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OSNACA Analytical Techniques

Ore samples are crushed in a steel jaw crusher with barren quartz processed between each sample. They are then pulverised to -75µm in a steel mill, with barren quartz rinse milled in between each sample. 

Analytical solutions for reading by ICP-MS and ICP-OES are prepared by the following methods.  

1  Pb Fire assay       Au, Pt, Pd

40 grams of sample is weighed (lower weights down to 10 grams can be used if there is insufficient sample or for highly sulphidic samples). The sample is then fused for one hour at 1100 oC in a ceramic pot with 165 grams of flux (a mix of borax , soda ash, flour ,litharge and a small amount of Silver ) with additional soda ash for silica rich samples, additional borax for basic samples, and additional KNO3 and litharge for some sulphidic samples. During the fusion the litharge( PbO) is reduced to Pb collecting the precious metals. Molten samples are poured into steel conical moulds where a lead button sinks to the bottom separating from the glass slag. The lead button is then placed on a cupel and heated to 990 oC for 45 minutes during which time the Pb oxidises back to PbO flowing into the absorbent cupel leaving behind a silver prill containing all Au,Pt & Pd. The prill is then dissolved in 1 ml of concentrated nitric acid followed by 1 ml of concentrated hydrochloric acid at 110 oC. The resultant solution is then made up to 10 ml with deionised water.

2  Peroxide Fusion     B, Cr, Hf, Pb, S, Sb, Si, Sn, Ta, Ti, W, Zr

0.2500 grams of sample is fused with 1.5 g of Sodium Peroxide in an alumina crucible for 30 minutes at 650 oC.  For Carbon rich samples the temperature is raised slowly to 650 oC to allow ashing for 1 hour prior to fusion.  The crucible is allowed to cool and then agitated in 100 ml of deionised water for one hour, before adding 100 ml of hydrochloric acid and agitating every 10 minutes for 30 minutes in a waterbath at 95 oC. The solution is made up to 500 ml with de-ionised water.

3  Aqua Regia      Au, Bi, Te, Hg, Ag, Sb

4.000 grams of sample is placed in a polythene vial before 5 ml of concentrated nitric acid is added in 1 ml increments checking for violent reactions before agitating and adding a further 5 ml of nitric acid.  10 ml of concentrated hydrochloric acid is added, agitated, and left to stand for 30 minutes before heating to 95 oC for two hours in a waterbath. The solution is made up to 50 ml with deionised water.

4  Mixed Acid (Hot Box)    Ag, Al, As, Bi, Ca, Cd, Co, Cs, Cu, Fe, In, K, Li, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Rb, Re, Sc, Se, Sn, Sr, Te,  Th, Tl, U, V, W, Y, Zn, REEs

0.1500 grams of sample is placed in a teflon tube and dissolved in 10 ml of a concentrated mixture of hydrofluoric, nitric and perchloric acids (hot-box mix). This solution is brought up to 250 oC and back down to 150 oC over a period of 12 hours. After allowing to cool for 5 minutes, 10 ml of concentrated hydrochloric acid is added and the sample is re-heated to 150 oC for 15 minutes. 10 ml of de-ionised water is added before decanting into a polystyrene tube. The solution is made up to 20 ml with deionised water and mixed thoroughly by agitating.

DATA SELECTION

Whole rock geochemical data are presented as raw laboratory files, and in tabular format that can be linked with metadata by sample number. In the tabular data files, a “preferred value” has been selected for those elements which have been analysed by two different techniques; a robust and a sensitive technique. For the element tabulated below, the robust technique ensures complete digestion for samples that contain high concentrations, and the sensitive technique allows readings to lower detection limits where concentrations are low. The “robust cut-off” is the value below which the sensitive technique is used in preference to the robust technique. Combining the data in this way ensures the most reliable value for each of these elements over a very wide range of concentrations.

Element

Robust Technique

Robust LLD (ppm)

Robust cut-off (ppm)

Sensitive Technique

Sensitive LLD (ppm)

Ag

Mixed Acid

0.5

Lab selected

Aqua Regia

0.05

Bi

Mixed Acid

0.1

Lab selected

Aqua Regia

0.02

Te

Mixed Acid

0.2

Lab selected

Aqua Regia

0.02

Pb

Peroxide Fusion

100

<1000

Mixed Acid

1

Sb

Peroxide Fusion

2

Lab selected

Aqua Regia

0.02

Sn

Peroxide Fusion

5

<5

Mixed Acid

1

W

Peroxide Fusion

5

<5

Mixed Acid

0.5

   

For Bi, Te, Ag and Sb a lot or repeat assaying was performed, particularly for samples that followed samples with very high values. Fusion, mixed acid and aqua regia data were all considered before selecting the preferred value. In general, mixed acid data for Ag and fusion data for Sb were found to be reliable right down to their respective detection limits, whereas aqua-regia data have been preferred for Bi and Te at values up to five times the mixed acid detection limit. Very high results for Ag, Bi and Te have been confirmed by fusion.

Bureau Veritas – Ultratrace have confirmed data for many other elements by comparing values by different techniques. These internal checks show, for example, that the REEs have been fully digested by the mixed acid technique when the data are compared to peroxide fusion data.  Gold has been analysed by fire assay and aqua-regia to the same detection limit and fire assay values are used in preference.

For those interested in more information on data selection for Ag, Bi, Te and Sb (particularly those values that have not been published) further details can be provided by Alex Christ from Bureau-Veritas   alex.christ@au.bureauveritas.com .