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. Higher-quality mines have higher ore grades, and lower-quality mines have lower grades. When gold ore has a high grade, it takes relatively less effort to extract an ounce of gold from the ground; less ore has to be dug out, which reduces input costs for the gold mining company.

Another, less common definition of high grading refers to the concealment of precious metals ore by workers who have mined it. This definition had greater applicability and was used more frequently during the 19th century, when so-called high-graders would often hide gold ore in their lunch pails. To combat this problem, mining companies often required their employees to open their lunch pails, turn out their pockets and shower following their shifts.

Understanding Precious Metals, Gold Grading

All precious metals are graded by the proportion of metal in the ore. Grams per tonne, or g/t, is the most common metric used to represent the grade of an ore. The value of a precious metals mine is determined by its total estimated weight, the grade of the ore, and how difficult it is to extract and distribute.

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.

The World Gold Council on Grading

The standards of high- or low-grade gold ore are set by the World Gold Council, which is a non-profit organization that was established to promote the use of gold and gold products internationally. In addition to marketing and supporting gold producers, the World Gold Council also researches gold mines and creates standards by which to evaluate gold mining prospects.

The World Gold Council defines a high-quality underground mine as having a gold ore density between 8 and 10 g/t (grams per ton), 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 necessary to obtain them. The council recommends using cost per ounce, not gold ore grading, to evaluate a gold mine.

Highest Grade Gold Mines in the World

The highest-grade gold mine in the world is the Fire Creek mine in Battle Mountain, Nev., according to 2015 data. With a gold ore density of 44.1 g/t, Fire Creek falls high above Macassa, the second highest grade mine at 22.2 g/t, which is located in Kirkland Lake, Ontario. Of the 10 highest-grade gold mines, a majority are located in the U.S., Russia and Peru, according to the 2015 data.