PRECIOUS METAL ANALYSIS

Accurate and precise precious metal analysis can be performed by using fire assay, cyanide leach, aqua regia or INAA.

Precious metal analysis is usually determined by using either a lead or nickel sulphide fire assay pre-concentration step prior to analysis. This is required to separate the precious metals from the matrix and what could be interfering elements. The lead collection method (using Ag as a collector) is used routinely for gold analysis as well as platinum and palladium, or with no collector for silver, or with gold as a collector for Rh. Various types of analytical finishes can be used including INAA, atomic absorption, Gravimetric, ICP and ICP/MS. Gold and multi-element analysis can also be determined by INAA using a 30g sub-sample. In order to do the full platinum group element suite a nickel sulphide fire assay is required with either an INAA or ICP/MS finish.

Gold Analysis – Geochemistry

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.      

INAA Finish   

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)

Element Detection LimitUpper Limit
Au120,000

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.

References:
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.

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.

AA Finish

The entire Ag dore bead is dissolved in aqua regia and the gold content is determined by AA (Atomic Absorption). AA is an instrumental method of determining element concentration by introducing an element in its atomic form, to a light beam of appropriate wavelength causing the atom to absorb light. The reduction in the intensity of the light beam directly correlates with the concentration of the elemental atomic species.On each tray of 42 samples there is two blanks, three sample duplicates and 2 certified reference materials, one high and one low (QC 7 out of 42 samples). We generally rerun all gold by fire assay gravimetric over 5,000 ppb to ensure accurate values

 1A2   (Fire Assay-AA) Detection Limits (ppb)

Element Detection LimitUpper Limit
Au55,000

Note: If value exceeds upper limit, reanalysis by Fire Assay-Gravimetric (Code 1A3) is recommended.

Reference:

Hoffman, E.L., Clark, J.R. and Yeager, J.R. 1998. Gold analysis – Fire Assaying and alternative methods. Exploration and Mining Geology, Volume 7, pp. 155-160.

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. 

AA Finish

The entire Ag dore bead is dissolved in aqua regia and the gold content is determined by AA (Atomic Absorption). AA is an instrumental method of determining element concentration by introducing an element in its atomic form, to a light beam of appropriate wavelength causing the atom to absorb light. The reduction in the intensity of the light beam directly correlates with the concentration of the elemental atomic species.On each tray of 42 samples there is two blanks, three sample duplicates and 2 certified reference materials, one high and one low (QC 7 out of 42 samples). We generally rerun all gold by fire assay gravimetric over 5,000 ppb to ensure accurate values

Detection Limits (ppb)

Element Detection LimitUpper Limit
Au510,000

Note: If value exceeds upper limit, reanalysis by Fire Assay-Gravimetric (Code 1A3) is recommended. 

Reference:

Hoffman, E.L., Clark, J.R. and Yeager, J.R. 1998. Gold analysis – Fire Assaying and alternative methods. Exploration and Mining Geology, Volume 7, pp. 155-160.

Fire Assay

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 preheated at 850°C, intermediate 950 °C and finish 1060 °C, the entire fusion process should last 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.

ICP-OES

The Ag doré bead is digested in hot (95°C) HNO3  + HCl. After cooling for 2 hours the sample solution is analyzed for Au by ICP-OES using a Varian 735 ICP. 

Detection Limits (ppb)

Element Detection LimitUpper Limit
Au230,000

Fire Assay

A sample size of 5 to 50 grams can be used but the routine size is 30g 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 preheated at 850°C, intermediate 950 °C and finish 1060 °C, the entire fusion process should last 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.

ICP-OES

The Ag doré bead is digested in hot (95°C) HNO3  + HCl. After cooling for 2 hours the sample solution is analyzed for Au by a Perkin Elmer Sciex ELAN 6000, 6100 or 9000 ICP-MS.  On each tray of 42 samples there are two method blanks, three sample duplicates, and 2 certified reference materials.

Detection Limits (ppb)

Element Detection LimitUpper Limit
Au0.530,000

BLEG (Bulk Liquid Extractable Gold) involves weighing a 1-2 kg sample into a polyethylene bottle, adding an appropriate cyanide solution (0.25% to 1% NaCN) and bottle rolling for a determined period of time. A pH of 10 or greater is maintained during leaching.  The gold is dissolved through formation of a cyanide complex. The resultant cyanide solution is diluted and analyzed by a Perkin Elmer Sciex ELAN 6000, 6100 or 9000 ICP/MS. A blank is run every 40 samples after instrument recalibration. The method can be made multielement for other elements which are cyanide extractible. The presence of carbonaceous material may result in preg-robbing (absorption of gold). Roasting of the sample prior to the BLEG process may be desirable for carbonaceous material or high sulphide samples.

