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It is desirable to use material which has as nearly the same metallurgical characteristics as the samples with which the standards will be included. This is usually a difficult chore. For many reasons, including the particle size at which a significant amount of the gold mineral is liberated, the sampling characteristics of even -150 mesh material may preclude the use of geologically and metallurgically similar ore as a standard. It is usually easier to get material with desirable grade characteristics with the necessary sampling properties, than to find geologically and metallurgically similar material with the required sampling characteristics. High grade standards are especially difficult to find and prepare. This is because as grade increases the size of the gold particles usually increases. Larger gold particles are liberated and tend to segregate during comminution and the homogeneity of the material cannot be maintained. For grades much above 3 g/t it is very difficult to find material which has the proper sampling properties.
Likely candidate material for assay standards is old mill tailings. Some of these have sufficiently homogeneous mineral content so that the sampling errors can be effectively controlled. Where mill tailings are either not available or are unacceptable, mineralization that has exhibited homogeneous results in re-assay of pulp material is also a good candidate for the standard. Finally, the mineralized rock being sampled may and should be used if adequate homogeneity in the -150 mesh material exists. (“Adequate” homogeneity or “Acceptable” homogeneity is defined in a later section).
Once candidate material has been chosen, a lot (lot as used here and what follows means the batch of material) must be collected and prepared for testing. A lot to be used as a standard must contain sufficient material. This means that at least 50 kg must be available for use as the standard lot. Lots used for pilot testing of the sampling characteristics of the fine material may be made up of as little as 10 kg. The lot should be collected to minimize the inclusion of foreign or undesirable material. For example, mill tails are typically layered in the tailings impoundment. Material tends to be more homogenous within a layer than across layers. Therefore, when the tailings are collected, the collection of a lot should be confined to a single layer.
Once the lot is collected, the entire mass must be reduced in particle size to -150 mesh (the 150 mesh size assumes that a sampling nomograph has been produced and the relative standard deviation for sampling error at 150 mesh is less than two percent as given by that nomograph). The fine material should be screened through a 150 mesh screen and material not passing the screen should be returned to the pulverizer for further comminution. After two iterations material retained on the screen may be discarded (remember – this is not acceptable sampling practice for determining the grade of the lot, but that is not the objective in this case).
When the lot is reduced in particle size, it must be divided for analysis. This is probably best done using the fractional shoveling technique. The lot should be divided to produce 100 sub-samples of 35-40 g per sample (This weight is based on the assumption of using a one assay-ton portion for fire assay. The weight may be adjusted according to the weight to be used for the fire assay.) The weight of the sub-sample is not absolutely critical so it may vary within a few grams, but it probably should not exceed 40 g or be much less than 35 g. Restricting the amount of material to 35-40 g precludes the testing lab from duplicating assays and reporting an “average” value. Averages have a different distribution of errors than individual samples. Including at least 35 g insures that even if a small amount of material is lost in handling, there will be sufficient pulp for a one assay-ton lot for testing.
The 100 samples should be assigned to one of five groups using a systematic random sampling procedure. When the assignment is complete, each group should have 20 samples. Each sample should then be assigned a sample number designating the group to which it belongs and its number in the group and so labeled.