Concentration, Grade and UnitsThe concentration of gold in a rock is key to any gold project. If the gold concentration is high, it means a relatively small amount of rock needs to be mined and processed in order to gain certain amount of gold. If the concentration of gold is low, a lot more rock would need to be mined and processed to gain the same amount of gold. The more rock that has to be mined and processed the higher the costs and lower the profits. Gold bearing rock that has enough gold concentration to be mined profitably is called 'ore'.
The gold concentration is usually referred to as the gold 'grade'. and is measured as 'grams per tonne', written alongside results as 'g/t Au'. In some cases this same ratio will be listed as 'parts per million', written as 'ppm' (ie. there are 1 million grams in 1 tonne). The average gold grade of mined ore in 2009 was 1.8 g/t Au (Schrodde, 2011). In early stage exploration the levels of gold that are significant are much smaller than 1 g/t Au because very small levels of gold may be indicative of a larger body somewhere nearby, even if it was not directly sampled. This is why stream sediment and soil sampling require results in parts per billion, ‘ppb Au’. |
The bricks of gold produced by a mine and further refined at a refinery are measured in percent gold purity. This measure is never used in exploration or mining.
Terms used to describe the gold ratio in jewellery describe the gold as a ratio compared to other metals. For example pure gold jewellery is 24 carats. 14 parts gold and 10 parts silver is 14 carat gold. These units are never used in gold exploration or mining. Curiously in the West although assay results are reported in g/t Au, resource, reserves and production figures are reported in Troy Ounces. One Troy Ounce is 31.1034768 grams. Countries of the former Soviet Union and China report resources, reserves and production tonnes of gold. |