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Grade and recovery are different measurements
Gold grade is a concentration: the mass of gold in a unit mass of ore. A result such as 3 grams per tonne means that, on average, one tonne of the tested material contains 3 grams of gold. The result is meaningful only if the sample represents the material being described and the laboratory method can measure its concentration reliably.
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Metallurgical recovery is a process result. It is the percentage of contained gold that reports to the products a plant considers recovered, such as a gravity concentrate, doré, a flotation concentrate, or a leach solution. The basic calculation is:
Recovery (%) = gold recovered in products ÷ gold contained in feed × 100
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A high-grade sample can have poor recovery if gold is locked in sulfide minerals or protected by carbonaceous matter. A lower-grade ore can be attractive when its gold is readily liberated and recovered. Investors, mine planners, and royalty analysts therefore need both a defensible head grade and testwork that demonstrates how much of that gold a process can actually capture.
How a mine laboratory measures grade
The laboratory workflow is designed to turn a field sample into a small, homogeneous charge that can be analyzed. Each preparation step is a potential source of bias, so laboratories document it as part of the result.
1. Receive, identify, and dry the sample
The laboratory logs the sample, checks the chain of custody, and records its condition. Moist material is dried under controlled conditions before weighing and crushing. Dry-mass reporting matters because water changes the apparent mass and can distort both grade and the subsequent metal balance.
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2. Crush and split a representative portion
The entire submitted sample is crushed to a specified size. A riffle splitter or another validated device then divides it into portions. The split used for analysis must preserve the source material’s distribution of gold; taking a convenient handful is not a substitute for representative splitting.
3. Pulverize the assay portion
The selected portion is pulverized to a fine, specified particle size. Pulverizing improves homogeneity so that the weighed analytical charge is more likely to contain the same average gold concentration as the prepared portion. Laboratories retain a coarse reject and a pulverized pulp so that questionable results can be checked or rerun.
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4. Weigh the analytical charge
A measured mass of pulp is weighed for the assay. Charge size, particle size, and the method’s detection limit are recorded with the result. For very low concentrations, the mass analyzed can determine whether the method is sensitive enough to distinguish gold from analytical background.
What happens in a fire assay
Fire assay remains a reference approach for gold in many geological materials. USGS Bulletin 1445, Methods for the Fire Assay of Noble Metals in Geological Materials, is a dedicated procedural reference. In a conventional fusion, the laboratory mixes the weighed charge with flux and heats it until the minerals melt. The flux chemistry collects precious metals in a molten lead button while separating most of the rock-forming material into slag.
Fusion, cupellation, and the bead
- Fusion: The charge and flux are heated in a crucible. Gold and silver report to a lead button at the bottom of the melt.
- Cupellation: The button is heated in a porous cupel. Lead oxidizes and is absorbed, leaving a gold-silver bead, often called a prill.
- Finish: The bead is weighed directly for gravimetric determination or dissolved and measured instrumentally, commonly by atomic absorption (AAS) or inductively coupled plasma (ICP).
Fire assay is valued because the fusion can collect gold from a relatively large charge and because the method is supported by long-established controls. ASTM E1805 describes fire-assay gravimetry as a reference and referee method for gold in copper concentrates, including metallurgical accounting, process control, and concentrate evaluation.
Instrumental alternatives and detection limits
USGS Circular 544 describes two trace-gold approaches: cyanide extraction followed by atomic-absorption measurement, and fire-assay atomic absorption after dissolving the gold-silver bead. It reports that either method can determine concentrations as low as 50 parts per billion in a 15-gram sample. That sensitivity does not mean every 15-gram sample is representative of every deposit; coarse gold, heterogeneous mineralization, and preparation errors can dominate the uncertainty even when the instrument is sensitive.
Why coarse gold can make an assay difficult
Visible or coarse native gold may be distributed unevenly through a rock. A small split can contain a gold particle and return an unusually high value, while an adjacent split misses it and returns a low value. This sampling variability is commonly called the nugget effect. Crushing and pulverizing reduce, but do not automatically eliminate, the problem.
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Screened-metallics analysis
When coarse gold is suspected, laboratories may use a screened-metallics procedure. SGS’s 2026 Geochemistry Guide describes submitting a 500-gram or 1,000-gram sample, screening it to a defined size such as 75, 106, or 212 micrometres, and assaying the entire coarse fraction. The undersize fraction is typically analyzed in duplicate. The laboratory then combines the gold calculated from each size fraction to produce a total result.
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When to request a larger or repeat sample
- Visible gold occurs in the core, chip, or reject.
- Duplicate assays differ widely without an obvious preparation explanation.
- Results show isolated extreme values surrounded by much lower assays.
- Gold occurs in veins, nuggety alluvial material, or coarse mineralized clasts.
- The economic decision is highly sensitive to a small change in grade.
A laboratory should define the sample mass, screen aperture, fraction treatment, and calculation before work starts. Simply ordering a second conventional assay may reproduce the same sampling bias.
Quality assurance and quality control (QA/QC)
A defensible assay campaign is more than a number on a certificate. It is a documented chain from field collection to interpretation. USGS’s QA/QC guidance is organized from presampling through final interpretation and warns that inadequate controls can produce erroneous results.
Controls that belong in a campaign
- Sampling plan: Define locations, intervals, mass, sample boundaries, and how material will be selected.
- Chain of custody: Record transfers, seals, identifiers, and receipt condition.
- Preparation records: Document drying, crushing, screen sizes, split ratios, pulverizer settings, and contamination-cleaning procedures.
- Blanks: Insert material expected to contain little or no gold to detect contamination or carryover.
- Certified reference materials: Include standards with known concentrations to test accuracy and drift.
