Porgera is a large-scale open-cut operation and lies in the Highlands of Papua New Guinea. The

Question

deposit is hosted by dioritic stocks of Miocene age and includes lode gold. Porgera produced 32 tonnes of
gold and 5.6 tonnes of silver in 2004 from a large open-cut and a small underground operation beneath
the pit. Total material mined is 208,000 tpd and mill throughput is 15,600 tpd. Reserves stand at 7.2
million ounces grading 5 700 000 000 nanogram per tonne gold (DoM., 2005). What is the gold grade in;
i) grams per tonne (g/t)?
ii) parts per million (ppm)?

Answer 1

To convert the gold grade from nanograms per tonne to grams per tonne and parts per million, we first need to understand the units:

1. Gold grade in nanograms per tonne (ng/t):

   • You've stated the reserve grade is 5,700,000,000 ng/t.

2. Conversion from ng/t to g/t:

   • 1 gram = 1,000,000,000 nanograms (since there are \(10^9\) ng in 1 g).
   • To convert ng/t to g/t, divide the value in ng/t by \(1,000,000,000\):

   \[ \text{Gold grade in g/t} = \frac{5,700,000,000 \text{ ng/t}}{1,000,000,000} = 5.7 \text{ g/t} \]

3. Conversion from g/t to parts per million (ppm):

   • In this context, 1 g/t is equivalent to 1 ppm because 1 ppm refers to 1 part of a substance in 1 million parts, and 1 g/t signifies 1 gram of gold in a tonne (1,000,000 grams).

Therefore, we can conclude:

i) The gold grade is 5.7 grams per tonne (g/t).
ii) The gold grade is also 5.7 parts per million (ppm).