What is an Assay?
The assay is an analysis of a sample from a prospect or an orebody to determine the quantity of elements present in the sample. The assay is used to determine the mineral resource estimate of a prospect or a project.
A typical assay result will look like 20m @ 5.08g/t Au from 8m (including 1.67m @ 35g/t Au) from XYZ hole. The result can be interpreted as there is a zone of 20m starting from 8m below the surface, which contains 5.08g of gold if one tonne of the material is tripped. There is a subzone of 1.67m thickness (usually lesser depth) in that particular location that contains gold up to 35g in one tonne of material.
The 5.08g/t and 35g/t is the concentration of gold in samples from a particular depth range.
Assay results are important to estimate the total resource potential of the project. Potential investors take their decision of investment based on these results. Also, most of the time, it has been seen a bonanza grade assay results trigger the price rise of the shares of the mining companies.
How are the assay results determined?
There are several techniques and methods to determine the assay result of a sample. Some mining companies have on-site labs established to check assay of the stripped materials, while some sent their samples to commercial labs for thorough assay results.
Two of the most common methods which are widely used for assays are Fire Assay and Wet Chemistry Assay.
The fire assay method involves grinding the sample into fine powder. Then this powder is mix with a dry chemical powder. To determine which dry chemical is to be used, the XRF method is used to know the chemical composition of the fine powder sample. Once a reactive chemical is decided, the sample is weighed, and then chemicals are added for the further testing process.
Now the fine-grained sample and dry powder mixture are heated to an extreme temperature in a ceramic or metal container. At this temperature, the mixture melts, and a chemical reaction between the sample and chemical powder occurs. The container in which the sample is heated is called a crucible, and the chemical reaction is termed crucible fusion.
The superheated mixture is now allowed to cool and put into a mould once the reaction is over. Precious metals like gold and silver are extensively assayed by this method. Platinum group metals are a little difficult to assay through this method. Compounds like copper-nickel sulphide are added to the mixture to carry on the fire assay method on platinum group metals.
Once the contents are cool enough, they are put in a fire assay cupel. A cupel is a shallow cup made up of phosphorous, calcium phosphate or bone ash. A small bead of metal will be revealed in the cupel, which then weighed and analysed. This small bead will be formed from the precious metal present in the sample.
The bead is further sent for more accurate analysis using other analysis techniques like ICP-AES.
Flow diagram of Fire Assay method Copyright © 2021 Kalkine Media Pty Ltd.
Wet Chemistry Method
The wet chemistry assay method employs dissolution, titration, precipitation and gravimetric analysis method to determine the contents or concentration of the sample. Usually, a combination of all mentioned methods or more chemical reaction methods will be used to separate the contents of the sample.
The XRF method will be employed to know the basic chemistry of the sample. The appropriate reagents will be used to separate the elements and compounds from the sample. A precipitation reaction will be used in a typical analysis to filter the metal which is of interest. Then the precipitate material will be filtered, washed and dried for more separation. The obtained material is then weighed and compared with the determined mass to estimate the content of the metal present.
- Assay result represents the average grade across the deposit to know the commercial viability of the project.
- Fire assay and wet chemical assay method are the most common type of assaying techniques employed by mining companies.
- The fire assay method is used for precious metals like gold and silver. Platinum group metals are difficult to analyse by this method.
How assay results help in making an investment decision?
For any mining company, once the resource is discovered on the tenement, it is really important to know the resource and reserve the potential of the discovery. If the prospect does not exhibit economic reserves, then it is not advisable to move further on the development or extraction of the metal. For this purpose, the miners assign a cut-off grade on the reserves. Anything above the cut-off grade will ensure good returns on the investment.
The mineralisation grade of the deposit renders the project attractive for mining purposes. The exploration companies try to estimate the average grade across the deposit using the assay result. If the assay result is above the 'cut-off' grade, then the company will decide to infuse more investment, and investors will also put their money in the stocks of the company.
Mining is a risky business. The junior mining companies normally have low-grade tenements which do not interest the mining majors. Investors should carefully analyse their reports that bear this grade information and data to invest in these junior mining companies.
Is a decent assay result enough to decide for investment?
A good assay result could be used with other parameters of investment in the company. Commercial extraction of the mineral depends on various factors ranging from the size of the deposit to ease of processing of the stripped material, availability of mining infrastructure in close vicinity of the project, macroeconomic factors and many more.
Investors should use assay results as one of the additional tools for analysing the company's stock potential. They should not entirely depend on good assay results for the investment. Suppose no mining infrastructure is present and a company discovers a good reserve of gold. Processing in such cases can cost more and will decrease the overall profitability of the project.