- Duplicates: Use field, preparation, and pulp duplicates to separate sampling variation from laboratory variation.
- Detection limits and reruns: Record reporting limits, over-range procedures, and criteria for reanalysis.
- Data review: Investigate failed controls before accepting or using affected results.
QA/QC does not make an unrepresentative sample representative. It shows whether the laboratory and preparation process behaved as expected and identifies results that require investigation.
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How miners measure metallurgical recovery
Recovery testing starts with material that represents the ore types a plant will treat. Programs may use composites for an average response and variability samples to test changes by domain, alteration, oxidation state, or depth. The flowsheet is run under controlled conditions, and every relevant stream is assayed.
Build a gold balance
The test records feed mass and feed grade, then measures gold in each product and waste stream. Typical streams include a gravity concentrate, flotation concentrate, leach solution, residue, and final tailings. Results should reconcile on a consistent dry-mass and contained-gold basis. If feed gold and product-plus-tailings gold do not balance within an explained tolerance, the recovery figure is not yet reliable.
Flowsheets commonly compared
| Test route | What it examines | When it can be informative |
|---|---|---|
| Gravity concentration | Recovery of liberated, sufficiently dense gold particles | Coarse free gold that can be captured before chemical treatment |
| Whole-ore cyanidation | Dissolution of cyanide-accessible gold from the entire feed | Oxide or other ores where gold is exposed to the lixiviant |
| Flotation followed by leaching | Concentration of sulfide-associated gold before dissolution | Ore where flotation can reject barren gangue and enrich sulfides |
| Pretreatment followed by cyanidation | Oxidation or other treatment before leaching | Refractory ores in which sulfides or carbonaceous matter block cyanide access |
These are test routes, not guaranteed plant recoveries. Grinding size, reagent conditions, residence time, mineralogy, water chemistry, and operating control can all change the result.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Mineralogy explains why recovery changes
Gold grade alone does not reveal how gold will behave in a plant. SGS process-mineralogy guidance recommends determining gold and silver grade by fire assay, then examining gold size, liberation, and associations with sulfides, oxides, silicates, and carbonaceous matter. Those observations help estimate whether gravity, flotation, direct cyanidation, or a combination is appropriate.
Free and liberated gold
Gold that is physically liberated during grinding is generally more accessible to gravity or leaching. Particle size still matters: very fine gold may not respond well to a gravity device designed for coarse particles, while overly coarse gold may not be liberated at the selected grind.
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Sulfide-associated and refractory gold
Gold enclosed in sulfide minerals can remain inaccessible to cyanide. Refractory ores may require pre-oxidation before cyanidation. If a feasibility study reports a recovery without identifying the mineralogical basis and pretreatment assumptions, the number should be treated as a flowsheet-specific result rather than a universal property of the deposit.
Carbonaceous matter and preg-robbing
Carbonaceous material can adsorb dissolved gold and reduce the amount reaching the intended recovery stage. Mineralogical examination can identify this association and guide the choice of pretreatment or process controls.
How to compare assay and recovery results
When reviewing a technical report, a laboratory certificate, or a project presentation, compare like with like. The following checklist prevents common misreadings:
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- Sample mass and representativeness: Was the material a routine split, a large screened-metallics sample, a composite, or a variability sample?
- Detection limit and range: Can the method resolve the reported concentration, and were any values over-range?
- Coarse-gold risk: Is there visible gold or evidence of a nugget effect?
- Mineralogy: Is gold free, sulfide-hosted, oxide-associated, or linked to carbonaceous matter?
- Result type: Is the number a head grade, concentrate grade, tailings grade, solution grade, or calculated recovery?
- Test conditions: What grind, reagent regime, residence time, and pretreatment produced the recovery?
- QA/QC: Are blanks, standards, duplicates, detection limits, reruns, and data review documented?
- Laboratory competence: Is the laboratory’s accreditation and method scope stated?
Never compare a fire-assay head grade from one sample with a process recovery from another and treat the pair as a single measured property. Recovery must be tied to the feed used in the test and to a defined process.
Why two assays of the same rock can differ
- Heterogeneous gold distribution: One split contains more coarse gold than another.
- Different sample masses: A larger screened-metallics sample averages more particles and can reduce nugget-related variance.
- Preparation differences: Incomplete crushing, segregation, or contamination changes the portion that reaches the pulverizer.
- Method differences: Fire assay, cyanide extraction, and instrumental finishes have different chemistry and may target different gold populations.
- Over-range or detection-limit effects: A result near a reporting limit or above an instrument’s calibrated range carries additional uncertainty.
- QA/QC failures: Failed blanks, standards, or duplicates can indicate contamination, drift, or poor precision.
A difference is not automatically evidence that one laboratory is wrong. The correct response is to examine sample mass, preparation records, duplicates, controls, and mineralogy, then select a method that addresses the identified source of variation.
A practical interpretation example
Suppose a dry test feed weighs 100 tonnes at 2 g/t. Its contained gold is 200 grams (100 × 2). If the selected flowsheet recovers 150 grams into saleable products and the remaining gold is accounted for in tailings and other streams, the calculated recovery is 75%. If a later test reports 3 g/t on a small split but uses a different, more selective sample, that higher grade does not prove the entire 100-tonne feed is 3 g/t. It may reflect coarse-gold sampling variance.
The example also shows why grade and recovery must remain separate inputs: changing the grade changes contained gold, while changing the flowsheet changes the proportion recovered.
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A useful assay or recovery result identifies the material tested, its dry mass, preparation route, analytical method, units, detection limit, and relevant QA/QC. A recovery result additionally identifies the flowsheet, operating conditions, stream masses, stream assays, and the reconciliation basis. Without those details, a precise-looking number can be less decision-useful than a qualified result with a clearly stated uncertainty.
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