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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchGold grade is the concentration of gold in a representative, prepared sample. Recovery is the percentage of that contained gold that a defined processing flowsheet captures in saleable products. Miners measure the two separately: a fire assay or related analytical method answers “how much gold is in this material?”, while metallurgical testing answers “how much can this process recover?”
Reliable results depend less on a dramatic laboratory instrument than on representative sampling, adequate sample mass, controlled preparation, documented quality control, and a gold balance that reconciles feed, products, solutions, and tailings.
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Grade and recovery answer different questions
Assay grade is normally reported as a concentration, such as grams of gold per tonne of dry ore or parts per billion (ppb). It describes the material submitted to the laboratory after a sample has been collected, dried, crushed, split and pulverized. A grade result is therefore only as representative as the field sample and the portion finally weighed into the analytical charge.
Recovery is a process result. It is the fraction of the gold contained in a defined feed that appears in the process products. A high-grade ore can have poor recovery if its gold is locked in sulfides or associated with carbonaceous matter. A lower-grade ore can give good recovery when its gold is liberated and readily dissolves. Substituting an assay grade for recovery, or treating a recovery percentage as an ore grade, produces an invalid economic or metallurgical conclusion.
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The recovery equation
Use a consistent dry-mass and contained-gold basis:
Recovery (%) = gold recovered in products ÷ gold contained in feed × 100
The numerator can include gravity concentrate, flotation concentrate, precipitated solution gold, or other defined products. Tailings, intermediate streams and solution assays are needed to check whether the balance closes; a single product assay is not a complete recovery measurement.
How a mine laboratory measures gold grade
A routine laboratory workflow turns a field sample into a small, homogeneous charge while preserving a documented chain of custody.
- Receive and document the sample. The laboratory records the sample identity, mass, condition and custody history, then assigns preparation and analytical work orders.
- Dry the sample. Drying removes variable moisture before mass reduction and allows results to be reported on a consistent dry basis.
- Crush the entire submitted sample. Crushing reduces particle size so that gold-bearing fragments are distributed more evenly before splitting.
- Split a representative portion. A riffle splitter or equivalent procedure produces a laboratory portion without simply selecting convenient fragments. The retained reject can support checks or reruns.
- Pulverize the laboratory portion. Fine grinding improves homogeneity and exposes gold-bearing particles to the analytical procedure.
- Weigh the analytical charge. The laboratory records the charge mass and method, because charge size affects the chance of including coarse gold.
- Perform the analysis and review quality controls. The result is released with method, detection limit, blanks, reference materials, duplicates and any rerun notes.
USGS Bulletin 1445 is a dedicated procedural reference for fire assaying noble metals in geological materials. The USGS analytical-chemistry program also lists methods for gold and ore-grade gold, including fire-assay and instrumental finishes.
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What happens in a fire assay
Fire assay remains a reference approach because it collects gold from a relatively large, weighed charge rather than relying on a tiny dissolved aliquot alone. The charge is mixed with a flux and heated until it fuses. Precious metals collect in a lead button. The button is then cupelled: lead oxidizes and is absorbed by the cupel, leaving a gold-silver bead. The bead is finished by weighing (gravimetry) or by dissolving it and measuring the gold instrumentally, commonly with atomic absorption or inductively coupled plasma techniques.
Fire assay is not one universal result. The report should identify the charge mass, fusion and finish, detection limit, and whether the value is a screened-metallics result, a routine assay, a check assay or a referee determination. ASTM E1805 describes fire-assay gravimetry as a reference and referee method for gold in copper concentrates and supports its use in metallurgical accounting, process control and concentrate evaluation.
Instrumental methods described by the USGS
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 states: “By either method concentrations as low as 50 parts per billion of gold can be determined in a 15-gram sample.” That capability is a method-specific detection statement for the described 15-gram sample, not a guarantee that every laboratory, matrix or charge will achieve the same limit.
Why coarse gold needs special treatment
Visible or coarse native gold can be distributed extremely unevenly. A small split may contain a nugget while an adjacent split contains none, creating a large “nugget effect.” Repeating the same small-charge fire assay can therefore produce materially different numbers even when the laboratory procedure is correctly followed.
For this situation, a screened-metallics procedure uses a much larger sample and handles size fractions separately:
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- Submit a 500-gram or 1,000-gram sample.
- Screen it at a defined size, often 75, 106 or 212 micrometres.
