How do I convert gold grades from oz/ton to g/t?
The short answer is 1 oz/ton = 34.28 g/t. See Abbreviations and Symbols in the Mining Glossary for a complete list of symbols commonly used in mining. The terms ounce and ton do, however, need some explanation.
The imperial unit of ounces per ton (oz/ton) is used to measure gold grade, chiefly in North America. It is a frequent cause of confusion to those in the metric world, particularly when converting into the metric unit of grams per tonne (g/t). The confusion arises because “ton” and “ounce” are ambiguous.
By way of demonstration, which weighs more: an ounce of gold or an ounce of feathers? The answer is an ounce of gold. And that’s because there are two ounces: troy and avoirdupois (or standard). We use troy for precious metals (mainly gold, silver, platinum, palladium, and rhodium) and avoirdupois (avdp) for most everything else.
In mining, ton means short (US) ton (= 907.18 kg). There’s also a long ton, which is used in some industries, but rarely in mining. And there’s the metric ton, which we know as the tonne (t).
Seems incredible, but oz/ton mixes both troy and avoirdupois in the same expression. Spelled out, it means “one troy ounce of metal per short (US) ton of rock”. But we measure the mass of rock in standard units, where 1 short ton = 2000 lb = 32,000 oz avdp (not troy).
Converting to SI units, 1 oz/ton equals 31.10 g (1 oz troy) per 0.9072 tonne (1 short ton), which multiplies out as 34.28 g/t.
Also note that some metal prices, reserves, and stock levels are quoted in (standard) pounds—copper and uranium, in particular. And I hesitate to mention the treacherous troy pound (= 12 oz troy), which crops up in wholesale silver prices.
October 8, 2015 / Tim McAuley / 3