GEOLOGY 101 — Placer gold deposits, Pt. 1

Placer gold deposits form as a result of the breakdown and weathering of existing gold concentrations, erosion of the weathered material and, ultimately, the concentration of that material at a variable distance from its source.

The term “placer” derives from the Spanish word for sand bank or stream eddy. Placer deposits, in the strictest sense, are formed in river systems, but the term is typically used to describe deposits formed in glacial and beach environments.

Placer gold deposits are formed when gold is carried from its source to its site of deposition and concentration by a surface erosional force such as rivers, glaciers, oceans and (rarely) wind.

The formation of gold placers is predicated by two fundamental physical properties of gold. To begin with, gold is dense, with a specific gravity of 19.3 grams per cubic cm — among the highest for all known minerals or native elements. Also, gold is a native element rather than a mineral (the latter being a naturally occurring inorganic chemical compound), and does not readily react with other elements.

A corollary of this second point is that gold is difficult to dissolve out of rock or minerals. The original source of the gold is unimportant, ranging, as it does, from mesothermal lode deposits to massive sulphide deposits to disseminated sulphides in bedrock to pre-existing placer systems.

Placer deposits depend on an original pre-concentration of gold which can be liberated through weathering. Eluvial, or residual, placers are a type of placer deposit in which gold has undergone little transport and actually formed on, or near, the original source through the weathering or erosion of host rock. Owing to its relative chemical inertness, gold remains behind while the surrounding material is removed, essentially concentrating the gold in the weathered remnants.

Gold in placer systems is transported as discreet grains as a result of the metal’s inertness. Such grains are said to be detrital, as they are derived from the physical weathering and breakdown of their host rock or mineral, a process known as detrition.

Because of the high density of gold grains relative to other rock and mineral material (detritus) carried in the same erosional system, the gold grains must be transported by erosional agents operating with relatively higher energy than that needed to transport normal rock detritus. When the energy exerted by the erosional agent decreases, the gold and other dense detritus will stop moving.

In the case of fluvial (or river) placer systems, detrital gold grains are concentrated in those areas where the current of the stream slows, such as on the slow sides of bends in the river, on the downstream sides of islands or near sand bars. Gold grains move when energy is exerted on them by the transporting medium. The grains will continue to move until the medium loses sufficient energy, whereupon the gold grains will settle out of the transporting medium.

An example of a fluvial placer gold deposit is a mature stream in a valley floor into which numerous subsidiary streams flow. In glacial tills, gold is transported along with other detrital material until the glacier ceases to move, dropping the gold and detritus. The driving mechanism for the formation of placer deposits, therefore, is gravity.

Another innate feature of placer gold deposits is that the material that hosts the gold is unconsolidated sediment (particulate rock that is not cemented together). The host sediment can range from gravels to sand in fluvial systems, as well as to various types of till in glacial deposits or beach sands.

A “pay streak” is the layer of sediment in a placer deposit which is enriched in particulate gold. In fluvial examples, the pay streak frequently occurs in sediments that lie directly on top of bedrock. The pay streak will also contain other dense, hard or inert minerals, such as magnetite, zircon, garnet or chromite.

There is some debate as to whether nuggets in fluvial systems represent purely detrital fragments that were rounded in transport or are the nuclei upon which dissolved gold in the stream precipitated and grew. In some instances, gold grains have greater fineness (ratio of gold to silver) towards the rim, suggesting either preferential removal of silver or precipitation of new gold.

— The author is a geology professor at Memorial University in St. John’s, Nfld.