Precious Metals Analysis

Gold Analysis

There are many different methods available to analytical chemists for the determination of gold. These include:

• The classical fire assay fusion collection followed by a gravimetric spectrometric finish to determine ore grade.
• Cyanide Leach to determine the recovery potential for mine development samples. This may also be used as a very sensitive and cost effective grassroots exploration tool ?
• Aqua regia digestion for soil and stream sediment exploration samples.
• A "metallics" or screen fire assay for samples containing "coarse" gold.

Each of these methods has its application and these will be outlined below. All methods have limitations to some degree. For example, an aqua regia digestion for gold is suitable for soils and sediments because the gold is expected to be largely available for chemical attack. The same cannot be said for an aqua regia digestion of rocks and drill cuttings where the gold may be retained in an insoluble quartz matrix or otherwise encapsulated. For rocks and drill cuttings, a fire assay fusion is clearly the superior method of gold analysis.

Fire Assay Process

Lead Collection

The standard fire assay procedure has been used for millennia to dissolve and separate gold, silver and other precious metals. In the first part of the fire assay, precious metals are dissolved using an aggressive fusion mixture consisting of litharge (lead oxide) and a variety of other fluxes such as sodium carbonate, borax, silica, potassium nitrate and household flour. During the complex reactions that occur between sample and the flux mixture, the litharge is reduced to molten lead and the silica within the sample is oxidized to a borosilicate slag. The molten lead that is produced within the reaction mixture forms as tiny droplets throughout. Because of the high specific gravity of the lead droplets, they filter down through the reaction mixture, dissolving and collecting the precious metals as they do so. In an ideal fusion, the end result is a clean two-phase melt in which the barren borosilicate slag floats on top of the molten lead containing the precious metals. When this two-phase melt is poured into an iron mold to cool, the lead solidifies and can be recovered. The subsequent separation of lead and precious metals occurs during the next step known as cupellation.

Cupellation

Cupellation most commonly refers to that part of the fire assay process. Following a successful fusion, the analyst is left with a lead "button" which contains all the precious metals from a particular sample. Cupellation is the process by which the lead is separated from the precious metals. Cupellation is considered "total" if the lead is removed in its entirety and "partial" if it is not. For the determination of gold, silver, platinum, palladium, a total cupellation is standard. In this case, the lead button is placed on a magnesia cupel in a furnace at 960-1000?C. At this temperature, the lead melts and is simultaneously oxidised. Part of the lead is volatilised and part is drawn into the cupel by capillary attraction. Eventually the lead is entirely removed and what remains behind is a small precious metal bead that represents the entire precious metal content of the original sample. This bead can then be analysed by a variety of methods

A partial cupellation can be used for the analysis of platinum group metals (PGM) following a lead collection. A total cupellation cannot be used as losses in ruthenium, rhodium, osmium and iridium would result. However, if the cupellation is partial then the bulk of the lead is removed and losses of PGM can be avoided. Further chemical manipulations are required however to dissolve and remove the remainder of the lead

The Fire Assay-Gravimetric Procedure for Ore Grade Samples

Gravimetric methods involve the use of balances to weigh the element of interest, either in its pure elemental form or as a chemical compound. One of the most common gravimetric determinations is that of gold and silver following a fire Assay Fusion cupellation. The precious metal bead that remains following cupellation is an alloy of silver and gold. Weighing this bead will give the total weight of silver and gold. If the bead is then treated with dilute nitric acid, it is possible to remove the silver quantitatively. The residual mass consists of pure gold which can then be weighed separately, thus allowing the silver to be determined by difference. The balances used for this purpose are microbalances capable of weighing to the nearest microgram (one millionth of a gram). Analysis of Bullion for gold, silver and base metal content is another common procedure. The classical technique for determining gold is the fire assay fusion followed by cupellation and a gravimetric finish (method codes Au-GRA21, Au-GRA22and Au-GRA24).This is still the preferred procedure for the analysis of high grade ores. There is no upper quantitative limit applied for these procedures but clients should note that the detection limit is significantly higher than for procedures that use spectroscopic measurement techniques.

