Many folks in the U.S. celebrate St. Patrick’s Day with the retelling of the Irish myths about leprechauns and pots of gold. There is even an Irish Folklore Commission that was set up in 1935 by the Irish Government to study and collect information on the folklore, including those of leprechauns, and the traditions of Ireland. In many of those tales, leprechauns are in possession of pots of gold and hide their treasures at the end of a rainbow. However, the leprechauns are tricky and humans inevitably fail at finding the precious metal.
Many gold miners probably feel the same way. Gold remains difficult to find because it is relatively scarce in the earth and occurs in a variety of rocks and geological environments. Typical prospecting involves examining samples throughout the mine site to determine if the region is worth exploring. To enhance their chances of finding gold, geologists may use geophysical methods to measure variations in the physical properties of rocks that may indicate the presence of gold, such as density, magnetism, electrical conductivity, or natural radioactivity. Although these geophysical methods can be crucial for gold exploration, geochemical methods, including using X-ray fluorescence spectroscopy are the methods often used to measure concentrations of gold and other associated elements.
X-ray fluorescence spectroscopy (XRF) is a non-destructive analytical technique used to determine the elemental composition of materials. XRF analyzers work by measuring the fluorescent (or secondary) X-rays emitted from a sample when excited by a primary X-ray source. Each of the elements present in a sample produces a set of characteristic fluorescent X-rays, or “unique fingerprints.” These fingerprints are distinct for each element, making XRF analysis an excellent tool for quantitative and qualitative measurements.
XRF sampling techniques are aimed at mapping the distribution of gold and in particular, the various elements associated with gold known as the pathfinder elements (silver, copper, zinc, nickel, mercury, arsenic and barium). Pathfinder elements are very important in finding gold because they help focus the search area. It’s much easier to find the pathfinder elements, due to their higher concentration, than it is to find gold, and once found they can help determine if gold is nearby. Portable XRF analyzers are used in other stages of gold exploration and mining including core logging, identification of lithologies, and even grade control.
The USGS reports that about 6% of domestic gold was recovered as a byproduct of processing domestic base-metal ores, chiefly copper ores, and the top 28 operations yielded about 98% of the mined gold produced in the United States. When it comes to other countries, the USGS reports that “import of ores and concentrates over the past few years came from Canada, 89%; Greece, 9%; and Germany, 2%. Dore: Mexico, 45%; Colombia, 12%; Peru, 8%; Nicaragua, 7%; and other, 28%….An assessment of U.S. gold resources indicated 33,000 tons of gold in identified (15,000 tons) and undiscovered (18,000 tons) resources. Nearly one-quarter of the gold in undiscovered resources was estimated to be contained in porphyry copper deposits. The gold resources in the United States, however, are only a small portion of global gold resources.”
Another trick that is sometimes played on gold miners is that one of the most common sulfide minerals found in the earth, pyrite, is not a precious metal at all but it is referred to as “Fool’s Gold” because it resembles gold to the untrained eye. While pyrite has a brass-yellow color and metallic luster similar to gold, pyrite is brittle and will break rather than bend as gold does. Gold leaves a yellow streak, while pyrite’s streak is brownish black. Portable x-ray fluorescence (XRF) analyzers are also an important tool in this effort because in just a few seconds, you can identify that ‘foolish rock’.
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- Read more about pyrite and its properties
- Application note: Application of the Thermo Scientific Portable XRF Analyzer in PGE Exploration
Editor’s Note: Some parts of this article were taken from previously published articles.
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