Placer Gold Deposits, Part 1,
by DEREK WILTON
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.
Placer Gold Deposits, Part 2
by Derek Wilton
Although placer gold deposits have yielded less than 10% of Canada's gold production, their discovery played an important role in attracting settlers to remote areas of the country.
For example, the Cariboo gold rush of the 1860s led many people to the interior of British Columbia, and the Klondike gold rush of 1897-98 did the same for the Yukon. Placer gold deposits were attractive to those early settlers because of their simplicity -- they occur when ore from a bedrock source is milled and concentrated, by natural forces, in the pay streak. Because of the unconsolidated nature of the host sediment, placer gold can be separated easily through simple techniques.
Gold, including nuggets, was first recovered from placer sources thousands of years ago by sifting through sand or gravel horizons. More sophisticated techniques, based on gravity separation and gold's higher density, were developed later.
In Greek mythology, the Golden Fleece sought by Jason and the Argonauts was actually a form of ancient sluice for the separation of detrital gold grains from river gravels; gold-bearing river gravels were flooded over sheep skins, and gold grains were entrapped in the wool.
Placer gold deposits, because of their variable grades and tonnages, are difficult to develop into large commercial operations. Conversely, the ease with which gold can be collected from the sediment makes placer deposits unusual in that individual prospectors with pans can still recover economic concentrations. Gold placers are mined in Siberia, Australia, Colombia and other areas around the world. Placer production in Canada and the U.S., however, has been curtailed somewhat as a result of stricter environmental regulations.
The most typical means of placer production is mining the sediment containing the pay streak and using gravity processing to collect the gold. The sorting usually involves flushing sediment over a separator table (mechanical trap), which collects the gold. These techniques use a considerable amount of water. The mining of placer deposits occurs mainly on surface, but, in the case of deep pay streaks, shafts are sunk through sediment accumulations. With respect to fluvial systems, mining essentially digs up the stream bed. Gold grains can form a plastic mixture, called amalgam, with mercury, which is in liquid form at room temperature. At some deposits, pay streak material can be passed through mercury baths, which removes gold particles. The amalgam is then collected and the mercury driven off, leaving the gold behind. Cyanide solutions can also be percolated through placer gravels to collect gold.
Aside from primary production of gold, placers can also point the way to bedrock gold sources, which is what happened in the California gold rush of 1849.
By following fluvial placers back to their source, prospectors were able to locate richer bedrock mesothermal gold deposits. This practice has become more important in light of environmental concerns with respect to large-scale production from placer systems.
Because gold is a soft metal, the shape and size of gold found in placer deposits can vary. Detrital gold grains become more rounded the farther they travel from the source. For example, eluvial gold grains may take the form of wires reflecting the crystal or intergrowth shapes of the source gold. Abrasion and internal grinding are intrinsic to an erosional system, thus the gold grains will be worked into rounded, nodular shapes as they travel greater distances.
The revelations surrounding the scandal at the Busang deposit of Bre-X Minerals in Indonesia illustrates this principle -- the gold added to core there had the rounded, nugget shape of placer gold, which was reportedly collected from a fluvial placer system. Gold grains from a bedrock source would have had the mineralogical or crystal shapes typical of intercrystalline formation.
In Ontario, Quebec and Newfoundland, geologists have been able to locate bedrock sources of gold by following patterns in the shape of till trains, coupled with determinations of the direction of ice flow in the till. -- The author is a professor of geology at Memorial University in St. John's, Nfld.