Assaying is an essential component in determining the amount of gold in a mineral deposit. Assaying is conducted in a laboratory where experts choose the appropriate methods to determine the concentration of valuable minerals in a rock sample. Most often, the rock sample is a core, obtained through drilling; however, in some cases hand/grab samples may also be used.
Assay results are often mapped, and used to pin-point future drilling programs. The assay values at particular locations can help exploration companies learn about the distribution of gold and the potential of the deposit. When the gold values are extremely high and the others are very different, the high values must be studied carefully because these assays may not reflect the gold grade around the sample point location. Also, assay values are generally reported to the public, and strong assay values can result on a share-price jump.
There are several methods that can determine the concentration of gold, silver and other metals from any given rock samples. The fire assay is the method that is the accepted standard applied for valuing gold ore as well as gold and silver bullion at major refineries and gold mining companies. Another assay method is the "wet assay" which consists in dissolving the samples and metals are recovered using chemical reagents.
The Fire Assay
The fire assay is the preferred method of determining the mineral grading in a deposit because it is the most accurate. If the assay is performed on the ore materials using fusion and followed by cupellation separation, detection of mineralization may be in parts per billion. However, accuracy on ore material is typically limited to 3 to 5 percent of reported value. The downfall of fire assays is that it is very disruptive; however, if performed on exploration-level properties this does not pose a problem.
The first step in the fire assay process is to fuse or dissolve the rock sample in question in a crucible using a lead glass flux. The flux is a mixture of materials, usually- sodium bicarbonate, potassium carbonate, borax, litharge (lead oxide) and flour that is selected by the laboratory chemist. The appropriate flux is mixed with the crushed ore (rock sample). The mixture is then heated to encourage a reaction. The reaction is allowed to go to completion, and then the crucible is removed. The flux mixture and rock sample have now reacted to form two parts- a molten glass that is on top of metal at the bottom of the crucible. The molten glass is poured off, as this part of the material does not contain any valuable metals. The remaining metal is poured into a mold, and allowed to cool until it solidifies. Once cooled, the solid metal is removed from the mold. This metal generally contains lead, gold and silver. The mass of lead metal is then placed on a cupel made of bone ash- a material that readily absorbs lead oxide, but does not absorb metal. The metal is then heated; the lead metal is oxidized to lead oxide. The oxide then is either absorbed into the cupel, or volatized off into the atmosphere. What is left is a tiny bead containing all of the gold and silver that was in the original sample. The bead is then weighed, and the total weight of the gold and silver together is known.
The final step is to separate the gold and silver (in the rare case of a sample without silver the separating process may be skipped.) The gold and silver are parted by flattening the bead and putting it in a solution of dilute nitric acid. The acid will dissolve the silver but not the gold. The remaining purified gold is then weighed. The weight of the silver in the assay is calculated by subtracting the weight of the gold from the weight of the bead that contained both the gold and the silver. Using the known original weight of sample that was used in the assay, the weight of the gold and the weight of the silver, the contents of the ore sample in ounces per tonne can be calculated.
Assay results are instrumental in estimating approximate grade and tonnage reserve estimates . As mentioned above, the assay results are generally converted to grades by using the amount of metal found in a sample and the sample size . If the gold assay results come from drill cores, they must be written in terms of metal concentration over a given length; for example, 15 g/t of gold over 2 meters. While these are only two dimensional snapshots of a gold mineralized area, enough of them in one vicinity are combined to form a three dimensional graph of the tonnage and grade of the gold deposit.
Not only are grades what investors are most likely to respond to- they play an instrumental role in the behind the scenes financial analyses of a mining project. All mining operations come at a cost- and this cost can vary widely according to several factors, such as the accessibility of a deposit, and the current and projected price of the metals contained in the deposit. An estimated gold ore reserve, the average gold grade of the deposit and the cut-off grade are used to evaluate if a deposit hosts favourable economics.
Exploration companies generally compare two key measurements when estimating project economics- the average grade of a deposit (determined by a sampling and assaying program), and the cut-off grade. The cut-off grade is the grade of material below which mining is uneconomic. Calculating the cut-off grade involves a mini-feasibility study in which all the known and potential costs of the project are accounted for. The larger the average grade is in relation to the cut-off grade, the more profitable the mine. Properties whose cut-off grade is higher than its average project grade will not see production, unless the numbers used to calculate cut-off grade change in favour of lower operating costs. The cut-off grade is a dynamic number, and adjustments routinely happen, most commonly, in response to a change in the market price of a metal.
Introduction to Gold Assaying: What's in a Gold Grade?originally posted ongoldinvestingnews.com