Here’s your 3-minute quick read of Fresh Thinking by Snowden Optiro’s podcast Episode 106.
We’ve pulled out the key insights and takeaways – perfect for when you want the value, minus the headphones.

Smarter Mineral Resource Classification Strategies

In this episode of Fresh Thinking, Snowden Optiro’s Ian Glacken and Graeme Lyall explore the nuts and bolts of mineral resource classification – from traditional drill spacing techniques to more robust methods like the two-indicator approach. This is part one of a two-part series diving into the evolving best practices for resource estimation.

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Why Classification Matters

Resource classification – breaking down resources into inferred, indicated, and measured categories – is a common thread across most international reporting codes. The goal? To provide confidence levels that underpin future mine planning and investment decisions. As Ian points out:

“Resource classification is one of the things that’s fairly consistent across just about all the codes… but the real challenge is how we take our blocks and classify them into those categories.”

The discussion introduces the 90-15 approach, a method developed by Dohm and Harry Parker in the 1990s. This approach focuses on ensuring that production parcels – such as one- or three-month volumes – have a 90% confidence interval with less than 15% error. It’s widely adopted by major mining companies and provides a robust starting point for classification.

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Moving Beyond Simple Drill Spacing

While drill spacing remains a common metric, Graeme stresses that modern approaches demand more rigour. Rather than relying on irregular drill grids, he recommends calculating an equivalent grid by assessing the distance to the nearest three drill holes – a method widely used in the industry.

“What we’re saying is not ‘don’t use drill spacing.’ Absolutely not – you should use it, but use it in a structured way, as a starting point for your classification,” Graeme explains.

This technique provides smoother and more continuous classification results, reducing the risk of the dreaded “spotted dog” effect – patchy, impractical classifications.

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The Two-Indicator Approach

Graeme also introduces the two-indicator approach, developed with the late Dr Harry Parker. This method combines a tonnage indicator and a metal indicator to assess both geological continuity and grade uncertainty. By calculating these variables for large production units, the method identifies grid spacings that deliver acceptable levels of uncertainty (typically 15% error at 90% confidence).

“Dr Parker pioneered this long before simulation became common. We used his two-indicator recipe as far back as the early 90s – and it still works today,” Graeme recalls.

While this technique is particularly effective for porphyries and IOCG systems, they note that other deposit styles, like structurally hosted gold or laterites, may require different approaches.

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A Foundation, Not a Finish Line

Both Ian and Graeme emphasise that classification techniques – whether drill spacing, 90-15, or the two-indicator method – serve as starting points. A competent or qualified person must still apply their professional judgement, considering geological, mineralogical, and economic factors.

This episode ends with a preview of part two, where the team will explore the 90-15 method in more detail and introduce the scorecard approach – another practical tool for resource classification.

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Missed the podcast?
Listen to Episode 106 of Fresh Thinking to hear Ian and Graeme break down these strategies in depth – and discover how modern resource estimation blends data, confidence intervals, and professional expertise.our project and help maximise NPV.

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🎧 Catch the full conversation in Episode 106 on your favourite podcast platform, or share this summary with colleagues.

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Watch or listen to the full episode
Available now on YouTube, Spotify, Apple Podcasts, and all major platforms.

Stay tuned for more 3-minute articles from our Fresh Thinking 3-Minute Read series.

If you would like to contact either Ian or Graeme: contact@snowdenoptiro.com

Here is a LINK to our Snowden Optiro YouTube channel.

Snowden Optiro is a resources consulting and advisory group that provides independent advice, consulting and training to mining and exploration companies, their advisors and investors.

We help mine developers to advance their projects, mining companies to improve their operations and their professionals, and investors to de-risk their investments by the provision of quality advice, training and software in the field of Mineral Resources and Mineral/Ore Reserves.

LINK to Snowden Optiro website