woods point geology, page-59

WW111,

See if this analogy works for you.

Two kids have a physical disagreement in the sandpit. One of them drops a $2 coin and so breaks off the argument. Unfortunately the coin has buried itself in the sand and can't be found. The kids borrow a strainer from the kitchen and start sieving the sand. They need to sieve lots of sand before they have a good chance of finding the coin. If they didn't know the coin was there, they would soon give up because almost every sieve would have zero coins.

This is the nugget effect. A tonne of ore might contain one 10g nugget, ten 1g nuggets, a hundred 0.1g specks and a thousand 0.01g flecks. This would be rich ground with 40g of gold/tonne but chances are you would need ten 100g samples before getting just one fleck. To make matters worse, that one fleck would give an inflated grade of 100g/tonne unless you took account of all the barren samples. A 1kg sample size would give you a better chance of picking up a fleck but you would need to sample 10kg to get one of the specks and the whole tonne to be sure of picking up the 10g nugget.

It actually gets worse because the gold is likely to be concentrated in zones rather than dispersed. We need to carefully choose where we sample. A bit like the two kids really. They would soon get sick of sieving sand and leave the strainer to rust. Then they would go to the laundry and mine Dad's pockets.