Detection Limits (ppb)

Element Detection LimitUpper Limit
Au0.110,000

Reference:

Hoffman, Eric L., Clark, John R. and Yeager, James R., 1998.  Gold Analysis – Fire Assaying and Alternative Methods. Explor. Mining Geology , Volume 7, Nos. 1 and 2, pp. 155-160.

A 30g sample is digested using aqua regia leach at 95 °C, followed by analysis by ICP-MS using a Perkin Elmer Sciex Elan 9000. This method may not report total gold concentrations.

Detection Limits (ppb)

Element Detection LimitUpper Limit
Au0.22,000

Silver Analysis – Geochemistry

A 0.5g sample is digested using aqua regia, followed by analysis by a Varian 735 ICP. QC for the digestion is 15% for each batch, 2 method reagent blanks, 6 in-house controls, 8 sample duplicates and 5 certified reference materials. An additional 20% QC is performed as part of the instrumental analysis to ensure quality in the areas of instrumental drift.

Detection Limits (ppm)

Element Detection LimitUpper Limit
Au0.2100

Gold and Multi-element Analyses – Geochemistry

Determining rock types, alteration patterns and levels of pathfinder elements are key for the geologist to assess exploration potential. Actlabs’ “Au+34” (1D) is a cost effective multi-element approach to Au, PGE and base metal exploration. 1D Enhanced, with enhanced detection limits, is also available.

INAA (Instrumental Neutron Activation Analysis) 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. Routine multi-element analyses by INAA are performed on practically any material from the smallest sample which can be weighed accurately to very large samples.

A 30g aliquot, if available, 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 with resolution of better than 1.7 KeV for the 1332 KeV Co-60 photopeak. Using the flux wires and control standards, the decay-corrected activities are compared to a calibration developed from multiple certified international reference materials. For values exceeding the upper limits, assays are recommended. One standard is run for every 11 samples. One blank is analyzed per work order. Duplicates are analyzed when sample material is available.

1D (Au+34) Elements and Detection Limits (ppm)

Element Detection LimitUpper Limit
Ag5100,000
As210,000
Au5 ppb
30,000 ppb
Ba100500,000
Br11,000
Ca1%50%
Ce310,000
Co55,000
Cr10100,000
Cs210,000
Eu0.22,000
Fe0.02%75%
Element Detection LimitUpper Limit
Hf1500
Hg11,000
Ir5 ppb10,000 ppb
La110,000
Lu0.051,000
Mo510,000
Na0.05%10%
Nd510,000
Ni5010,000
Rb3010,000
Sb0.210,000
Sc0.1200
Element Detection LimitUpper Limit
Se510,000
Sm0.110,000
Sn0.05%10%
Sr0.1%40%
Ta110,000
Tb0.51,000
Th0.510,000
U0.510,000
W410,000
Yb0.21,000
Zn50100,000

1D Enhanced (Au+34) Elements and Detection Limits (ppm)

Element Detection LimitUpper Limit
Ag5100,000
As0.510,000
Au2 ppb30,000 ppb
Ba50500,000
Br0.51,000
Ca1%50%
Ce310,000
Co15,000
Cr5100,000
Cs110,000
Eu0.22,000
Fe0.01%75%
Element Detection LimitUpper Limit

Hf

1500
Hg11,000
Ir5 ppb10,000 ppb
La0.510,000
Lu0.051,000
Mo110,000
Na0.01%10%
Nd510,000
Ni2010,000
Rb1510,000
Sb0.110,000
Sc0.1200
Element Detection LimitUpper Limit
Se310,000
Sm0.110,000
Sn0.02%10%
Sr0.05%40%
Ta0.510,000
Tb0.51000
Th0.210,000
U0.510,000
W110,000
Yb0.21,000
Zn50100,000

Reference:

Hoffman, E.L., 1992. Instrumental Neutron Activation in Geoanalysis. Journal of Geochemical Exploration, volume 44, pp. 297-319.

Gold and Silver Analyses – Assay

Fire Assay

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.      

