Gold grade

The term gold grade is commonly confused with gold purity, with the two meaning something quite different.

Gold grade is a term used in gold mining, and should be used as a measure of the quality of gold ore – that is the raw material obtained from mining.

Gold purity measures the amount of natural gold mixed with other metals in finished gold products or, more strictly, gold alloy products such as jewellery.

The grade of mined, raw gold ore is measured in grams of gold per tonne of ore. High-grade gold ore contains more gold per tonne; this means that more gold is obtained from every tonne mined. This greatly increases the profitability of a mine. Indeed, the high cost of processing extremely low-grade gold ore may actually make mining financially unfeasible.

There are other key factors besides the grade of gold ore that contributes to mining feasibility. The depth of the ore deposits, and the composition of the ore, will also affect whether a company decides to mine.

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Underground and open-pit mining

Underground mining is far more costly than open-pit mining. It follows, then, that deep deposits of low grade ore may not be worth an underground mining operation. The same quality ore grade near the surface however could be deemed profitable for open-pit excavation.

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Composition of ore

It is rare that ore will contain only one valuable metal. The common conception of a gold mine exclusively producing gold is far from true. The majority of mines extract polymetallic ore - a mix of various metals.

Gold deposits are, for instance, often found with other base and precious metals, like silver in electrum ore. This is relatively easy to process and separate. By contrast, volcanogenic-massive-sulphide ore – or VMS – is made up of lead, zinc, copper, gold, and silver. This makes it very complex to process.

The high value of silver and gold in electrum may be profitably mined. There is by comparison little profit to be made from the zinc and copper in VMS processing, reducing its profitability.

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The highest grade of gold

The grade of gold, and composition of the ore that can be practically excavated, are different for open-pit and underground mines. The grams per tonne for a low grade open-pit is then different for underground mining.

Technically speaking, the highest grade of gold would be nuggets, which are some of the only naturally occurring deposits of gold. Even these tend not to be pure 24 carat gold, but are certainly the highest grade of gold found naturally.

For underground mining, the World Gold Council defines between 8 and 10 grams per tonne as ‘high-quality’, and 1 to 4 grams per tonne as ‘low-quality’. There is no exact definition of what is considered as high or low grade. The list below is therefore a rough guide to what may be expected.

Open Pit:

Low grade	0 - 0.5 gold grams per tonne
Average grade	0.5 - 1.5 gold grams per tonne
High grade	1.5 + gold grams per tonne

Underground Mine:

Low grade	0 - 5 gold grams per tonne
Average grade	5 - 8 gold grams per tonne
High grade	8 + gold grams per tonne
'Bonanza' grade	Troy Ounces (31.1 grams) per tonne

Reliable up-to-date figures for grades of gold produced by various mines are difficult to find. The US Fire Creek underground mine in Battle Mountain, Nevada, has consistently produced high grade ore. In 2015, ore from the mine had an impressive density of 44.1 grams per tonne, putting it very high on the scale, and potentially the highest grade of gold currently being mined.

In the same year, the Macassa mine in Kirkland Lake, Ontario came second, and far below with 22.2 grams per tonne.

The grade of gold being mined however, is only one of many factors that impacts upon the profitability of a mine. Besides the composition and depth of the ore there are also labour costs, technological investment and management factors to consider. Plus, mines are subject to many national and political influences.

• Gold grade defines the amount of gold in gold ore
• The gold grades are different for open-pit and underground mines
• Gold grade is a major factor that determines the feasibility of mining