Geology Reference
In-Depth Information
centuries mined the gold at Las Medulas in Spain after they conquered the
region in 25 BC. Similar deposits triggered the gold rushes to the USA,
Australia and the Yukon of Canada. Stories of miners making their fortune
after finding enormous nuggets attracted hundreds of thousands of prospectors
to the alluvial gold fields of California (1848-1852), Victoria (1851) and the
Klondike (1898-1899). The “Welcome Stranger”, the largest single lump of
gold ever found, measured 61 cm by 31 cm and weighed almost 70 kg. This
nugget was found accidently in a cart track in Victoria in 1869. Artisanal
mining of placer gold continues to the present day in many parts of the world,
notably in parts of South America (where the use of cyanide and mercury to
extract the gold causes major pollution of river systems).
In these areas, which are located in or adjacent to young mountain ranges,
the gold particles originally formed as lode gold deposits (Chap. 4) and were
subsequently concentrated in the beds of streams and rivers that flowed
rapidly in these hilly to mountainous terrains. Another types of placer deposit
formed through alluvial processes in desert sands in the flat dry plains around
Kalgoorlie and Coolgardie in Australia, fueling a gold rush to this region at
the end of the nineteenth century. This gold came from enormous lode-gold
deposits in the Archean greenstone belts of this region (Chap. 4), which have
been mined continuously using conventional methods since that time. Simi-
larly large and rich deposits in Canadian greenstone belts such as those around
Timmins or Kirkland Lake yielded very little placer gold and triggered only
minor gold rushes (the Porcupine gold rush in 1909-1911). This was in part
due to the hostile winter climate but mainly because continental glaciers
scoured this region in geologically recent time, removing any existing placer
deposits and leaving a subdued topography in which modern rivers flow only
sluggishly and concentrate little gold.
Young gold placers
consist of accumulations of gold particles in Quaternary and
Tertiary gravel, sand or soil, and their consolidated equivalents. Two broad types
can be distinguished; alluvial deposits, in which the gold is transported by river or
ocean currents and is separated from grains of other minerals by the action of these
currents; and eluvial deposits, in which the gold remains more or less in place at
the site of exposure of the primary deposit while the other minerals are removed.
One type of eluvial deposit forms on hill slope immediately below the exposures of
gold-bearing veins: gravity sliding or the action of wind or water removes the
lighter components leaving the denser gold particles where they are. Another type
forms on desert plains where winds or occasional floods remove the less-dense or
soluble minerals.
In alluvial placers in river or stream beds, the flowage of water concentrates gold
at locations where the velocity decreases markedly or where currents with
contrasting flow rates or flow styles (laminar or turbulent) are juxtaposed. Examples
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