Au is separated from the Ag in the doré bead by parting with nitric acid. The resulting gold flake is annealed using a torch.  The gold flake remaining is weighed gravimetrically on a microbalance.

Detection Limits (g/T)

Element Detection LimitUpper Limit
Au

0.03 (30g)

0.02 (50g)

10,000

Reference:

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.

Fire Assay

A sample size of 5 to 50 grams can be used but the routine size is 30g for rock pulps, soils or sediments (exploration samples). The sample is mixed with fire assay fluxes (borax, soda ash, silica, litharge), which contain no silver, 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 cupellation of the bead is controlled by the volatility of the silver. The Ag bead is weighed and Ag value calculated from the weight. Au is separated from the Ag in the doré bead by parting with nitric acid. The gold (roasting) flake remaining is weighed gravimetrically on a micro balance for Au.

Elements and Detection Limits (g/tonne)

Element Detection LimitUpper Limit
Au

0.03

10,000
Ag

3

10,000

Metallic Screen

A representative 500g split (1,000g for 1A4-1000) is sieved at 100 mesh (149 micron) with fire assays performed on the entire +100 mesh and 2 splits on the -100 mesh fraction. The total amount of sample and the +100 mesh and -100 mesh fraction is weighed for assay reconciliation. Measured amounts of cleaner sand are used between samples and saved to test for possible plating out of gold on the mill. Alternative sieving mesh sizes are available but the user is warned that the finer the grind the more likelihood of gold loss by plating out on the mill.

 Fire Assay

A sample size of 5 to 50 grams can be used but the routine size is 30g 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.

Au is separated from the Ag in the doré bead by parting with nitric acid. The gold (roasting) flake remaining is weighed gravimetrically on a microbalance. Two splits on the -150 micron fraction are weighted and analyzed by fire assay with a gravimetric finish. A final assay is calculated based on the weight of each separated fraction and obtained Au values.

Detection Limits (g/T)

Element Detection Limit
Au

0.03

Reference:

Hoffman, E.L. , Clark, J.R. and Yeager, J.R. 1998. Gold analysis-Fire Assaying and alternative methods. Exploration and Mining Geology, Volume 7, pp. 155-160.

A sample size of 10 to 50 grams can be used, but the routine 30g size is applied for rock pulps. The sample is mixed with fire assay fluxes (borax, soda ash, silica, litharge), which contain no silver. The mixture is placed in a fire clay crucible, and is preheated to 850°C, raised to an intermediate 950 °C and finished at 1060 °C. The entire fusion process lasts 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.

Element and Detection Limits (g/tonne)

Element Detection LimitUpper Limit
Ag

3

10,000

Gold Platinum, Palladium, and Rhodium

A sample size of 5 to 50 grams can be used but the routine size is 30g 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, Pt and Pd.

The Ag doré bead is digested in hot (95°C) HNO 3 + HCl.  After cooling for 2 hours the sample solution is analyzed for Au, Pt, Pd by a Perkin Elmer Sciex ELAN 6000, 6100 or 9000 ICP-MS.  On each tray of 42 samples there are two method blanks, three sample duplicates, and 2 certified reference materials.

For vegetation ash samples, a lower weight can be used but will result in elevated detection limits.  Smaller sample splits are used for high chromite or sulphide samples to ensure proper fluxing and metal recoveries.

If values exceed upper limits, reanalysis by fire assay Au, Pt, Pd (Code 8) is recommended.

Elements and Detection Limits (ppb)

Element Detection LimitUpper Limit
Au

2

30,000
Pt

1

30,000
Pd

1

30,000

References:

Hoffman, Eric L. and Dunn, Bernie, 2002.  Sample Preparation and Bulk Analytical Methods for PGE.CIM Special Volume 54 The Geology, Geochemistry and Mineral Beneficiation of Platinum Group Elements Edited by Louis J. Cabri, pp.1-11.

Hoffman, Eric L., Clark, John R. and Yeager, James R., 1998.  Gold Analysis – Fire Assaying and Alternative Methods. Explor. Mining Geology , Volume 7, Nos. 1 and 2, pp. 155-160. 

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, Pt and Pd.

The Ag doré bead is digested in hot (95°C) HNO3 + HCl.  After cooling for 2 hours the sample solution is analyzed for Au, Pt, Pd by a Perkin Elmer Sciex ELAN 6000, 6100 or 9000 ICP/MS.  On each tray of 42 samples there are two method blanks, three sample duplicates, and 2 certified reference materials.