- Assay the entire coarse fraction rather than taking a small split from it.
- Analyze the undersize, typically in duplicate.
- Calculate total gold from the coarse and undersize results using their measured masses.
The finish may be atomic absorption, inductively coupled plasma optical-emission spectrometry, or gravimetry, depending on concentration. The report must state the screen size, masses, fraction results and calculation; without those details, a “metallics” value cannot be evaluated for representativeness.
How accurate is a gold assay?
Accuracy has several components. Analytical bias is the laboratory method’s departure from a suitable reference value. Precision is the spread among repeat determinations. Sampling error is the difference between the submitted material and the true lot because gold is unevenly distributed. In coarse-gold deposits, sampling error can dominate the uncertainty even when the instrument and fire-assay finish are precise.
Sources of differing results from the same rock
- Different fragments were selected. Gold grains are not uniformly distributed, so two small portions can have different contained gold.
- Sample mass differed. A 15-gram charge has a different chance of capturing a coarse particle than a 500-gram or 1,000-gram screened-metallics program.
- Preparation was inconsistent. Incomplete drying, crushing, splitting or pulverizing can segregate gold-bearing particles.
- Methods measured different fractions. A routine split assay, screened metallics assay, cyanide extraction and fire-assay finish do not necessarily dissolve or include exactly the same gold.
- The material was heterogeneous. Veins, sulfide bands, oxide zones and carbonaceous patches can vary over short distances.
- Quality-control failures occurred. Contamination, sample swaps, poor calibration or unrecognized detection-limit issues can create false agreement or disagreement.
When results conflict, compare the submitted masses, preparation records, screen sizes, charge sizes, analytical finishes and quality-control data before declaring one result wrong. A larger, properly documented sample is often more informative than an additional repeat of the same small split.
Quality assurance and quality control that make results defensible
A defensible campaign records controls from field collection through final interpretation. The following checklist should be visible in the project data package:
- Sampling objectives, locations, intervals, nominal masses and a plan for coarse or visible gold.
- Chain-of-custody transfers and unique sample identifiers.
- Drying temperature or procedure, crushing specifications, split method and pulverization target.
- Analytical method, charge mass, finish, calibration and detection limit.
- Method blanks to detect contamination.
- Certified reference materials with expected values and control limits.
- Preparation and analytical duplicates to measure precision.
- Rerun rules for failed controls, outliers, over-range results or visible gold.
- Review of batch acceptance, data validation and final interpretation.
The USGS quality-assurance and quality-control primer is organized from presampling through final interpretation and warns that inadequate QA/QC can produce erroneous results. A laboratory certificate without the associated control results is not enough to judge whether a number is fit for a resource estimate, mine reconciliation or process decision.
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How miners measure metallurgical recovery
Recovery testing starts with representative composites or variability samples, not an arbitrary high-grade fragment. The laboratory or pilot facility defines a flowsheet and measures every important stream on a consistent basis. Typical programs compare gravity concentration, whole-ore cyanidation, flotation followed by leaching, and pretreatment for refractory ores.
A practical recovery-test sequence
- Characterize the feed. Determine head grade, moisture, mineralogy, particle-size distribution and the presence of coarse gold, sulfides, oxides, silicates or carbonaceous matter.
- Select representative composites. Combine samples to reflect the ore domain or variability being evaluated; document the mass and compositing rule.
- Run candidate flowsheets. Test gravity, direct cyanidation, flotation-plus-leaching and, where indicated, a pretreatment route.
- Sample all streams. Measure feed, concentrates, solutions, intermediates and tailings, including their dry masses where applicable.
- Assay and calculate contained gold. Apply the recovery equation to each test and reconcile the gold balance.
- Investigate discrepancies. Recheck moisture corrections, stream masses, sampling representativeness, assay precision and unmeasured solution or inventory gold.
Recovery is a property of the ore and the tested process conditions together. Grind size, reagent conditions, residence time, concentrate handling and pretreatment can change the result; a recovery percentage must therefore be reported with its flowsheet and test conditions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Mineralogy explains why recovery changes
Fire assay establishes total gold, but it does not by itself show where that gold resides. SGS process-mineralogy guidance pairs fire-assay grade with examination of gold size, liberation, association with sulfides, oxides and silicates, and association with carbonaceous matter. Those observations help estimate whether gold should respond to gravity, flotation, direct cyanidation or a combination.
