Mineral resource estimates are often widely promoted by mining companies. Some lead to sharp stock market gains and make it easier to secure new financing. These announcements play a central role in the mining industry, yet remain poorly understood by non-specialist observers. Here is a closer look.

In September 2025, Australian company Resolute Mining released an updated mineral resource estimate for its future Doropo gold mine in Côte d’Ivoire. The company reported a 28% increase in the project’s mineral resources compared with the previous estimate published in October 2023, following new drilling work. The project is now estimated to host 4.36 million ounces of gold, a figure that remains provisional and subject to change.

• What is a mineral resource estimate and what does it actually measure?

A mineral resource estimate is a technical assessment of the geological potential of a deposit, based on data available at a given point in time. It aims to quantify the amount of ore present underground and its average metal grade, using information mainly drawn from drilling results and laboratory analyses.

Such an estimate does not assess the economic viability of a project or its ability to enter production. It only reflects what is considered to be present in the ground, with varying levels of confidence depending on the density and quality of available data. This leads to a classification into three resource categories.

“Inferred” resources represent the lowest confidence level. Geological indications and samples exist, but they are insufficient to confirm the continuity of the deposit and grades, resulting in high uncertainty. “Indicated” resources rely on more extensive data, allowing continuity to be reasonably assumed. “Measured” resources reflect the highest level of confidence, with geological and grade continuity confirmed by detailed and reliable data.

Of the 4.36 million ounces estimated at Doropo in September 2025, only 78,000 ounces fall into the measured category. Indicated resources account for the largest share, at 3.6 million ounces, while inferred resources total 680,000 ounces. As exploration work progresses, the understanding of the deposit improves and the resource estimate becomes more precise.

• Reference frameworks and what they guarantee

Several international frameworks are used to report mineral resource estimates, depending on countries and markets. Canada applies the NI 43-101 standard, Australia uses the JORC Code, South Africa follows the SAMREC Code, while Europe often refers to the PERC standard. For mining companies, especially those that are listed or seeking to attract investors, these estimates must follow strict reporting and accountability rules. The goal is to allow projects to be compared and to limit misleading disclosures.

These frameworks impose a common reporting structure and require transparency on technical and economic assumptions. They also mandate the involvement of qualified professionals. Geologists certify the reliability of the estimates by signing off on the reports. In Africa, most projects targeting international investors publish their estimates under the Australian JORC Code or Canada’s NI 43-101 standard, as these are the most widely recognized frameworks on the stock exchanges where foreign mining companies raise capital.

• Why and when mining companies publish resource estimates

The first estimate released by a mining company is known as a “maiden resource.” It represents a key milestone in a project’s development, as it provides the first quantified view of the deposit or deposits. At this stage, data are usually limited, and resources are often classified in the lower confidence categories.

As the project advances through its life cycle, subsequent estimates refine or expand the initial assessment. Additional drilling and data collection help improve confidence levels and better define the scale and quality of the resource.

Companies publish resource estimates because they serve as the official reference point for describing a mining project. Without an estimate, it is difficult to assess the potential size of a deposit, compare an asset with other projects, or explain the relevance of ongoing exploration work. Resource estimates also support credibility and transparency, especially for listed companies. They formalize technical progress and allow companies to communicate within a regulated framework, rather than through vague claims of “positive drilling results.”

A resource estimate is updated throughout a project’s life cycle, both before a mine enters production and during the operating phase. As drilling programs expand, companies can extend the explored area, better define the limits of the deposit, refine grades, and reduce uncertainty. Successive updates highlight what has changed since the previous version. This may include larger volumes, improved average grades, a shift between inferred, indicated, and measured resources, or depletion linked to mining activity.

In the October 2023 estimate for Doropo, indicated resources stood at 3.02 million ounces and have since increased. Each new estimate therefore reflects the state of geological knowledge at a given point in time, not a definitive conclusion.

• Key elements for interpreting a resource estimate

To interpret a resource estimate correctly, the first step is to examine the breakdown between inferred, indicated, and measured resources. Changes do not carry the same meaning depending on the category affected. Ore quality is also as important as quantity. Two projects may report the same total volume, but one may host higher-grade ore while the other is more diluted. An estimate that grows in volume but declines in grade does not have the same implications as one that improves on both fronts.

Another key point to check is the “cut-off grade.” This is the threshold used to decide what qualifies as ore in the estimate. In practice, the company sets a limit. Below it, the rock is considered too low-grade to be included. Above it, it counts toward the resource. A stricter cut-off can reduce the reported resource size, while a more flexible threshold can increase it.

• What comes next?

A resource estimate marks the transition from exploration to a more concrete assessment of a deposit. Once this benchmark is established, the company must determine what this geological potential can translate into in practical terms. The project then moves toward economic studies designed to quantify future operations. These studies assess required investment, expected production levels, potential revenues, and key constraints, including environmental impacts.

They also rely on the conversion of part of the resources into reserves. Reserves represent a more robust estimate of a project’s potential, based on operational and economic parameters. They usually provide a closer indication of the volumes that will ultimately be produced and are used to estimate a mine’s lifespan. However, this conversion process, and what it implies, deserves a separate article.

Emiliano Tossou