Assay
A test that measures the amount of one or more specific metals or minerals in a sample of ore or rock, such as a fire assay for gold.
Detailed Definition
An assay is a test that measures how much of a particular metal or mineral a sample contains. The U.S. Bureau of Mines' Dictionary of Mining, Mineral, and Related Terms (1968) describes it as "an examination of a mineral, an ore, or an alloy differing from a complete analysis in that it determines only certain ingredients in the substance examined, whereas an analysis determines everything it contains."
Assay versus analysis
An assay answers a narrow question, such as how much gold or copper is in a sample. A full analysis identifies everything in it. In mining, assays are the everyday test because the question is almost always the value of specific metals.
Methods
BLM's handbook for mineral validity reports asks examiners to name the laboratory method used and names four: "atomic absorption, neutron activation, wet chemical, fire assay." It also asks them to explain why the method was chosen. Fire assay is the traditional method for gold and silver, which the 1968 dictionary notes "require an additional process called cupelling" to separate them from base metals.
Samples and chain of custody
An assay is only as good as the sample. BLM's mineral examiners must describe how samples were collected and handled, "including details such as sample surface preparation, sample type, security procedures, chain of custody." The handbook states that there are "legal reasons" for chain of custody, and asks examiners to explain how they know core splits "were not tampered with."
BLM runs its own National Minerals Testing Lab, which crushes, pulverizes, and splits hardrock samples and ships them to an outside assay lab "meeting the custody and control requirements of the Handbook for Mineral Examiners."
How assays are used
- Establishing the grade of a deposit for a mineral resource or reserve estimate
- Testing whether a mining claim is supported by a discovery of a valuable mineral deposit
- Deciding whether material goes to the mill or the waste dump during mining
- Settling the value of ore sold to a smelter
Why it matters in research
Historical assay results appear in mineral surveys, patent files, company reports, and old mine records. They can show what a tract was prospected for and how rich it was thought to be. When they are relied on, it matters who took the samples, how they were handled, and which lab and method produced the numbers.
Related Terms
Mineral Resource
A concentration of material of economic interest in such form, grade, and quantity that there are reasonable prospects for economic extraction, classed as inferred, indicated, or measured.
Ore
Rock or mineral material containing a metal or mineral in enough quantity and grade that it can be mined and processed at a profit.
Grade
The concentration of a metal or mineral in rock, such as ounces of gold per ton or percent copper, which together with tonnage decides whether a deposit can be mined at a profit.
Core Sample
A cylinder of rock cut by a hollow drill bit and brought to the surface, or a split part of that core taken for assay, showing the rock as it lies underground.
Exploration
Work that tests ground for a mineral deposit, such as sampling, trenching, and drilling. BLM defines it as disturbance greater than casual use to evaluate mineral values.