We are often asked “How much, and what form of, drilling do I need to do to be representative?” Like many questions, the answer is: “It depends”.
Drilling requirements change for each project and sometimes even within a project. How and why does it change?
Representivity means that the sample of ore you are taking has the same qualities of the original ore source. Therefore, to know the requirements for representivity you need to know the original source. You need to know the number and quantity of each ore types under consideration, their location and the mine schedule. We normally nominate that if an ore type is more than 10 per cent of the ore body it needs representation in the testwork program.
The first decision is the diamond core vs RC chip decision. This decision is easy for the Metallurgist. We always need to use diamond core for a representative sample. RC drilling markedly breaks down the ore, by its very action, so this energy put into the ore matrix cannot allow the sample to be metallurgically representative.
Diamond core, in the main, still represents the original structure of the ore. Diamond core is also larger diameter so, as a lot of the testwork done is size based, we need the large ore particles. For instance, there is no way to determine coarse breakage on fine samples.
The test program requirement determines the amount of ore to be tested. Most programs are a two-stage process. The main work and flowsheet development use a compositing philosophy to test the optimum processing conditions, which are tested on a range of samples, taken from all over the project, in a variability philosophy.
Project resource and reserve drilling
In the composite testing stage a mixture of similar ores are combined to make a composite (or number of) which represent the expected plant feed. The testwork performed on these composites develops the main knowledge base for the project.
Thus, we must ensure enough samples of each ore type to be in the plant feed, in its correct proportion, is present in these composites. If an ore type is a large part of the deposit then it needs more drill holes to represent it. Simply taking more ore from one hole is not representative.
Following the composite based testwork, we need to ensure that the composite does not mask individual ore zones issues. If there is a troublesome quality in the mix, but it is all in one section of the project, then it may not be an issue when diluted in a composite, but may be a major issue when treated alone. Say a small zone of
refractory gold ore may not influence the overall recovery but may render that section of the deposit uneconomic. This is the role of variability testing.
For variability testing, a selection of samples in and out of the major composite intervals is required to ensure that the various ore types respond to the selected flowsheet in an acceptable manner. For this section, a range of samples that correspond to minor and major ore types are needed so that any bad performing
sections are not included in reserve calculations.
From this approach, we can see that to develop a good metallurgical program testwork samples from each ore type are needed and enough of each to represent its importance in the resource. Additional samples representing minor ore zones considered in the mine need consideration to develop the reserve profile.
Therefore, the geology, mining and metallurgy disciplines all have a part to play in the drilling selection and sample development process and one cannot really progress without the others. Each decision depends on the others.
Mineral Engineering Technical Services
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