Gold mines and gold deposits are often categorized by the average proportion of gold contained in the ore at the site. This is also known as the ore grade. When gold ore has a high grade, it has a high density of gold within the ore.
While higher-quality mines have higher ore grades, lower-quality mines have lower grades.
Mines with ore that has a lower grade due to lower gold density can still be profitable if less effort and, therefore, cost is required to extract the gold.
- All precious metals are graded by the amount of metal within the ore.
- A higher grade means that the ore has a higher density of gold.
- Gold ore density is measured by grams per tonne, or g/t.
- Higher quality mines have densities of 8 to 10 g/t while lower quality mines have densities of 1 to 4 g/t.
- Some of the world's highest-grade mines have been in the U.S., Russia, and Peru.
Understanding Gold Mine Grading
All precious metals are graded by the proportion of metal in the ore. Grams per tonne of milled ore, or g/t, is the most common metric used to represent the grade of ore.
Precious metals are found in a wide range of geological sites. They show up in open pits, underground, under bodies of water, and in single nuggets resting on the ground. Grading is usually only applied to open pits and underground deposits.
Gold grade and gold purity are two different things. Grade refers to the quality of the gold ore (high-grade ores have more gold). Purity refers to the amount of gold in finished products.
Gold Ore Density
A high-quality underground mine has a gold ore density between 8 and 10 g/t, while a low-quality underground mine has a gold ore density of 1 to 4 g/t.
Open-pit mines tend to have a lower grade, but they can be considered very valuable because of the lower average operating costs involved in obtaining the gold. Cost per ounce rather than gold ore grading is a recommended way to evaluate a gold mine.
Based on the g/t standard, some of the world's high-grade gold mines have been located in the United States, Russia, and Peru. For instance, the Fire Creek mine in Battle Mountain, Nev. was reported to have one of the highest grades at 44.1 g/t. Others making the list include the Macassa mine in Canada, the Kedrovka mine in Russia, and the Toguraci mine in Indonesia.
How Is Gold Purity Rated in Karats?
The purity of finished gold (e.g., jewelry) is often related in karats. A karat represents a 1/24th unit of the metal present in the alloy. Therefore, 24-karat gold is the purest (all 24 units are composed of gold) and is rated as 99.5% or higher pure gold. On the other hand, 18-karat gold is 75% gold and 25% other alloy metal such as tin or copper.
What Are the Biggest Gold Producing Nations?
Historically, most of mined gold has come from China, Australia, South Africa, and the United States.
How Much Gold Has Ever Been Discovered?
All the gold that has ever been mined, plus proven discovered reserves, amounts to around 244,000 metric tons of the metal. If all of that were melted down and fashioned into a cube, it would measure 28 meters (92 feet) per side, which could fit inside an Olympic-sized swimming pool.