GEOLOGY 101 — Placer gold deposits, Pt. 2

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.