Issues Concerning the Quality of Assay Results
“It is necessary that the assayer who is testing ore or metals should be prepared and instructed in all things necessary in assaying, and that he should close the doors of the room in which the assay furnace stands, lest anyone coming at an inopportune moment might disturb his thoughts when they are intent on the work.” Agricola
Phillip L Hellman Hellman & Schofield Pty Ltd, Suite 6, 3 Trelawney St, Eastwood NSW 2122 Australia
It should not be assumed that assays of samples collected during activities associated with mineral exploration, drilling and metallurgical testwork will be either accurate or precise. The onus of responsibility of monitoring quality should be on those who submit samples.
Assumptions of quality that depend upon, inter alia:
- Certification or affiliation of the laboratory,
- Use of internal standards by the laboratory,
- Apparent accuracy of internal standards as reported by the laboratory,
- Agreement between original assays and repeat assays by a second, third or subsequent laboratory,
- Agreement between Calculated Heads and Head Assays in metallurgical testwork should not be made.
Numerous examples are presented highlighting problems such as:
- cross contamination of gold,
- incorrect assay technique leading to under-statement of gold,
- background analytical error resulting in delineation of waste as ore,
- assay bias induced by lithology and presence of coarse gold and
- incorrect calibrations.
These issues reinforce the need for the submission of control samples such as blanks and standards, as well as properly designed check assay campaigns, to:
- provide proof of accuracy and precision,
- provide early warning signals of assay problems,
- identify or eliminate the source of error when issues arise such as poor reconciliations (eg between resource model vs grade control, grade control vs mill),
- minimise risks associated with resource development.