Natural Resources

Gold Occurences

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.

Gold symbol

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

Boyle, R. W.

1979: The geochemistry of gold and its deposits. Geological Survey of Canada, Bulletin 280, 583 pages.

Brimhall, G.

1991: The genesis of ores. Scientific American, May, p. 84-91.

Rona, P. A.

1992: Deep-sea geysers of the Atlantic. National Geographic, vol. 184, no. 3, p. 105-109.

Roberts, R. G., and Sheahan, P. A.

1988: Ore deposit models. Geoscience Canada Reprint, Series 3, 194 pages.

Swinden, H. S.

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.

Tuach, J.

1990: List of gold occurrences and deposits in Newfoundland. Newfoundland Department of Mines and energy, Open File 1928, 72 pages.

 
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