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Gold by fire assay

Low detection level gold analysis by fire assay

Low detection levels are not only advantageous in exploration applications where the geochemical expression of mineralised systems may be very subtle, in mature deposits, but also when looking at classification of ore and waste. A low detection limit improves the ability to detect subtle signals in exploration; and improves precision to provide confidence in gold grade, particularly near cut-off, which can have large economic impacts on a project. By removing all but the precious metals during fire assay, consistent low detection levels, down to 1ppb, can be reported via sensitive instrumentation such as ICP-AES and ICP-MS. The ability to report every sample to the same low detection limit can be important for the interpretation of geochemical data. The matrix removal also carries the advantage of eliminating all potential interferences before measurement to give the most accurate, robust gold results regardless of geological type.

Ouro por ensaio de fogo

Fire assay for gold analysis

Fire Assaying has been practiced for millennia and has been considered the benchmark for gold analysis since the mid-twentieth century. This well-understood technique provides accurate results across geological sample types. However, practiced skill is essential to achieving a successful fire assay. ALS's expertise and innovations coupled with high quality standards provide confidence in the data produced. As standard practice, the geological matrix of each sample is identified to customise the specifically formulated base flux for optimal fusion recovery on every sample type. Multiple quality control verifications are carried out to ensure a successful outcome at each stage of the process, with provisions to re-assay if they are not met. In addition, natural matrix reference materials are utilised as batch quality controls to accurately reflect geological samples.

A range of fire assay methods are available with 30g or 50g sample aliquots and multiple options for detection ranges to suit the needs of any project. Our lowest detection limits are provided via ICP-AES or ICP-MS instruments, trace and mid-range detection levels utilise an atomic absorption instrument and high grade samples are assayed gravimetrically.


Method Analyte Range (ppm) Description
Trace Level
Au 0.001-10 Au by fire assay and ICP-AES
30g sample
50g sample
Au 0.005-10 Au by fire assay and AAS
30g sample
50g sample
Ore Grade
Au 0.01-100 Au by fire assay and AAS
30g sample
50g sample
Au 0.05-10,000 Au by fire assay and gravimetric finish
30g sample
50g sample

Fire assay explained

Fire assay has been used for centuries as a total decomposition technique to determine the amount of gold present within a sample. Homogenised and pulverised samples are mixed with flux composed of PbO and SiO2 with variable amounts of borax, soda ash and other reagents. The flux and sample are mixed, then heated at high temperature (>1,000°C) to decompose rock lattices and allow gold within the sample to be collected into a lead button. The button is placed in a porous cupel and heated again in an oxidising environment to convert lead to lead oxide that is absorbed into the cupel, leaving the precious metals behind as a doré bead or prill. The gold content of the prill is then determined either gravimetrically or via aqua regia digestion and spectroscopy.



Improved Fire Assay Technology


The Importance of Low Detection Levels for Gold