructions and in areas of lower fluid velocity. Examples of gold concentrations processes in fluvial placers are given in Fig. 2.2.

Marine placers are formed by the natural sorting action of a beach environment which concentrates the valuable minerals. Gold in such deposits is often associated with other dense minerals such as iron (magnetite), titanium (ilmenite, rutile) or tin (cassiterite) minerals. Gold accumulations can occur in beach terraces as a result of a drop in sea level relative to the land mass, e.g. South Island (New Zealand), or are presented as submerged alluvials resulting from a net increase in sea level