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It has been accepted almost as dogma that sampling contributes most of the error to fluctuations in gold fire assays. Differences in assay results on pulp material from the same sample interval are frequently regarded as evidence of the presence of the so called “nugget effect” or relatively coarse gold particles. It is true that if coarse gold particles are present they contribute to substantial sampling fluctuations, but while the process of sampling is probably the major source, the analytical process cannot be completely ignored as a possible contributor to erratic assay results.
In order to maintain a stable assay process, the analytic part of the system must be kept in control as well as the sampling and preparation part. One way of monitoring the performance of the analytic system is to systematically assay standard material whose sampling characteristics are carefully controlled.
Gold assay standards are not prepared nor can they be prepared to account for both sampling and analytical errors. It is not possible to send coarse material to a lab for preparation (which includes both the processes of comminution and splitting) followed by fire assay, to make inferences only about the fire assay process. Because most gold ores are very heterogeneous, sampling errors would in most cases completely mask the contribution of the analytical errors. Assay standard material is prepared only to assess the accuracy and variability in the fire assay process.
Because the objective of the assay standard is to provide information about the fire assaying, it is necessary to control the sampling error of the standard material so that it is only a minor constituent of the discrepancies observed in any assay results. To do this requires that the particle size of the standard material be reduced to a point where the relative standard deviation of the sampling error (i.e., the standard deviation of the errors divided by the average gold content of the material) is two percent or less. For all but very homogeneous mineralization this means that the material must be reduced in size to 100% -150 mesh before the sampling errors are adequately controlled. Even reducing the particle size can contribute to sampling problems. Liberation of gold may cause segregation that can cause large sampling fluctuations which are not easily controlled while maintaining the desired grade.
Since in most cases the standard material would already be in the “pulp” state when it is submitted to a lab for assay, it is not possible to entirely conceal the nature of the sample from the lab. This is a problem of using assay standard material. Because of the contribution of sampling to error generation in the assay process, the use of “coarse” material does not solve the problem of submitting a totally “blind” standard to the lab.
In the sections which follow, the selection, preparation, testing and use of gold fire assay standard material is discussed. While some may dismiss the production of standard material as folly, it is possible to produce and utilize standard material to stabilize and improve the fire assay process to produce more reliable assay results.