Freely liberated coarse gold may be recovered by gravity before leaching. Fine gold attached to sulfides may require flotation to make a smaller, richer stream for subsequent treatment. Gold enclosed in sulfide minerals can be refractory to direct cyanidation; pre-oxidation may be required to expose it. Carbonaceous material can adsorb dissolved gold and cause preg-robbing, reducing the apparent cyanidation recovery unless the flowsheet addresses it.
Advanced microbeam work can add detail, but conventional fire assay, gravity concentration and ore microscopy remain important. As SGS Minerals Services puts it, “It is worthwhile to point out that some conventional techniques such as fire assaying, gravity concentration, and ore microscopy are just as important as advanced microbeam techniques.”
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Choosing an assay or recovery program
Compare programs against the actual decision you need to make. A resource estimate, a shipment settlement, a plant-control question and a refractory-ore flowsheet study require different samples and outputs.
| Program or method | What it measures | Key treatment | Main limitation or use |
|---|---|---|---|
| Routine fire assay | Total gold in a prepared charge | Fusion, lead button, cupellation, gravimetric or instrumental finish | Small charges can be vulnerable to coarse-gold sampling error |
| Screened-metallics assay | Total gold calculated from coarse and fine fractions | 500 g or 1,000 g sample; screen commonly at 75, 106 or 212 µm; assay all coarse material and duplicate undersize | Designed for visible or coarse gold and requires fraction-specific reporting |
| Cyanide extraction with atomic absorption | Gold extracted under the specified cyanide conditions | Cyanide extraction followed by atomic-absorption measurement | Method response depends on extraction conditions and mineralogy |
| Fire-assay atomic absorption | Gold in the fire-assay bead | Fusion and cupellation, bead dissolution, atomic-absorption measurement | Useful trace-gold approach; charge and detection limit must be stated |
| Metallurgical recovery test | Fraction of feed gold captured by a defined process | Controlled gravity, cyanidation, flotation, leaching or pretreatment test with a stream-by-stream balance | Result applies to the tested feed and flowsheet, not automatically to every ore type |
Reporting results so they can be used correctly
A useful report keeps grade, recovery and uncertainty visible rather than presenting one impressive number. Include the sample description and mass, dry-basis convention, preparation steps, charge mass, analytical method, finish, detection limit, screen size if applicable, QA/QC outcomes and any reruns. For a recovery test, add the flowsheet, operating conditions, stream masses, stream assays, contained-gold calculations and reconciliation.
Use “head grade” for the feed assay used in a metallurgical test, “product grade” for a concentrate or other product, and “tailings grade” for the residue. State whether a recovery is mass-balanced, assay-based, or calculated from a particular product definition. This vocabulary prevents a concentrate grade from being mistaken for plant recovery or an assay value from being treated as a process guarantee.
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Common shortcuts that fail
- Relying on a visible speck. A hand specimen can be attractive but is not a representative grade sample.
- Repeating tiny charges indefinitely. More repeats do not remove a nugget effect if every repeat is too small.
- Using a hobby acid kit or handheld X-ray fluorescence device as a validated substitute. Such tools should not replace a commercial, method-specific assay without evidence that the matrix, calibration, detection limit and sampling procedure are suitable.
- Quoting recovery without a flowsheet. “90% recovery” has no transferable meaning unless the feed, products, conditions and balance are defined.
- Ignoring failed controls. A batch with contaminated blanks, drifting standards or unexplained duplicate differences needs investigation and possibly reruns before interpretation.
The most reliable answer to “what is this ore worth?” is therefore a linked set of measurements: representative grade, mineralogical explanation, tested flowsheet recovery and documented QA/QC. Each answers a different part of the decision.
Frequently Asked Questions
Why can two assays of the same rock differ so much?
They may have used different fragments, charge masses, preparation splits or analytical methods. Coarse native gold creates a nugget effect, so a larger screened-metallics sample may be more representative than repeated small charges.
Does fire assay measure recoverable gold?
No. Fire assay measures gold concentration in the submitted material. Recoverable gold depends on liberation, mineral associations and the tested processing flowsheet.
When should a mine request screened-metallics analysis?
When visible or coarse gold is present or routine assays show erratic results. The procedure uses a 500 g or 1,000 g sample, a defined screen size and separate analysis of coarse and fine fractions.
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What must accompany a recovery percentage?
The feed and product definitions, dry-mass basis, flowsheet and operating conditions, stream assays and masses, and a reconciled contained-gold balance.
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