Fire Assay Fusion
A sample size of 5 to 50 grams can be used, but the routine size is 30 g for rock pulps, soils or sediments (exploration samples). The sample is mixed with fire assay fluxes (borax, soda ash, silica, litharge) and with Ag added as a collector and the mixture is placed in a fire clay crucible. The mixture is then preheated at 850°C, intermediate 950°C and finish 1060°C with the entire fusion process lasting 60 minutes. The crucibles are then removed from the assay furnace and the molten slag (lighter material) is carefully poured from the crucible into a mould, leaving a lead button at the base of the mould. The lead button is then placed in a preheated cupel which absorbs the lead when cupelled at 950°C to recover the Ag (doré bead) + Au.
The gold content of the resultant bead is directly determined by INAA (Instrumental Neutron Activation Analysis). A minimum of one blank and two standards are analyzed with every work order. Duplicates are analyzed when samples are provided.
INAA is an analytical technique dependent on measuring gamma radiation induced in the sample by irradiation with neutrons. The primary source of neutrons for irradiation is usually a nuclear reactor. Each activated element emits a “fingerprint” of gamma radiation which can be measured and quantified.
The sample is encapsulated in a polyethylene vial and irradiated along with flux wires at a thermal neutron flux of 7 x 10 12 ncm-2 s-1. After a 7-day period to allow Na-24 to decay, the samples are counted on a high purity Ge detector. Using the flux wires and control standards, the decay-corrected activities are compared to a calibration developed from multiple certified international reference materials.
Detection Limits (ppb)
Note: If values exceed the upper limit, reanalysis by Fire Assay-Gravimetric (Code 1A3) is recommended. This method is only recommended when low level gold values less than 100 ppb are required. Otherwise code 1A2 will be more applicable for the range of 5-5000 ppb.
Further details are available on isotopes and gamma-ray energies.
Hoffman, E.L., 1992. Instrumental Neutron Activation in Geoanalysis. Journal of Geochemical Exploration, volume 44, pp. 297-319.
Hoffman, E.L., Clark, J.R. and Yeager, J.R. 1998. Gold analysis-Fire Assaying and alternative methods. Exploration and Mining Geology, Volume 7, p.155-160.