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Miners measure gold grade by analyzing a representative, prepared portion of ore and reporting the concentration of gold it contains. They measure recovery separately by running that ore through a defined processing flowsheet and calculating the percentage of contained gold captured in saleable products or other designated streams. A fire assay is the traditional reference measurement for grade, while recovery is a metallurgical result that depends on liberation, mineral associations, reagent response and operating conditions.
Those two numbers answer different questions. Grade tells you how much gold is present; recovery tells you how much a particular process can extract. Reliable decisions require both, supported by representative sampling and documented quality control.
Grade and recovery are different measurements
Grade is the concentration of gold in a defined sample, commonly expressed in grams per tonne (g/t) for mine ore or in parts per million (ppm). The value is meaningful only when the sample represents the material being evaluated and the laboratory method is suitable for its concentration and texture.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesRecovery is the fraction of the gold contained in a feed that is recovered by a specified process. It is calculated on a contained-gold basis:
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Recovery (%) = gold recovered in products ÷ gold contained in feed × 100
A 5 g/t head grade does not imply 5 g/t in a concentrate, nor does it predict a 90% recovery. Recovery must be measured or modelled for a stated flowsheet, such as gravity concentration, whole-ore cyanidation, flotation followed by leaching, or pretreatment followed by cyanidation.
How a mine laboratory measures gold grade
1. Receive, dry and document the geological sample
The laboratory logs the sample and its chain of custody before preparation. The sampling plan should identify the material, location, interval or production source, intended analysis and any special instructions. Moisture and unusual pieces are recorded because the reported grade is normally based on a consistent dry-mass basis.
2. Crush and split a representative portion
The entire submitted sample is dried and crushed, then reduced with a controlled splitting procedure. The purpose is not simply to make a smaller sample; it is to preserve the average composition while reducing mass. A small, poorly selected split can miss a visible gold particle and produce a result that is not representative of the lot.
3. Pulverize the laboratory portion
The selected portion is pulverized to a specified particle size so that gold is distributed more uniformly through the analytical charge. Preparation records should include crushing, splitting and pulverizing conditions, equipment cleaning and any material retained for checks or reruns.
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4. Weigh an analytical charge
The laboratory weighs a measured charge from the pulverized pulp. Charge mass, particle-size specification and the chosen analytical method are part of the result’s definition. Coarse or nuggety gold may require a substantially larger mass or a different protocol rather than a routine small-charge assay.
What happens in a fire assay?
Fire assay fuses the weighed charge with a flux. During fusion, precious metals are collected in a lead button. The button is then cupelled: lead and other base metals oxidize and are absorbed, leaving a gold-silver bead. The bead is finished either by gravimetry (weighing it) or by dissolving it for an instrumental measurement such as atomic absorption (AAS) or inductively coupled plasma (ICP) analysis.
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Fire assay is not a single universal recipe. The laboratory selects charge size, flux, finish and reporting range for the matrix and expected concentration. Results should therefore state the method and any over-limit procedure, rather than presenting an unexplained number.
How low concentrations can be measured
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, by either method, concentrations as low as 50 parts per billion can be determined in a 15-gram sample. That capability is method- and sample-specific; it is not a guarantee that every matrix or laboratory will achieve the same detection limit.
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When a result is near a detection limit, the report should identify the limit, units and analytical finish. A value below a detection limit is not equivalent to zero gold, and combining censored results without a defined statistical or resource-estimation procedure can bias an average.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchWhy coarse gold causes unusually variable assays
Visible or coarse native gold can be distributed unevenly, creating a pronounced nugget effect. Two small pulverized charges from the same field sample may contain different numbers of gold particles, so their assays can differ substantially even when the preparation and instrument are functioning correctly.
The first response is usually to improve representativeness rather than to average away the problem. Options include submitting a larger initial mass, using more replicates, examining the sample for visible gold and adopting a method designed for coarse particles.
Screened-metallics analysis
SGS’s 2026 Geochemistry Guide describes screened-metallics analysis for this situation. A 500 g or 1,000 g sample is screened to a defined size, often 75, 106 or 212 micrometres. The entire coarse fraction is assayed, while the undersize is analyzed—typically in duplicate—and the size fractions are combined mathematically to calculate total gold.
The finish for the fractions may be AAS, ICP-OES or gravimetry, depending on concentration. Because the coarse fraction is not reduced to a tiny split, this procedure can better account for gold particles that would otherwise be missed. It also costs more material, preparation time and laboratory work than a routine small-charge assay.
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Quality assurance and quality control make an assay defensible
A number without evidence of control is difficult to use in a resource estimate, mine reconciliation or investment decision. A defensible campaign documents the following:
- Sampling plan, sample identifiers and chain of custody.
- Drying, crushing, splitting and pulverizing conditions.
- Analytical method, charge mass, finish and detection limits.
- Calibration records and instrument checks.
- Blanks to reveal contamination or carryover.
- Certified reference materials (standards) to test accuracy and bias.
- Duplicates to measure sampling and preparation precision.
- Rerun rules for failed controls, extreme values and suspected coarse gold.
- Data review, validation and an auditable final data set.
USGS’s QA/QC primer organizes controls from presampling through final interpretation and warns that inadequate QA/QC can produce erroneous results. Controls should be inserted at a planned rate and reviewed before results are accepted, not added only after an unexpected grade appears.