Fire Assay-Atomic Absorption procedures for Exploration and Low to Medium Grade Ore Samples

Exploration samples (particularly soils) may require a better detection limit than that offered by the above procedures. Method codes Au-AA21 and Au-AA22, which include a fire assay collection followed by cupellation, dissolution of the precious metal prill and a pre-concentration solvent extraction step. The final determination is by flame AAS, providing a detection limit of 1 ppb. It is a more expensive technique than the conventional fire assay /AAS procedure, but for explorers looking for the best resolution of low level gold anomalies, this procedure is excellent.

In recent times, we have turned to ICPMS technology to offer trace level gold. See method codes Au-ICP21 and Au-ICP22. In addition to a detection limit of 1ppb, the advantage offered by this technique is the ability to determine platinum and palladium together with gold.

Many samples arriving at our laboratories have "intermediate" levels of gold; that is in the range of 3-10 g/t (0.1-0.3 oz/ton). These samples are best analyzed using FA-AAS procedures Au-AA23, Au-AA24. If samples contain higher concentrations of gold, procedures Au-AA25 or Au-AA26 would be a more appropriate technique.

Advantages of the Fire Assay Process

•  A large sub-sample (10-50g or more) can be taken for analysis, helping to ensure that the analytical sub-sample is truly representative of the sample submitted to the laboratory \
• The fire assay fusion is considered to provide a "total" gold.
•  All samples are amenable to the fire assay procedure in the hands of a skilled assayer.
• The fire assay procedure is universally accepted as the definitive method for the analysis of gold.
•  The fire assay fusion quantitatively dissolves and extracts the entire platinum metal group in addition to gold and silver.

Limitations of the Fire Assay Process

• When a gravimetric finish is used, it is essential that the separation ("parting") of silver and gold is complete. If the silver is incompletely removed, then the gold results will be artificially high and the silver results will be low.
• A certain amount of silver (usually estimated to be in the range of 2%) is lost by volatilization during the cupellation process.
•  When an atomic absorption spectroscopy finish is selected, the upper reporting limit is set at 10 g/t (0.3 oz/ton) and samples higher than this must be re-analyzed using additional silver in the firing process and a larger dilution factor. Alternatively, gravimetric finish can be used.
•  Samples containing coarse gold can give erratic results making it difficult to determine the true ore grade; however, sample heterogeneity rather than the fire assay process causes this problem. • Soil samples (typically -180um(-80 mesh material)) can also give erratic results but again for the same reason
• It can take many years of experience before a fire assayer has the necessary degree of skill and knowledge to flux difficult ore types. • Some ores such as chromites and tellurides can be more difficult to fuse, resulting in the need to take smaller subsamples for analysis and consequently yielding higher detection limits than normal.

Cyanide Leach

Cyanide Extraction

One very important alkaline digestion is the cyanide leach for extractable gold. Not only is cyanide very efficient in extracting gold in an alkaline environment but it would be lethally dangerous in an acid environment due to the formation of deadly hydrogen cyanide.

BLEG for Trace Level Gold

The treatment of gold exploration samples with alkaline cyanide solutions can be successfully applied to soil and stream sediment samples. The cyanide solutions are very dilute and as a result it is economically feasible to leach very large samples (up to 2,500 g) in contrast to fire assay. The ability to leach very large samples (Bulk Liquid Extractable Gold-BLEG) helps counteract the problem of sample heterogeneity. The gold is dissolved through formation of its cyanide complex, which can be concentrated through the process of solvent extraction to an organic solvent. The combination of large sample weight and solvent extraction gives an extremely low detection limit. As low as 0.1 ppb. This method is a very cost effective grassroots exploration tool to detect low grade gold dispersions.

Cyanide Leach for Ore Grade Samples

Smaller sub-sample weights are generally taken for ore grade samples because of the greater amount of gold that will be dissolved in contrast to exploration samples.

In the case of ore grade samples, the solvent extraction step is omitted because low detection limits are not generally required. The leach solutions are analyzed directly by flame AAS. The main objective for testing ore grade samples is to determine the cyanide solubility and recovery of gold in a mining situation. Cyanide extraction using an accelerant may also be used in a grade control situation. 

Advantages of Cyanide Leach

• The combination of a large sub-sample and solvent extraction lead to a very low detection limit for gold (as low as 0.1 ppb) in exploration samples.
• The large sub-sample weights help to counteract heterogeneity problems.
• Cyanidation of ore grade samples closely approximates the gold plant leach process and permits the evaluation of recovery potentials.

