Ore Reserve Definition
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).