High Assays

The treatment accorded abnormally high assays will var\' with each property examined and with every engineer. The usual procedure is to reassay the sample one or more times, to determine if the high result is due to a rich speck in the pulp taken for assay. If the sample as a whole is found to be high, the cut should be resampled. If this result checks the first, some engineers recommend resampling halfway between the high sample and the adjacent samples, and using the average of these results in place of the high result; others advocate the omission of the high assay, using in its place the average of the other assays from the same exposure.

The most reasonable basis upon which to consider a high assay is in the light that it is due to the average grade of ore plus an extra amount of the valuable mineral, and to substitute for it the average of the higher samples from the same exposure the results of which have been accepted.

The Calculation of Results.—The foot-ounce method is the one generally adopted in the calculation of ore reserves. Several elaborate and complicated methods have been put forward by various engineers, but it seems probable that the results of calculation by the foot-ounce method are as accurate as the results of the samples themselves. In this method the length of each sample is multiplied by its assay value; the products from aU the samples in the block under consideration are added, and this total divided by the sum of the lengths, the quotient being the average value.

All calculations should be made in dollars for gold, in oimces for silver, and in percentages for other metals, and these values should not be translated into dollars per ton until the final result for a given block is obtained, when the market price used for the various metals should be set down also.

In calculating tonnages the specific gravity of the ore should be carefully determined; it is not unusual for engineers to assume an average specific gravity for the ore, a procedure that is likely to lead to serious error; this is apparent if it is remembered that the percentage error in the ore reserves is directly proportional to the percentage error in the guess at the specific gravity. With ores composed largely of heavy sulphides, the specific gravity may be determined by estimation, which requires that the percentage of iron be determined in addition to the other base metals; this method will yield accurate results and should be used where the specific gravity of the ore varies greatly, the calculation being applied to the individual sample results. The best method in most cases is to determine the specific gravity directly, weighing in air and afterward in water several batches of material from different parts of the ore-shoot under investigation.

The method of packing a box with ore and determining the weight of a known volume of broken ore, and then introducing a factor to represent the relative volumes of broken ore and ore in place, is open to objection; the factor introduced is a guess, and the result depends upon the ratio of voids to ore and therefore upon the tightness with which the ore is packed into the box. The determination of the specific gravity of a porous ore is a difficult matter; it may best be accomplished by weighing in air and determining the volume of the pieces weighed by overflow of a vessel full of water. If this is done rapidly the result is correct, but does not take into account vugs or open spaces, which must be allowed for.