Limitations of Cyanide Leach

•  The procedure is not as aggressive as fire assay procedures and so gold values will be lower; the recovery of gold is typically in the 90% range.
• The technique is generally ineffective for the leaching of encapsulated gold.
• Recovery potential can vary with leach temperature and leach time so caution is needed when comparing data from different sources.

Aqua regia acid digestion

The aqua regia digestion is an effective way of dissolving and measuring gold in soil and stream sediment samples. Gold in these sample types is typically amenable to an aqua regia acid attack and in many cases the data is comparable to that produced with fire assay procedures. The advantage of this method is primarily associated with its cheaper cost; acids being cheaper than fire assay fluxes. A disadvantage is that encapsulated gold will not be dissolved by aqua regia. Another limitation can be poor precision. This is especially true for samples with coarse gold, a common occurrence for these sample types which are usually only screened to -80 mesh (180 micron). We recommend that samples be pulverized to 85% -75microns(200 mesh) to avoid this problem.

The Problem of Coarse Gold Samples

Many exploration samples exhibit a pronounced "nugget" effect due to the presence of particulate gold in coarse fragments. The net result is a pronounced and unacceptable scatter in the gold analytical results making it difficult to assess the true gold concentration. There are a number of steps that can be taken to improve analytical reproducibility in these kinds of samples and usually it is possible to improve the situation, although perhaps not always to the degree sought by the client.

The first place to improve the analytical reproducibility is the sample preparation process. Essentially it is advantageous to prepare a larger and finer sub-sample so as to increase the chances that the sub-sample is more representative of the field sample and also more homogeneous. This can be achieved by crushing the sample to a degree finer than the standard 70% -2mm (10 mesh), or indeed pulverising the whole sample to a nominal 75 microns. See method code PUL-21. With the preparation of a finer and more representative sample, it is now possible to enhance the analytical process.

The ALS recommended analytical procedure is to select a 1000 gram "metallics" or screen fire assay (method codes Au-SCR21 (100 micron dry screen) or Au-SCR22 ( 75 micron wet screen)). In the Au-SCR22 procedure, 1000grams of the final prepared pulp is washed through a 75micron(200 mesh) screen to separate any coarse (+75 micron) material. Any +75micron material remaining on the screen is dried, weighed and analyzed in its entirety. The 75micron fraction is dried and homogenized. Duplicate sub-samples are analyzed using the standard fire assay procedures. The gold values for both +75micron and 75micron fractions are reported together with the weight of each fraction as well as the calculated total gold content of the sample. In this way a client can evaluate the magnitude of the coarse gold effect as demonstrated by the levels of the +75micron material.

An alternative to improve analytical reproducibility is to carry out an initial cyanide leach of a large subsample of ore followed by a fire assay analysis of the residual material. As much as 3000 g of ore grade material can be leached with cyanide (method codes Au-AA14 and Au-AA15 ) and typically around 90% of the gold will be extracted. The balance of the gold can be measured by carrying out a total gold fire assay fusion on the residue. It is then possible to calculate the total gold content of the original ore.

The Analysis of Problematic Ores such as Tellurides and Chromites

A number of ores such as tellurides and chromites present unique challenges to the fire assayer. Tellurium can cause low values for both silver and gold during the cupellation process by reducing the surface tension of the precious metal prill leading to losses into the cupel. However, if tellurium is prevented from being in the lead button in the first place, then the problem is eliminated. This is accomplished by ensuring that the flux mixture in the fire assay fusion is strongly oxidizing in order to oxidize tellurium and have it report to the slag component rather than to the lead component. If a sample containing significant tellurides is not recognized initially, the presence of remnant Te would be revealed at the cupellation stage by leaving a "color de rosa" residue on the cupel. The sample would then be re-analyzed with a different flux mixture.

Chromite ores, being highly refractory, are problematic because they do not melt readily with a normal flux mixture at 1000 deg. C. Furthermore, if the chromium oxidizes to a higher state (Cr 6+) then lead "shotting" occurs and it is impossible to bring about a two-phase separation into slag and molten lead. The analyst must therefore maintain a stronger reducing fusion environment by increasing the amount of reducing agents such as flour, minimizing oxidizing agents such as litharge (lead oxide) and substituting sodium carbonate and silica instead. This kind of modification is also helpful for ores containing tin or ores high in iron oxides.

