Here’s your 3-minute quick read of Fresh Thinking by Snowden Optiro’s podcast Episode 109.
We’ve pulled out the key insights and takeaways – perfect for when you want the value, minus the headphones.

Resource Classification in Practice – Beyond Drill Spacing

In this episode of our Fresh Thinking Podcast, Senior Consultant Dr Gregory Zhang sat down with Executive Consultant Ian Glacken to explore practical approaches to resource classification. Building on the first discussion in this series, Ian explained two widely used methods – the 90/15 approach and the scorecard method – and highlighted why classification goes far beyond drill spacing.

The 90/15 Approach

The 90/15 method, developed by Harry Parker and Christina Dohm in the 1990s, is now commonly applied across the mining industry.

As Ian explained:

“Essentially what we’re looking at is how often the 90% confidence interval is outside the mean plus or minus 15%. Those blocks that fail this test will receive a lower classification.”

Three elements define this approach:

• Confidence interval – how often estimates fall within range.
• Error tolerance – usually ±15%, reflecting what can be absorbed without damaging the business.
• Production parcel size – for example, quarterly parcels for measured resources and annual parcels for indicated.

While robust, Ian pointed out that 90/15 is not as straightforward as drill spacing and often requires simulation expertise:

“There’s still a widely held – and wrong – perception that drill spacing is all you need for resource classification. Drill spacing is the starting point, but this approach goes way beyond that.”

The Scorecard Method

The second technique discussed was the scorecard method, which formalises what competent persons already do – weighing up multiple factors when classifying resources.

Criteria such as drill spacing, kriging efficiency, QAQC, geological continuity, and geophysics can be included, with scores and weightings agreed by a team. This ensures a structured, transparent process, while still allowing for professional judgement.

As Ian noted:

“The classification from the primary criteria – drill spacing, 90/15, or whatever – is the pinnacle. From there, the only way is down.”

In other words, additional considerations such as data quality or structural complexity can downgrade the classification, but not upgrade it.

Applying the Methods in Practice

Ian shared a case study of applying 90/15 to a large porphyry copper deposit. The mechanical analysis initially suggested an optimistic classification, particularly for indicated resources. However, once the competent person’s overlay was applied – recognising insufficient drill coverage at depth – the resource was downgraded.

This example highlights the balance between quantitative methods and professional judgement:

“The 90/15 approach provides the starting point, but the ultimate classification requires geological and structural insight from the competent person.”

Key Takeaway

Resource classification is not about rigidly applying a single metric. Whether using 90/15, scorecards, or other methods, a combination of statistical rigour and geological judgement is essential. Or, as Ian put it:

“If you still believe that drill spacing alone is the only classification criterion – that’s not cutting the ice.”

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

The full episode of Fresh Thinking – Resource Classification in Practice is available on Spotify, Apple Podcasts, YouTube, and all major platforms. Watch the full episode on YouTube

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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.

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