Nugget effect in Au deposits along the Greenstone Belt in Quebec, Canada

Nugget effect in Au deposits along the Greenstone Belt in Quebec, Canada

As I say before, the nugget effect is present in ALL the Au deposits hosted in the Greenstone Belt in Quebec and also in Ontario. The effect intensity can vary from one place to another, but it is always significative, with very-high grade Au concentrations followed of low-grade or anomalous Au values in less than 2-meter distance along the ore zone. It definitely complicates the resource calculation becoming necessary the use of accurate geostatistical tools that consider the nugget effect. Try to ignore the importance of the nugget effect could be a disaster for exploitation planning and for the estimation of the market value of a project. Maybe the no-consideration of the nugget effect is the hidden reason why there are no more mines in production in Quebec according with the high Au potential of the region.

During decades the nugget effect in the Greenstone Belt Au deposits from Quebec and Ontario have been worked out with different methods. However the geological literature available practically never use the term “nugget effect” (the term in Spanish is more popular in South America, “efecto pepita”), we can see that they have dealt with it if we observe the variability of grades in the production zones, the no-continuation of the tunnels following ore zones, the cyclical closing and opening of mines and the explanations done for geologists to some results affected by the nugget effect. What were the solutions found for the problem? The first one and the most important was the positive effect of the nugget concentrations that compensated in a very-high-grade zone what they had lost in one or two poor zones. This compensation, statistically significative and proved with dynamite, was the “ace under the sleeve” that allowed to obtain rentability in a high variability environment. It must be done that this kind of solution is risky and inefficient at the long term, reducing the return of investment expected by the stakeholders. In these times of technology and improved education we can do it better, using geostatistical tools and adequate exploitation methods.

Other solutions were conceived by the mine engineering departments to attack the variability of grades. Some mines have augmented the size of the pit stops having a more-uniform-but-lower grade, other mines did a systematic blending of high and low grades before sending the ore to the metallurgical plant and other mines increased the cut-off grade (to 7 g/t Au, for example), reducing the volume extracted and sending only high grade to the plant, burying a percentage of the resources in the process. Solutions were successful, economically acceptable to survive or they provoked the closing of the mine, depending of the teams in charge.

The understanding of the nugget effect has an enormous impact in the evaluation of projects and the calculation of rentability of a mine. Since this effect is present in all the Greenstone Belt, it is a priority that geologists do not underestimate the impact. Everybody working in the same direction will have an important benefit for the mining industry in Quebec and Ontario.

Fernando Alvarez V., P. Geo, MBA

This article and their versions in French and Spanish are available at www.larocheverte.ca, in Blog section.

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