For all problematic ores, it is generally helpful to reduce the sample weight in order to increase the flux to sample ratio. This increases the likelihood of a successful fusion resulting in a clean 2-phase separation; however, the reduction in sample weight means that the detection limit will be elevated over the normal limits stated in our literature.

Quality Control Procedures for the Determination of Gold in Geological Samples

Fire Assay: Our assayers take numerous additional steps in the fire assay process. Fusion crucibles are carefully checked to ensure that no boilovers have occurred in the furnace. A boilover requires re-analysis of not just that sample but also of all its nearest neighbours in the furnace. We do visual checks of the fusion mixture to make sure that there has been a clean fusion with no lead "shotting". The size of the lead button is assessed and if it is either too small or too large, the fusion will be repeated. After cupellation, the precious metal bead is checked for size, colour and surface texture. A large bead or a gold hue will indicate a sample high in silver or gold, or both, and it must be handled with special care to control possible contamination. A pebbled bead surface can indicate the presence of platinum metals. A 'color de rosa' in the cupel can indicate the presence of tellurium, in which case the analysis will have to be repeated.

FAQs

Is it true that the fire assay procedure will not extract gold from certain types of samples?

Some ore types are very difficult to fuse, (see Problematic Ores) but in our experience a skilled fire assayer is always able to make the necessary adjustments to obtain a successful fusion. Unfortunately, unscrupulous promoters often make unsubstantiated claims to the effect that standard methods of fire assay do not work. We recommend that clients evaluate these claims by analyzing the samples by fire assay and also by aqua regia digestion.

Why do gold checks on my soil samples often show no gold even when the original analysis showed several hundred ppb gold?

This problem has been discussed in part in Coarse Gold Problems. Soils are typically screened to -180 micron (80 mesh) and this is a relatively coarse sample that can easily contain particulate gold. Hence, it is quite common for one subsample to contain a flake of gold whereas another does not. Pulverizing these samples may help reduce the variability of the assays. As part of our normal quality control procedures, we perform random checks on samples that have shown anomalous values. We choose to report these check values to the client (at no charge), even where the agreement with the original value is not as good as we would like to see, in order that the client may have the additional information that the gold present is likely to be of a particulate nature.

How much gold does an aqua regia digestion dissolve in comparison to a fire assay fusion?

In our experience, it depends entirely on how available the gold is for attack. If the gold is readily accessible, (free gold), then the aqua regia digestion can provide data that is virtually identical to that of a fire assay. However if the gold is encapsulated, or contained within a mineral that is not easily attacked by aqua regia, then the recovery of gold can be as low as about 80%. A typical recovery range would be from 85% to 95% but exceptions abound. The presence of carbon in the sample may lead to almost zero recovery of the gold.

Silver Analysis

Silver can be determined by a variety of analytical methods. Spectroscopic methods such as atomic absorption spectroscopy and inductively coupled plasma emission spectroscopy are most applicable for the trace silver analysis of exploration samples. The fire assay procedure for precious metals and ore grade acid digestion with atomic absorption finish are alternatives for the determination of silver in ore grade samples. The advantages and limitations of each of these techniques is outlined in their relevant sections.

Methods of Silver Dissolution

The standard digestion procedure for trace?and ore grade silver analyses is the aqua regia digestion. This digestion is effective for exploration and mineralised samples as is the triple acid digestion system of nitric-perchloric-hydrofluoric acids. Ordinarily the aqua regia digestion is effective for dissolving most forms of silver mineralisation. For high ore grade silver, fire assay gravimetric silver is applicable.

The Fire Assay Gravimetric Analysis for Silver

The fire assay gravimetric procedure is only applicable to the determination of silver in concentrates or high grade silver ores. It can not be used to determine low concentrations of silver due to the necessity to inquart with silver to obtain a acceptably sized prill. The uncertainty inherent in quantifying the silver recovery of the inquart would far exceed the accuracy requirements for this determination.

