Most geologists, whether students, in academia or in industry have heard of, or have submitted samples to have precious metal content analysed by lead fire assay. The lead fire assay method is one of a host of metallurgical analysis methods and is also often referred to as the fusion and critical cupellation (high temperature oxidation) step. It always helps to understand the analysis techniques used on samples so you can understand results and data better. Whether you are a geologist or not, I hope this fire assay photo essay will give you at least a basic understanding of the technique used by most precious metal explorers and miners to eventually come to a resource.
Drill core samples (or others) are received at the laboratory, crushed, milled and dried in the oven. Samples are then separated into bags, tagged and weighed.
Next, the samples are prepared for the fusion process, which requires a reducing environment and so the samples are mixed with flux, a carbon source (coal dust) and litharge (PbO).
The samples are then placed into a fusion/ refractory crucible, ready to go into the furnace. The high tempareture (1000 – 1200°C) melts the sample while the flux fuses with the gangue (non-precious material) and the now liquid lead collects all the precious metals. The newly-formed Pb-Au-Ag-PGE concoction migrates to the bottom of the crucible due to its high density.
As cooled lead sample is then separated from the glass waste (slag) with a hammer, and placed in a porous bone ash (MgO) crucible, ready for the cupellation step.
The sample is then placed in a muffle furnace at around 1000°C. The lead oxidises with the aid of the inflowing air from the furnace muffle and then immediately melts and is absorbed into the crucible, leaving behind what is known as a precious metal prill.
The prill is then analysed for its precious metal content through dissolving it in aqua regia (a mixture of hydrochloric and nitric acid) and the concentrations determined by spectroscopic methods (AAS, ICPAES, ICPMS).
Eventual detection limits are in the region of parts per billion but accuracy is usually in the region of 3 -5% for this metallurgical analytical technique.
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