What each control can reveal
A blank can show contamination introduced during preparation or analysis. A certified reference material can expose systematic bias or a failed calibration. A duplicate can show how much variability comes from sampling and preparation. None of these controls alone proves that a field sample is representative; they work together with the sampling design and chain-of-custody record.
How miners measure metallurgical recovery
Build representative test samples
Recovery testing starts with representative composites or variability samples. The samples should reflect the ore types, alteration, oxidation state and grade ranges that a plant is expected to treat. A single high-grade specimen cannot establish recovery for an entire deposit.
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Characterize mineralogy before choosing a flowsheet
Gold and silver grades are commonly determined by fire assay, then process mineralogy examines gold size, liberation and associations with sulfides, oxides, silicates and carbonaceous matter. SGS process-mineralogy guidance uses those observations to assess likely response to gravity, flotation, direct cyanidation or combinations of these methods.
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Liberation is critical. Gold locked inside sulfide minerals may not be accessible to a leach solution. Gold attached to coarse, dense particles may respond well to gravity. Carbonaceous matter can interfere with cyanidation by adsorbing dissolved gold. Refractory ores may require pre-oxidation before cyanidation.
Run controlled flowsheet tests
Typical programs compare:
- Gravity concentration: recovers suitably liberated, relatively dense gold particles into a concentrate.
- Whole-ore cyanidation: leaches accessible gold from the complete ground feed.
- Flotation followed by leaching: concentrates sulfide-associated gold before a leach step.
- Pretreatment plus cyanidation: uses pre-oxidation or another treatment for refractory mineralization.
Tests specify grind size, reagent conditions, residence time, pulp density and other operating variables. Changing those conditions changes the measured recovery, so a result is meaningful only with its test conditions.
Complete the gold balance
Each test tracks feed mass and grade against concentrate, solution and tailings assays. The balance should reconcile on a consistent dry-mass and contained-gold basis. In practice, analysts check whether the calculated gold entering the test is reasonably accounted for in products and residues; a poor balance signals sampling, assay, measurement or calculation problems that must be resolved before interpreting recovery.
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Comparing assay and recovery options
| Option or question | What it measures | Main strength | Important limitation |
|---|---|---|---|
| Routine fire assay | Gold concentration in a prepared charge | Reference approach with gravimetric or instrumental finish | Small charges can be vulnerable to coarse-gold sampling error |
| Trace-gold cyanide extraction with AAS | Low-level extractable gold in a defined sample | USGS Circular 544 reports determination to 50 ppb in a 15 g sample for its described method | Detection capability depends on matrix and method conditions |
| Screened-metallics assay | Total gold calculated from coarse and fine size fractions | Addresses nugget effect by assaying the entire coarse fraction | Requires 500 g or 1,000 g samples and more preparation |
| Gravity test | Gold recovered by a gravity circuit | Shows response of liberated dense gold | Does not represent cyanidation or flotation recovery by itself |
| Flotation followed by leaching | Recovery from a specified concentrate-and-leach flowsheet | Can target sulfide-associated gold | Results depend on mineralogy and operating conditions |
| Pretreatment plus cyanidation | Recovery from refractory ore after pretreatment | Can expose gold otherwise locked in sulfides or affected by refractory behavior | Adds process complexity and must be tested for the specific ore |
Why two assays of the same rock can differ
- Different portions were analyzed. A coarse gold particle may be present in one charge and absent from another.
- Sample preparation differed. Incomplete crushing, segregation, contamination or inconsistent pulverizing changes the portion delivered to analysis.
- Methods measure different populations. A fire assay, cyanide extraction and a screened-metallics calculation do not necessarily expose the same gold fraction in the same way.
- The material was not homogeneous. Veins, sulfide bands, oxidized zones and barren fragments can sit next to one another in one hand specimen.
- Quality controls identified a problem. Failed blanks, standards or duplicates may trigger reruns or invalidate a batch.
- Reporting conventions differed. Dry versus wet basis, units, detection limits, over-limit procedures and rounding can make apparently conflicting numbers incomparable.
When results disagree, compare sample mass, preparation, analytical charge, method, detection limit, units, dry-mass basis and QA/QC status before deciding that one laboratory is wrong. For suspected coarse gold, a larger mass and screened-metallics protocol may answer the question more reliably than repeated small charges.
A practical specification for an assay and recovery campaign
Before submitting samples, define the decision the data must support: resource estimation, grade control, concentrate valuation, plant design or troubleshooting. Then specify:
- Which material and grade ranges are represented.
- Required sample mass and whether coarse gold is expected.
- Preparation size, analytical charge and finish.
- Detection limits and over-limit procedures.
- Blank, standard and duplicate insertion rates.
- Whether results are head, concentrate, solution or tailings grades.
- Flowsheet conditions for any recovery test.
- Dry-mass and contained-gold conventions for the balance.
- Acceptance rules, reruns and independent data review.
For laboratory selection, compare sample representativeness, detection range, coarse-gold susceptibility, mineralogical coverage, turnaround, cost, accreditation and the laboratory’s ability to document QA/QC. A fast result with an unsuitable method is less useful than a slower result that answers the operational question.
What a trustworthy result should say
A useful report identifies the sample, preparation, analytical method, charge mass, units, detection limit, dry or wet basis, QA/QC outcome and any qualification such as visible gold, coarse-gold treatment or an over-limit rerun. A recovery report additionally identifies the flowsheet, test conditions, product streams, assays, masses and gold-balance reconciliation.
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