How to Read an Ore Assay: Grades, Payables and What Actually Gets Invoiced
KWE Metals Technical Team · · 8 min read
Grams per tonne, percentage copper, XRF versus fire assay, moisture and treatment charges — a practical guide to converting a certificate into a price.
An assay certificate is the single most important commercial document in a raw materials transaction. It states what is in the material. Everything else — price, payable terms, treatment and refining charges, penalties — follows from it. Yet assay documents are routinely misread, most often by treating a headline number as if it were the price basis.
Units: percent, g/t and ppm
Base metals are reported as a percentage by mass: 18% Cu means 180 kg of contained copper per tonne of material. Precious metals are reported in grams per tonne (g/t), which is identical to parts per million. A 37 g/t gold result therefore means 37 grams of gold in one tonne of rock — a strong result, but only meaningful when you also know the tonnage it represents and whether it is a single sample maximum or an average across a defined block.
XRF versus fire assay
Handheld and benchtop X-ray fluorescence is fast, non-destructive and excellent for field screening, grade control and sorting. It reads a surface, so it can be biased by a polished face, coating or heterogeneous specimen. Fire assay remains the reference method for gold and silver because it takes the entire sample charge into solution and collects the precious metals in a lead button. Serious commercial settlement uses fire assay with gravimetric or ICP finish, ideally from an accredited third-party laboratory with umpire provisions written into the contract.
From grade to invoice
- Dry metric tonne: wet weight less moisture. Moisture is never paid for.
- Contained metal: dry tonnes multiplied by grade.
- Payable percentage: the share of contained metal the buyer pays for, typically 96–98% for clean copper concentrate and lower for complex material.
- Treatment charge (TC) per dry tonne and refining charge (RC) per payable unit are deducted.
- Penalties apply to deleterious elements such as arsenic, antimony, bismuth, mercury, chlorine and fluorine above threshold.
Price the contained payable metal, not the pile. Two shipments of identical weight can differ by an order of magnitude in value.
KWE Metals publishes indexed pricing across the shop catalogue: each listing is recalculated against the live exchange benchmark for its principal metal, adjusted by the stated grade and payable factor. That keeps a quoted figure honest as the market moves, and it lets a buyer see precisely which assumption to challenge.