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Ore grade

Also known as
head grade, cut-off grade

Ore grade is the concentration of a valuable metal or mineral in ore, usually expressed as a percentage for base metals such as copper, or in grams per metric ton for precious metals such as gold. It determines how much rock must be mined and processed to produce each unit of metal.

What is ore grade?

Ore grade measures how rich a rock is in the metal or mineral being mined. Base metal grades are usually given as a percentage by weight: an ore grading 0.5 percent copper contains 5 kilograms of copper in each metric ton of rock. Precious metal grades are given in grams per metric ton. The rest of the rock is gangue.

Related terms

  • Head grade: the grade of the ore actually fed to the processing plant.
  • Cut-off grade: the lowest grade at which rock is worth processing as ore rather than treating as waste, which depends on metal prices, costs and recovery.
  • Concentrate grade: the grade of the product after processing, far higher than the ore grade.

Why ore grade matters

Grade drives mining economics. The lower the grade, the more rock must be mined, crushed, ground and processed, and the more tailings must be stored, for every unit of metal produced. Energy, water and reagent use rise accordingly.

Declining ore grades

Average grades at many mines, notably for copper, have fallen over time as the richest, most accessible deposits have been mined. Lower-grade and more complex ores often have finer mineral grains and more variable mineralogy, which makes mineral liberation harder and puts more weight on process efficiency. Small improvements in metallurgical recovery become more valuable as grades fall.

Frequently asked questions

Ore grade is the metal content of the rock going into the plant. Concentrate grade is the metal content of the product coming out, after gangue has been removed.

It is the minimum grade at which material is economically worth processing. Rock above it is ore; rock below it is waste. It changes with metal prices and costs.

  • Gangue

    Gangue is the commercially worthless material, usually minerals such as quartz, silicates, carbonates or unwanted sulfides, that surrounds or is mixed with the valuable minerals in an ore. Mineral processing separates the valuable minerals from the gangue.

  • Concentrate grade

    Concentrate grade is the concentration of the valuable metal or mineral in the concentrate produced by a mineral processing plant, usually expressed as a percentage by weight, such as percent copper. Higher grade means less gangue travels with each metric ton of metal.

  • Mineral liberation

    Mineral liberation is the degree to which grains of a valuable mineral have been freed from the surrounding gangue by crushing and grinding. Only liberated or mostly exposed mineral can be separated efficiently by flotation and other surface-based processes.

Sources

  1. Mineral commodity resources and terminology, U.S. Geological Survey
  2. SME Mineral Processing and Extractive Metallurgy Handbook, Society for Mining, Metallurgy and Exploration, 2019