Alchemy or Miscalculation, page-9

Further to stockpile grade irregularities;
The mined grade for the ore stored on the ROM pad comes from grade control sampling that is performed on all mines. The frequency on a per tonne basis sampling is usually close to the frequency of the head grade sampling as ore enters the plant.

My metallurgist mate told me that most modern processing plants have automatic samplers for their head grade sampling and that this analysed grade is used for the reconciliation. It has nothing to do with gold trapped in the plant nor the recovery in the plant. The recovery is calculated from a calculated head grade that takes into account the tailings grade and gold in circuit and may also be effected if there is a systematic bias in the analysis.

The facts are that the reconciliation indicates a 29% error between the mined grade and the back calculated stockpile grade; my metallurgy didn't need to use any complicated maths to work that fact out.