Ore Grade Digestions for Silver

Using the Ag-OG46 analytical procedure, higher grades (up to 1,500ppm(50 oz/t)) of silver can be digested with aqua regia. This method is suitable for most silver ores with the exception of halide salts, where we recommend Ag-OG62. Both are cheaper, quicker alternatives to fire assay procedures and equally accurate. In some cases, depending on mineralogy, there may be still precipitation of Silver as its Sulphate salt. We recommend sending in a test batch for verification by alternative methods before deciding on the most appropriate assay procedure.

Limitations of Silver Analytical Methodology

In the determination of silver using acid digestions, the analyst must be aware that silver has a propensity to precipate from solution in the presence of trace halides. Silver may also co-precipitate with insoluble sulfates. Strong hydrochloric acid will stabilise silver in solution or it may be complexed with sodium thiosulfate. For routine geochemical analysis it is advantageous to determine the silver as soon as possible after bulking to volume.

When silver is determined by ICP-AES, there can be a significant spectral interference from iron. If samples contain "normal" levels of iron, i.e. in the range of several percent, a successful correction can be made. However for samples containing elevated iron concentrations, we recommend that AAS techniques be used in preference to ICP. As part of our Quality Assurance program, we do carry out random AAS checks of ICP-generated silver data where it is suspected that elevated levels of iron may be present. The

limitations of the fire assay procedure have been discussed elsewhere on this website. The principal limitations in the measurement of silver are the inability to determine trace concentrations plus the silver losses that occur during cupellation. Cupelling at a lower temperature can reduce cupellation losses. The concurrent analysis of proof silver inquarts may be used to quantify these losses and corrections made based on these recoveries

How do I know if my samples require a total digestion for silver assay?

If your samples contain silver halide minerals and originate in the U.S. Southwest or Mexico, then it may be necessary to use a total digestion silver assay (Ag-OG62). We recommend talking to an ALS Client Services representative regarding the analysis of a limited batch of test samples.

When is an aqua regia digestion adequate for a silver assay?

TThe aqua regia digestion is ordinarily adequate for a reliable silver assay. However, if silver halide minerals are present, we recommend a total digestion. It is always possible to analyse a small test batch by both methods in order to confirm the validity of the aqua regia digestions.

Platinum Group Elements Analysis

Platinum group elements (PGE' s) are rarely found in significant concentrations in any kind of mineralisation. As a result, the fire assay fusion is the universally preferred method for collecting and concentrating platinum group elements.

The platinum group metals can be successfully collected using a lead collection fire assay fusion but several (Ir, Os, Ru) can be lost during the subsequent cupellation process. An alternative fusion process that does not require the cupellation step is a nickel sulphide fusion. In this fusion, samples are mixed with a variety of fluxes including nickel oxide, elemental sulfur and sodium carbonate and melted together in a high temperature furnace. Nickel sulphide is formed in this process and because it possesses a high affinity for the PGM, they are collected together with the nickel sulphide as it forms. The nickel sulphide-PGM matte can be analyzed by neutron activation analysis (NAA) directly or by plasma mass spectroscopy (ICP-MS) following chemical dissolution. It should be noted that gold is only qualitative by this process.

Fire Assay Lead Collection

All of the PGE's are collected in the lead button, but only gold, platinum and palladium are determined routinely using this technique. During cupellation, osmium and ruthenium are partly volatilised and iridium is partly absorbed by the cupel. Rhodium, although quantitatively retained in the prill, may only partially dissolve in the aqua regia unless additional gold is added prior to the fusion process. The PGE's in solution are determined by inductively coupled plasma mass spectroscopy (ICPMS)

Fire Assay Nickel Sulphide Collection

This is a modified fire assay technique for recovering all of the PGE's using a Nickel Sulphide Matte instead of lead. The nickel sulphide matte dissolved in hydrochloric acid. The undissolved PGE's and Gold sulphides are recovered, and then dissolved in Aqua Regia prior to ICPMS determinations. However, gold is not quantitative by this method due to the poor solubility of gold sulphide in hydrochloric acid. See method PGM-MS26 and PGM-NAA26

Limitations of PGE Analytical Methodology

PGM s occur most commonly in placer deposits or in ultrabasic intrusives with nickel sulfides. High nickel samples require special fluxing to ensure that cupellation is not impeded. Alluvial samples will usually require a large excess of silver to facilitate parting. Chromite may also be present with PGE s and will also require special fluxing. Unless the laboratory has the expertise to recognise and compensate for these situations poor recoveries will inevitably occur.