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Consulting Geologist, 8 Franklynn Suite, The Priory, Haywards Heath, West Sussex RH16 3LB, UK
This paper is concerned mainly with assaying of precious and noble metals, gold, silver, and platinum group metals.
The determination of the amount of saleable metals (or other mineral products), in an ore is the most important step in evaluation of a mineral deposit. Most assays, even if well done, will not tell you how the material occurs in the rock or how much of it can be recovered, but if the amount that is present in the first place is unknown, it is impossible to make any comment as to the efficiency of a recovery system. This may not be important because if a recovery can be made which will provide a large profit, that which is not recovered will not seriously affect the economics of the operation. However, where the grade, and thus the operation is marginal, this is a different matter.
Fire assaying, with its origins in antiquity, is widely regarded as the best, if not the only, way to determine precious metals in naturally occurring ores. There are a lot of misconceptions about the universal applicability of fire assay methods and the more recent combination of traditional fire assay collection techniques with laboratory instrumentation has given rise to a serious, if not ludicrous, situation in the case of some materials.
There are some naturally occurring materials which cannot be successfully determined by fire assay. Some of those same materials can only be determined by instrumental techniques if their chemistry is understood.
Even the most sophisticated instruments in the world will not yield the correct answers if the chemistry of the ores being tested is not taken into consideration in designing the sample preparation and instrumental procedures to be used. Such considerations can only be made if the chemistry is understood, and such proper understanding requires the availability of a mind open to the fact that there may indeed by something new (to man) under the sun. It is important to appreciate the implications of the fact that naturally occurring materials may be in an unusual form; hence the need for a clear as well as an open mind.
The evaluation of precious metal deposits of a certain type involves an appreciation of their setting in terms of plate tectonics models.
Case histories of assaying problems with ores from south-western United States are presented which illustrate all of the above points. Attention is drawn to the possible implications of the phenomena described for mineral exploration in the UK as well as world-wide.