For vegetation ash samples a lower weight can be used but will result in elevated detection limits.  Smaller sample splits are used for high chromite or sulphide samples to ensure proper fluxing and metal recoveries.

If values exceed upper limits, reanalysis by fire assay Au, Pt, Pd (Code 8) is recommended.

Elements and Detection Limits (ppb)

Element Detection LimitUpper Limit
Au

1

30,000
Pt

0.5

30,000
Pd

0.5

30,000

References:

Hoffman, Eric L. and Dunn, Bernie, 2002.  Sample Preparation and Bulk Analytical Methods for PGE.CIM Special Volume 54 The Geology, Geochemistry and Mineral Beneficiation of Platinum Group Elements Edited by Louis J. Cabri, pp.1-11.

Hoffman, Eric L., Clark, John R. and Yeager, James R., 1998.  Gold Analysis – Fire Assaying and Alternative Methods. Explor. Mining Geology , Volume 7, Nos. 1 and 2, pp. 155-160. 

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, Pt and Pd.

The Ag doré bead is digested in hot (95°C) HNO3 + HCl with a special complexing agent to prevent the Au, Pd and Pt from adsorbing onto the test tube.  After cooling for 2 hours the sample solution is analyzed for Au, Pt and Pd using a Perkin Elmer Sciex ELAN 6000, 6100 or 9000 ICP/MS. On each tray of 42 samples there are two method blanks, three sample duplicates, and 2 certified reference materials. The ICP/MS is recalibrated every 45 samples. Smaller sample splits are used for high chromite or sulphide samples.

For vegetation ash samples a lower weight can be used but will result in elevated detection limits.  For a 1 g sample the detection limit will be 3 ppb for Pt and Pd. Vegetation and humus can be analyzed directly by pre-ashing in the fire assay crucible at 475 degrees to eliminate organics which do not go through the fire assay procedure directly very well.

If values exceed the upper limits, reanalysis by fire assay Au, Pt, Pd (Code 8) is recommended.

Elements and Detection Limits (ppb)

Element Detection LimitUpper Limit
Au

1

30,000
Pt

0.1

30,000
Pd

0.1

30,000

Reference:

Hoffman, Eric L. and Dunn, Bernie, 2002.  Sample Preparation and Bulk Analytical Methods for PGE. CIM Special Volume 54: The Geology, Geochemistry and Mineral Beneficiation of Platinum Group Elements Edited by Louis J. Cabri, pp.1-11.

A sample size of 5 to 50 grams can be used but the routine size is 30g for rock pulps, soils or sediments (exploration samples). The sample is mixed with fire assay fluxes (borax, soda ash, silica, litharge) 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. Instead of using Ag as a collector, Au is used to avoid the problem of insoluble Rh Ag alloys.

After cooling the lead button is separated from the slag and cupelled at 950°C to recover the bead.  The bead is digested in hot (95°C) HNO3 + HCl. After cooling for 2 hours the sample solution is dissolved in acid and analyzed for Rh by using a Perkin Elmer Sciex ELAN 6100 or 9000 ICP/MS.   On each tray of 42 samples there are two method blanks, three sample duplicates, and 2 certified reference materials.

For vegetation ash samples a lower weight can be used but will result in elevated detection limits.

Detection Limits (ppb)

Element Detection LimitUpper Limit
Rh510,000

Reference:

Hoffman, Eric L., Clark, John R. and Yeager, James R., 1998.  Gold Analysis – Fire Assaying and Alternative Methods. Explor. Mining Geology, Volume 7, Nos. 1 and 2, pp. 155-160.  

A sample size of 5 to 50 grams can be used but the routine size is 30g 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, Pt and Pd.

The Ag doré bead is digested in hot (95°C) HNO 3 + HCl.  After cooling for 2 hours the sample solution is analyzed for Au, Pt, Pd by ICP/OES using a Varian 735 ICP.  The instrument is recalibrated every 45 samples. On each tray of 42 samples there are two method blanks, three sample duplicates, and 2 certified reference materials.

For vegetation ash samples a lower weight can be used but will result in elevated detection limits. Smaller sample splits are used for high chromite or sulphide samples to ensure proper fluxing and metal recoveries.

If values exceed upper limits, reanalysis by fire assay Au, Pt, Pd (Code 8) is recommended.

