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I've seen many papers on mineral deposits say something along the line of "a 100,000 tonne deposit of Zn, 1% grade." Does this mean there is 1% of 100,000 tonnes which is 1000 tonnes pure zinc, or does it mean there is a total zinc deposit of 100,000 which is 1% of the total giving 10,000,000 tonnes including the waste material?

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  • $\begingroup$ When I went to school , an ore was a deposit from which something could be produced at a profit. A mineral deposit was everything else. As economic conditions change , a deposit may change back and forth between "ore" and "mineral". $\endgroup$
    – blacksmith37
    Nov 25, 2019 at 20:42

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Ore reserves represent the economic portion of a mineral deposit. They are reported as tonnes and grade above a minimum value (cut-off grade).

Irrespective of whether the ore reserve will be mined by surface or underground methods the deposit is blocked out and the mass of each block and the mass of the valuable commodity (zinc, copper, gold, etc.) is calculated for each block. From this the grade of each block is also determined. The reserve is determined by summing the masses of all the blocks and determining the overall grade of the total.

In your example, a deposit of 100,000 tonnes grading 1% zinc means there is 100,000 tonnes of rock that can be mining and treated by a processing plant (mill). The amount of zinc metal contained within that 100,000 tonnes of rock is 1000 tonnes.

Similarly, a gold deposit that contains 2 Mt @ 8 g/t, contains 2 million tonnes of rock grading 8 grams per tonne, or 8 parts per million. The amount of gold contained in that 2 million tonnes is 16,000,000 g (16,000 kg or 16 t).

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It means there is an estimated 100,000 tonnes of recoverable zinc in the lifetime of the mine, and the ore it produces is 1% zinc. As zinc ores usually contain other metals such as lead, it is not necessarily the case that this ore is 99% waste material.

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  • $\begingroup$ As someone who has spent decades determining ore reserves for various commodities & mining methods, you answer is wrong. $\endgroup$
    – Fred
    Nov 25, 2019 at 4:43

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