Mining Dilution in Gold Projects: Why More Tonnes Can Mean Less Grade
Mining dilution occurs when waste or lower-grade material is extracted with ore and sent into the production stream. The added tonnes can make a mine appear busier while reducing the average gold grade delivered to the plant. For investors, dilution matters because it can affect processing capacity, unit costs, recovery, reserve conversion and the number of ounces ultimately produced.
This evergreen educational article does not describe GBDS resources, reserves, grades, dilution assumptions or operations. Public-company examples are dated illustrations from primary filings.
Dilution and ore loss are different
Dilution adds material that was not part of the intended ore—or adds material below the planned cut-off grade. Ore loss is valuable material that the mining process leaves behind or sends to waste. Both can reduce recovered ounces, but they affect the calculation differently. Investors should look for both assumptions instead of relying on a single “mining recovery” percentage.
The CIM Mineral Resources and Mineral Reserves guidance library and the SME Guide for reporting exploration results, mineral resources and mineral reserves provide the broader reporting framework: reserves represent the economically mineable portion after relevant modifying factors and allowances are considered.
The grade assigned to dilution matters
Diluting material is not always barren. It may contain low-grade gold, or it may be modeled conservatively at zero grade. Those choices can materially change the diluted reserve grade. Investors should ask whether the estimate uses internal dilution within the mineralized envelope, external waste outside it, operational overbreak, or a combination.
A January 2026 Donlin Gold technical report summary filed with the SEC described total dilution of 29% at 0.15 grams per tonne gold and separately quantified mine losses. By contrast, a March 2026 Haile Gold Mine technical report applied 10% zero-grade dilution to certain underground stopes, 15% to horizontal development, and a separate 94% stope recovery factor. These are project-specific methods, not industry benchmarks.
Hypothetical dilution example
Simplified hypothetical example: A mine plans to extract 100,000 tonnes of ore grading 4.0 grams per tonne. Before mining loss or plant recovery, that material contains 400,000 grams, or approximately 12,860 troy ounces of gold. If 10,000 tonnes of zero-grade waste are mixed in, the mill receives 110,000 tonnes at about 3.64 grams per tonne. Contained gold is unchanged, but the plant must process 10% more material for the same theoretical ounces.
If the operation also loses 5% of the planned ore, it instead sends 95,000 ore tonnes plus 10,000 waste tonnes to the plant. The blended grade falls to about 3.62 grams per tonne and contained gold falls to approximately 12,217 ounces. At a hypothetical 90% metallurgical recovery, recovered gold would be about 10,995 ounces. The example excludes stockpiles, moisture, payability, variable recovery and other real-world factors.
Why mine geometry and method change the result
Narrow underground veins can be sensitive to small amounts of overbreak because a few extra centimetres of wall rock may be large relative to the planned mining width. Large open-pit blocks may experience dilution at ore-waste contacts or through selective-mining limits. Equipment size, bench height, blast design, survey control, grade-control sampling and operator practices all influence the outcome.
What investors should verify
Read the reserve table together with the mine plan and reconciliation discussion. Check whether tonnes and grades are diluted or undiluted, what grade is assigned to added material, how mining recovery is treated, whether percentages vary by mining area, and whether actual mill feed is reconciling with the model. A lower dilution figure is not automatically better if its calculation excludes material counted elsewhere.
Educational disclaimer: This article is for general information only and is not investment, legal, tax or technical advice. Mineral estimates and mine plans are uncertain, and actual results may differ materially. Review current technical reports, regulatory filings and qualified professional advice before making decisions.