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compiled by R. James Weick, 1994
Gold, the noblest of metals, has been used by man for more than
5000 years. Its extreme softness or malleability, and resistance to
tarnish (oxidation), led to its earliest uses in art and currency. Gold
is the metal of choice for jewellery, and is often used in dentistry.
Gold has also been used successfully in many modern technological
applications. It is used as the electrical contacts of computer chips.
Minute quantities of gold (less than 3 micrograms) are vaporized to
mirror lens surfaces. The intrinsic value of gold offers an attractive
alternative to stocks and bonds for many investors. It remains the
principle medium for setting currency values and settling international
debts among the nations of the world.
Minor concentrations of
gold occurs in most natural substances. In seawater, for example, there
is approximately 0.012 parts per billion (ppb) of gold, and in fresh
water it is slightly higher at 0.02 ppb. Its average concentration in
the Earth's crust or lithosphere is approximately 5 ppb, and in certain
sedimentary rocks it may achieve concentrations of up to 2100 ppb or 2.1
parts per million (ppm). At these concentrations 20 or 30 tons of rock
must be processed to extract a single ounce of precious gold. As a
result, gold can only be mined profitably where it is highly
concentrated by natural chemical and physical processes.
Gold
occurs in many different geologic settings and its classification into
deposit types is complicated. However, two basic types of occurrences
or deposits are recognized, primary and secondary. Both rely on similar
chemical and physical processes to produce economic concentrations of
gold ore. Primary deposits form where gold precipitates during chemical
reactions between hydrothermal (hot fluids) mineralizing solutions
(metal-bearing)and rocks in the Earth's crust. Secondary deposits form
later during the chemical and mechanical processes of weathering and
erosion, and the physical reconcentration of gold-bearing sediment into
placer deposits.
Hydrothermal deposits can be classified as
either epigenetic (deposits that form after the formation of the
surrounding rocks and other events of mineralization) or syngenitic
(deposits that form the same time as surrounding rocks). In epigenetic
hydrothermal deposits gold may occur as the principle metal or as a
secondary mineral associated with other metals, such as iron, copper,
lead and zinc. In these epigenetic hydrothermal deposits. One variety of
epigenetic deposit (epithermal gold deposits) form at temperatures below
350°C by the convective circulation of fluids to depths of approximately
2 kilometres, usually near hot igneous bodies or plutons in volcanically
active regions. In this type of hydrothermal occurrence gold is
generally at relatively low concentrations. Hot springs are modern
examples of this type of mineralization process. Mesothermal gold
deposits, which form at temperatures above 350°C, occur along large
breaks or faults in continental crust. The origin of these is not
certain, but they form at depths of 3 to 5 kilometres below the Earth's
crust, and appear to be associated with the upward migration of fluids
from the Earth's mantle.
Gold is often extracted as a by-product
during smelting from volcanogenic massive sulphide (VMS) deposits
(syngenetic hyrdothermal deposits) which are generated by the
accumulation of metal-rich sediments near active volcanic centres on the
seafloor. Gold is also found in porphyry copper deposits, high volume
(up to 1000 million tons), low grade (0.7% Cu) deposits, formed by the
circulation of fluids through the Earth's crust during the volcanic
activity related to mountain building above active subduction zones.
Secondary gold occurrences or placer deposits are formed by the
deposition and reconcentration of gold-bearing sediments from primary
gold occurrences. Placer deposits are generally classified according to
their depositional environment. Marine placers occur offshore near
coastlines; fluvial placers occur in river and stream valleys in the
drainage basins which contain primary gold occurrences upstream. Some
studies suggest that gold is not only mechanically transported in
placers, but that it is also chemically transported. The unusual size
and purity of nuggets in some placer deposits supports this theory for
gold transportation.
Why is it important to continue the search
for gold and other metals? Aside from the obvious financial benefits
associated with the discovery and development of mineral deposits to
mining companies, there are many benefits to communities fortunate
enough to be located near producing mines. Exploration and mine
development are activities that create jobs. They require highly
trained professionals, and skilled technical personnel that may be found
in local communities. In addition to the manpower and labour
requirements, mineral property and mine development activities often
require additional materials and specialized technical services. These
are often provided by geologic and mining engineering companies, who
locate offices in local communities to participate in exploration and
mine development contracts. The economic "spin-offs" to communities
from these activities are often significant. Resource-based activities,
including mining and exploration often serve as the base for local and
regional economies.
Where is the gold in Newfoundland? Despite an
extensive exploration and mining history, gold exploration is a
relatively new activity in Newfoundland and Labrador. While the
gold-bearing base metal VMS deposits, such as at the former Buchans and
Rambler mines, are well known, exploration over the last two decades has
resulted in the recognition of numerous epigenetic gold deposits and
prospects in the province. Important recent discoveries include the Hope
Brook Mine, near Burgeo, and the Pine Cove and Nugget Pond deposits,
near Baie Verte. There is also excellent potential for the discovery of
marine placers in regions, such as the Baie Verte Peninsula, with an
abundance of primary base metal and gold deposits. The potential for
fluvial placers has not been investigated, but these may be discovered
in some of the larger river basins.
To summarize, recent
exploration in Newfoundland and Labrador has resulted in the discovery
of several new, significant gold prospects. While many of these are
sub-economic, a few have been successfully developed as mines. The
current economic recovery in Canada, and increases in the market price
of gold have already resulted in increases in the level of prospecting
and exploration in Newfoundland and Labrador, activities which may lead
to the development of these gold deposits. Gold should be seriously
considered as a mineral commodity of great importance to the development
and economic diversification of Newfoundland and Labrador.
This is the ancient alchemic symbol for gold. Innumerable experiments
which were focused on transforming base metals and other materials to
gold made significant contributions to the science of chemistry.
Further Reading
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Boyle, R. W.
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1979: The geochemistry of gold and its deposits. Geological Survey of
Canada, Bulletin 280, 583 pages.
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Brimhall, G.
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1991: The genesis of ores. Scientific American, May, p. 84-91.
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Rona, P. A.
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1992: Deep-sea geysers of the Atlantic. National Geographic, vol. 184,
no. 3, p. 105-109.
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Roberts, R. G., and Sheahan, P. A.
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1988: Ore deposit models. Geoscience Canada Reprint, Series 3, 194 pages.
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Swinden, H. S.
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1991: Regional geology and metallogeny of Central Newfoundland. In
Swinden, H. S., Evans, D. W. T., and Kean, B. F. (editors) Metallogenic
framework of base and precious metal deposits, Central and Western
Newfoundland (Field Trip 1), Geological Survey of Canada Open File 2156,
p. 7-19.
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Tuach, J.
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1990: List of gold occurrences and deposits in Newfoundland.
Newfoundland Department of Mines and energy, Open File 1928, 72 pages.
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