Precious metals analysis

Gold by Fire Assay

ALS has decades of experience with Fire Assay, the roots of which can be traced back as far as the twenty fifth century B.C., (Forbes, 1950) with pyro-metallurgical recovery of gold from ores practiced for more than 2000 years. With the advent of modern analytical instrumentation Fire Assay has become the benchmark method for precious metal analysis in ore and exploration samples since the 1960’s. These decades of routine application have given rise to a method with proven, consistent performance and reliability across geological sample types.

Improved Precision

Our advanced in-house technology applied to the ancient, well-understood fire assay technique provides accuracy and precision improvements. During Fire Assay the sample matrix is removed, leaving only the precious metals for analysis which eliminates all potential interferences before measurement. This matrix removal also results in the concentration of the precious metals, allowing for low detection levels.

Metallic Screening

Gold is found in many forms from disseminated fine-grained gold up to very coarse gold visible with the naked eye. These variations in gold deportment necessitate early ore characterisation. A well-designed screen fire assay program paired with robust geology models can aid in identifying the proportion and location of coarse gold within a deposit. This information will aid in planning appropriate sampling programs and provide an early indication of the process methodology, such as the proportion of gravity recoverable gold.

Screen Fire Assays

Traditional Fire Assays use a 30-50g aliquot of pulverised sample to provide a gold value for a sample. Screen Fire Assay methods use a larger volume of sample, typically 1 kg. The larger sample is sieved, and the fine fraction is analysed in duplicate. As the fine fraction is considered to be reasonably homogeneous this duplicate analysis is used to represent the fine fraction of the sample. The coarse material that didn't pass through the sieve is assayed in its entirety to determine the contribution of coarse gold.

Gold by PhotonAssay

PhotonAssay analysis is a relatively new technology for gold analysis. This technique is well-suited for coarse gold mineralisation due to its large sample analysis size. PhotonAssay uses a 500g sample aliquot and is a non-destructive method for gold analysis that utilises high energy X-Rays to excite the nuclei of gold to a higher energy state. As the nuclei drop to their unexcited state, they emit gamma rays at a signature energy that can be measured.

Silver

Trace level and low-grade silver samples may be analysed by acid digestion for maximum recovery and precision. Because silver can suffer from nugget effect, occasional duplicate analysis may help detect sampling error at low levels. At higher grades, Fire Assay with larger aliquot weights may be preferable.

Platinum Group Elements

Platinum, palladium, and rhodium, along with gold, may be determined by standard lead collection fire assay and AAS, ICP-MS or ICP-AES finish. But the other Platinum group elements are often volatised when a prill is digested or are not fully collected by the molten lead. For analysis of the full platinum group of elements nickel sulphide collection fire assay is required for a quantitative analysis.

Iridium, Ruthenium and Osmium

The iridium group of platinum group elements includes Iridium, Osmium, and Ruthenium has a greater affinity for Ni and Fe sulphide portions of an ore system whereas the palladium group (Rh, Pt, Pd) are more concentrated in the Cu sulphide portion of a deposit. For this reason, using Pt and Pd concentration as an indication of the concentration of all PGE’s may miss economic occurrence of Os, Ir, and Ru.

Gold Cyanidation

In mining and exploration applications, cyanide leaches are used to establish potential cyanide extraction efficiency for gold and silver. Another advantage of utilising a cyanide leach is the lowest detection limit (0.02ppb) available with our super trace methods. This is possible due to the stability of the gold cyanide complex formed, coupled with lower matrix effects and lower dilution factors compared to aqua regia digestions.

Issues to consider with cyanide methods

High concentrations of some sulphides particularly chalcopyrite, can negatively impact gold extraction. For samples that are expected to contain high copper sulphide concentrations it can be advantageous to follow cyanide digestion with a fire assay method. Solvent digestions can also lose gold to adsorption onto the original sample when certain forms of carbon and sulphide minerals are present in a process called preg-robbing.

Bulk Leach Extractable Gold

Bulk Leach Extractable Gold (BLEG) is a cyanide leach on a large bulk sample, often a stream sediment sample and rolled during digestion. The advantage of BLEG is the much larger amount of sample that is digested, with the most common methods using 500g or 1 kg. The large sample size utilised by BLEG methods can help decrease the influence of nuggety gold making results more reproducible. Bottle rolling also ensures that the whole sample interacts with reagents to ensure the most representative result.

Cyanide digestion

Cyanide is effective at taking gold into solution and will also digest secondary copper sulphides and bornites. Larger nuggets or gold enclosed in silicate minerals may not be fully dissolved even with excess cyanide reagent.

Super Trace/ Low Level Gold and Multi-Element

Gold plus multi-element packages use a single aqua regia digest on a 25g or 50g aliquot, with either Low Level or Super Trace detection options. Gold in conjunction with a large suite of base metal and pathfinder elements are determined from the same solution via a combination of ICP-MS and ICP-AES. Exploration for many types of deposits can often benefit from using groups of pathfinder elements frequently including mercury and gold. Both elements are effectively digested and maintained in aqua regia solution due to the presence of both an oxidising and complexing agent plus relatively low temperature heating.

The lowest Au detection limit in the industry

Super Trace and Low Level gold methods use larger aliquots of sample to produce a quantitative gold values for soil and sediment samples. The Super Trace cyanide gold method is the lowest detection limit in the industry, ensuring that background is well characterised.