by the Mineral Prospector
Prospecting for Minerals and Metals

Techniques for Mineral Sample Assaying

    There are a number of assaying techniques and it is not within the scope of this section to describe them all. The two most common techniques are Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and fire assay.

    In the ICP methodology a small sub-sample of the material delivered to the laboratory is dissolved in a variety of acids and the resulting liquid is vaporized at very high temperature in a very expensive instrument called a mass spectrometer. The combination of high temperature and magnetic induction in the mass spectrometer results in the breaking of all metallic and molecular bonds. The trajectories of the ionized metals are bent by the magnetic field in characteristic ways and the equipment is capable of creating a visual image or mass spectrum of the material. Reading the peaks of the mass spectrum output allows the operator to determine which metals are present and at what concentration.

    The number of elements that can be assayed from a sample depends upon the type of mass spectrometer being used and the acids used to digest the mineral sample. The ICP method can be varied in order to read minute quantities of elements (parts per trillion) or relatively larger concentrations of elements (%). Often the samples are all subjected to a single type of assay technique and then those samples which are beyond the detection limits (upper or lower) are re-assayed using a different technique.

    Fire assay techniques are the most accurate way of determining gold and silver content in mineral samples. Proponents of an “ore deposit” may insist that their particular gold deposit cannot be accurately assayed using fire assay techniques but this is an indication of a scam rather than unusual mineralogy.  In the fire assay procedure the samples are mixed with fluxing agents, including lead oxide, and fused at high temperature. The lead oxide is reduced to lead, which collects the precious metals. When the fused mixture is cooled, the lead remains at the bottom, while a glass-like slag remains at the top. The precious metals are separated from the lead in a secondary procedure called cupellation. The lead button is placed on a magnesia cupel in a furnace at 960-1000