RE: west wall2 Jan 2021 21:18
Mining Analyst Kees Dekker opinion
The specified maximum pit wall angles are comparatively low, which could be due to a number of factors: substantial unconsolidated overburden and/or unfavourable fault and joint structures relative to the pit wall. Being a dry climate, the groundwater level should be deep and hydrostatic pressure on the pit wall should not be an issue. What is exactly behind the pit wall failure is not clear. Either there are shortcomings in the study and guidelines, or management has cut corners and did not stick to the protocols and guidelines.
The volume of the deposit is not determined by number crunching, but the geological model defined for the deposit (wire frames) within which the number crunching of the samples are performed.
The model can include several domains for which there are different geostatistical parameters when doing the number crunching.
For a resource estimation to be reliable it is important that the geostatistical parameters are valid. This is in addition to the deposit having inherent characteristics making it suitable for modelling.
For example, if the coefficient of variation (= standard deviation / mean) is very high, the average grade is determined by only a very few, very high values. Resource estimates for such type of deposits are much more risky than for deposits with a low CV, even when the competent person did a lot of top cutting and/or other measures to limit the impact of these very high values.
In the case of Sukari the competent person for open pit resources used multiple indicator kriging (“MIK”). This is typically used for deposits with a very high CV. Ordinary kriging does not work well, requiring usually a CV below 2, achieved by manipulating the data by cutting the top values.
If the promoting company relies on the few very high value for a decent average grade they will push for another estimating technique.
MIK is used to allow inclusion of the very high values.
Geostatistical analysts will argue that the method is valid, but I have my doubts. The Moose River, Brucejack and Sukari deposits for which they have been used, all gave reconciliation problems.