Guest article by David Price, CEO of Rockfire Resources

Evaluating a mining exploration stock requires some level of knowledge of the terminology used within the sector and the ability to review and understand assay results.

Assaying is the process of determining the quantity of each element within an individual sample, which is representative of a prospect or an ore body. Assaying is usually performed by independent, commercial laboratories, however, some active mine sites will have their own assaying laboratory at the mine to minimise on-going costs of many hundreds of thousands of assays.

The term “prospect” is generally used to describe mineral occurrences and early-stage exploration targets. The term “Project” normally refers to a prospect which has been defined to the point that it is justified for significant funds to be allocated to the prospect and the Prospect becomes a Project. An ore body is a specific term used only when the economic parameters of potential mining have been taken into account. In other words, an orebody has to demonstrate potential to become economic at some point in the future. Without the economic component, mineral accumulations should only be referred to as prospects or projects.

There are numerous factors that combine to determine whether a mineralisation discovery is worth developing. Generally, how easy it is to mine (mining method), the size of the find (tonnage), the ease of extraction (processing), and the concentration of the minerals (grade) are the main considerations. Proximity to existing processing facilities naturally increases the odds of a discovery becoming economic, and a site having such operations nearby is often referred to as having ‘nearology’.

Near-surface deposits are cheaper to extract via open pit mining, as long as the orebody is large enough to be mined in bulk. Generally, open pit mines are no more than 300 metres deep and are several hundred metres wide. Open pit mines can be used in hard rock mining for ores such as metal ores, copper, gold, iron, aluminium, as well as coal mining and many other minerals.

As you might expect, the costs associated with underground mining are considerably higher owing to many factors including additional safety requirements, potentially harder rock, necessity for ventilation, escapeways and water/power provision at depth. As a comparison, if the cost of mining ore at an open pit mine is approximately $10 per tonne, an underground mine may cost around $25-$50 per tonne. The actual cost will vary from one project to another and factors such as rainfall, labour rates, snow depth, ground conditions, terrain, weathering of the rock, altitude, accessibility and fracture density in the rock will all contribute to variation in mining costs.

Does it make the grade?

Once a discovery is made, its economic viability needs to be assessed. Assuming the depth and size of the deposit make it attractive for development, the next thing to consider is its mineralisation grade. This is the proportion of target minerals within the rock and is usually expressed either as grams of target minerals per tonne of rock to be mined (g/t) in the case of precious metals (gold and silver), or a percentage (%) of target minerals within the ore in most other cases.

Analysis of gold content is generally performed by Fire Assay (FA). Each sample submitted by the company is firstly dried, crushed (to -5mm), then pulverized (to -80 micrometers) to create an homogenous sample. A sub-sample is then taken by the laboratory (either 30g or 50g) and generally, the larger the sub-sample, the more statistically accurate the result is likely to be.

A flux and lead bead is then added to the pulverized sub-sample and the sample is put into a furnace to be smelted to a liquid. The gold preferentially adheres to the lead, so as cooling occurs, the lead and gold separate out from the rest of the sample. The lead is ultimately absorbed into the crucible, leaving a small gold ball (prill) in the bottom of the crucible. This gold prill is then analysed using either Inductively Coupled Plasma (ICP) or Atomic Absorption Spectrometry (AAS), which are both spectrometry techniques to determine absolute gold values. 

From the assay results, exploration companies generally calculate the average grade across a deposit and apply what’s called the ‘cut-off grade’ – a minimum grade determined from possible economic parameters for the deposit. Grades beneath the cut-off grade are usually excluded from the data when calculating average grades. Similarly, high-grade cuts are usually statistical high anomalies which are excluded from the data when calculating average grades, to ensure anomalously high grades are not likely to contribute to an over-estimation of the average grade.

When reviewing grades, it is important to note the sample size. The more drill holes intersecting the orebody, the more reliable the data. It is also most important to note the individual sample size which is submitted to the laboratory. Sample sizes will vary from a 50g split to a 5kg sample. The larger the sample contributing to the preparation for fire assay, the more statistically accurate the sample is likely to be. For example, Rockfire Resources instructs the laboratory to crush and pulverises an entire 5kg sample which has been submitted to the lab, prior to a 50g sub-sample being taken by the laboratory for fire assay. This larger, homogenous sample increases the cost of each individual assay, but ensures the most statistically accurate sample is analysed.

Grades can vary quite significantly, with the highest grades frequently grabbing the headlines, but larger tonnages and higher average grades are the best indicators of a potential resource. Assay results are often mapped out in order to pinpoint likely targets for future drilling campaigns.

Author: David Price

Many thanks to David for this guest article. David is Chief Executive Officer and Managing Director of Rockfire Resources plc. David is an experienced geologist and senior executive with +30 years of experience in the global mining industry and has over 20 years’ experience in securing funding for exploration projects. David holds the highest category of membership as a Fellow of the Australasian Institute of Mining and Metallurgy (FAusIMM) and is a Competent Person for Mineral Exploration under the guidelines of the JORC Code. Rockfire Resources is advancing gold and copper projects in Queensland Australia, where historical drilling has already identified significant gold, copper and silver mineralization.