A few months ago I wrote a blog about different approaches that mining engineers are using to predict dilution in an open pit setting. You can read the blog at this link. Since that time I have been in touch with the author of a technical paper on dilution specifically related to underground operations. Given that my previous blog was from an open pit perspective, an underground discussion might be of interest and educational.

The underground paper is titled “Mining Dilution and Mineral Losses – An Underground Operator’s Perspective” by Paul Tim Whillans. You can download the paper at this link.

Here is the abstract

For the underground operator, dilution is often synonymous with over-break, which mining operations struggle to control. However, there are many additional factors impacting dilution which may surpass the importance of overbreak, and these also need to be considered when assessing a project. Among these, ore contour variability is an important component of both dilution and mineral losses which is often overlooked.  Mineral losses are often considered to be less important because it is considered that they will only have a small impact on net present value. This is not necessarily the case and in fact mineral losses may be much higher than indicated in mining studies due to aggregate factors and may have an important impact on shorter term economics.

My key takeaways

I am not going into detail on Paul’s paper, however some of my key takeaways are as follows. Download the paper to read the rationale behind these ideas.

• Over-break is a component of dilution but may not be the major cause of it. Other aspects are in play.

• While dilution may be calculated on a volumetric basis, the application of correct ore and waste densities is important. This applies less to gold deposits than base metal deposits, where ore and waste density differences can be greater.

• Benchmarking dilution at your mine site with published data may not be useful. Nobody likes to report excessively high dilution for various reasons, hence the published dilution numbers may not be entirely truthful.

• Ore loss factors are important but can be difficult to estimate. In open pit mining, ore losses are not typically given much consideration. However in underground mining they can have a great impact on the project life and economics.

• Mining method sketches can play a key role in understanding underground dilution and ore losses, even in today’s software driven mining world.

• Its possible that many mine operators are using cut-off grades that are too low in some situations.

• High grading, an unacceptable practice in the past, is now viewed differently due to its positive impact on NPV. (Its seems Mark Bristow at Barrick may be putting a stop to this approach).

• Inferred resources used in a PEA can often decrease significantly when upgraded to the measured and indicated classifications. If there is a likelihood of this happening, it should be factored into the PEA production tonnage.

• CIM Best Practice Guidelines do not require underground ore exposure for feasibility studies. However exposing the ore faces can have a significant impact on one’s understanding of the variability of the ore contacts and the properties of minor faults.

Conclusion

The bottom line is that not everyone will necessarily agree with all the conclusions of Paul’s paper on underground dilution. However it does raise many issues for technical consideration on your project.

All of us in the industry want to avoid some of the well publicized disappointments seen on recent underground projects. Several have experienced difficulty in delivering the ore tonnes and grades that were predicted in the feasibility studies. No doubt it can be an anxious time for management when commissioning a new underground mine.

Note: previously I had shared another one of Paul’s technical papers in a blog called “Underground Feasibility Forecasts vs Actuals”. It also provides some interesting insights about underground mining projects.

If you need more information, Paul Whillans website is at http://www.whillansminestudies.com/.

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