Elements and Detection Limits (ppb)

Element Detection LimitUpper Limit
Au230,000
Pt530,000
Pd530,000

References:

Hoffman, Eric L. and Dunn, Bernie, 2002.  Sample Preparation and Bulk Analytical Methods for PGE. CIM Special Volume 54 The Geology, Geochemistry and Mineral Beneficiation of Platinum Group Elements Edited by Louis J. Cabri, pp.1-11.

Hoffman, Eric L., Clark, John R. and Yeager, James R., 1998.  Gold Analysis – Fire Assaying and Alternative Methods. Explor. Mining Geology , Volume 7, Nos. 1 and 2, pp. 155-160. 

A 30 gram 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 preheated at 850 °C, intermediate 950 °C and finish 1060 °C, the entire fusion process should last 60 minutes.  After cooling, the lead button is separated from the slag and cupelled at 950 °C to recover the Ag (doré bead) + Au, Pt, Pd.

The Ag doré bead is digested in hot (95 °C) HNO3 + HCl.  After cooling for 2 hours the sample solution is analyzed for Au, Pt, Pd by ICP/OES using a Varian 735 ICP.  A blank and a digested standard are run every 15 samples. Instrument is recalibrated every 45 samples. All samples are analyzed in duplicate.      

Elements and Detection Limits g/t

Element Detection LimitUpper Limit
Au0.0011,000
Pt0.0011,000
Pd0.0011,000

References:

Hoffman, Eric L. and Dunn, Bernie, 2002.  Sample Preparation and Bulk Analytical Methods for PGE.CIM Special Volume 54. The Geology, Geochemistry and Mineral Beneficiation of Platinum Group Elements Edited by Louis J. Cabri, pp.1-11.

Hoffman, Eric L., Clark, John R. and Yeager, James R., 1998.  Gold Analysis – Fire Assaying and Alternative Methods. Explor. Mining Geology, Volume 7, Nos. 1 and 2, pp. 155-160.

Platinum Group Elements

Samples up to 25 g in size are fire assayed using a nickel sulphide (NiS) fire assay procedure. The nickel sulphide button is dissolved in concentrated HCl and the resulting residue which contains all the PGE and Au are collected on a filter paper. One batch of 34 samples includes 2 blanks, 3 certified standards and 3 duplicates. This residue undergoes 2 irradiations and 3 separate counts to measure all the PGE and Au. 

INAA Finish

Instrumental Neutron Activation Analysis (INAA) is an analytical technique which is dependent on measuring gamma radiation induced in the sample by irradiation with neutrons. The primary source of neutrons for irradiation is a nuclear reactor. Each element which is activated emits a “fingerprint” of gamma radiation which can be measured and quantified.  Multi-element analyses of practically any material, from the smallest sample which can be weighed accurately to very large samples, have been analyzed routinely by INAA.  

(NiS Fire Assay-INAA) Elements and Detection Limits (ppb)

Element Detection LimitUpper Limit
Os220,000
Ir0.110,000
Ru550,000
Rh0.220,000
Pt5100,000
Pd2100,000
Au0.520,000

Reference:

Hoffman, E.L., 1992.  Instrumental Neutron Activation in Geoanalysis.  Journal of Geochemical Exploration, volume 44, pp. 297-319.

Hoffman, E.L., Naldrett, A.J., Van Loon, J.C., Hancock, R.G.V. and Manson, A., 1978. The determination of all the platinum group elements and gold in rocks and ore by neutron activation analysis after preconcentrating by a nickel sulphide fire-assay technique on large samples.  Anal. Chim. Acta, volume 102, pp. 157-166.

Samples up to 50g in size are fire assayed using a nickel sulphide (NiS) fire assay procedure.  Residue is dissolved, diluted and analyzed by a Perkin Elmer Sciex ELAN 6000, 6100 or 9000 ICP/MS.  A blank and a digested standard are run every 15 samples. The instrument is recalibrated every 45 samples. One batch of 34 samples includes 2 blanks, 3 certified standards and 3 duplicates.

Elements and Detection Limits (ppb)

Element Detection LimitUpper Limit
Ir110,000
Ru110,000
Rh110,000
Pt1100,000
Pd1100,000
Au1100,000

Reference:

Hoffman, Eric L. and Dunn, Bernie, Sample Preparation and Bulk Analytical Methods for PGE.