# Reserves, stocks and depletion: What a reserves figure measures Source page: https://www.materialinsights.org/learn/commodities/reserves-stocks-depletion/p1/ [Understanding Commodities](https://www.materialinsights.org/learn/commodities/) / [Module 4: Reserves, stocks and depletion](https://www.materialinsights.org/learn/commodities/reserves-stocks-depletion/) Part 1 of 5 ## What a reserves figure measures Every mineral tile quotes a reserves figure, and most readers take it as the amount left in the ground. It is something narrower. This part explains how deposits are classified, why reserves behave like an inventory that companies choose how far to fill, and why national figures follow different rules. ### 1.1 Reserves sit in one corner of a two-way classification The US Geological Survey sorts mineral deposits along two axes at once. One is geological certainty: how well a deposit's size, grade and shape are known. The other is economic feasibility: whether it can be worked at a profit with today's prices and technology. The scheme grew out of work by Vincent McKelvey, a former USGS director, and the diagram is called the McKelvey box. On the certainty axis, identified resources are deposits whose location, grade and tonnage rest on specific evidence such as drill holes, trenches and samples. They divide into measured (sampling so close that the body is well defined), indicated (wider spacing, but enough to assume continuity) and inferred (extrapolated beyond the sampled zone). Undiscovered resources are only postulated, either in districts that already produce or in settings where nothing has yet been found. On the economic axis, material is economic, marginal or subeconomic. Reserves are the part of the demonstrated (measured plus indicated) resource that could be extracted profitably when the estimate was made. The USGS also defines restricted reserves, which would qualify except that a law forbids mining them. Tilton and Guzmán add the resource base: all of an element in the Earth's crust, at any grade. For copper they put it at about 1.5 million billion tonnes, against reserves of 0.7 billion tonnes in 2015. The box makes one point plain. Material can cross from subeconomic to reserves without anyone touching the rock, if prices rise or a cheaper process appears, and it can cross back if costs rise. Reserves describe where geology, technology and price meet on one date. In the data #### Copper in each box The USGS puts world copper reserves at 980 million tonnes, against mine output of 23 million tonnes in 2025. Its most recent global assessment, as of 2015, found 1.5 billion tonnes of copper in identified resources not yet mined and estimated a further 3.5 billion tonnes in undiscovered deposits. So reserves are about two-thirds of identified resources and under a fifth of identified plus undiscovered copper. The crustal resource base is over a million times larger than reserves, though almost all of it is too dilute to mine. - USGS Mineral Commodity Summaries 2026, Appendix C [web ↗](https://pubs.usgs.gov/publication/mcs2026) · pdf pp. 221-222 - USGS Mineral Commodity Summaries 2026, Copper [web ↗](https://pubs.usgs.gov/publication/mcs2026) · pdf p. 77 - Tilton & Guzmán (2016), Mineral Economics and Policy · pdf pp. 225-227 ### 1.2 Reserves are a working inventory, so they grow while being mined The USGS itself calls reserves a working inventory, limited by drilling costs, taxes, prices and demand. Its own example: world copper reserves were about 280 million tonnes in 1970. Some 712 million tonnes were mined over the next half century, yet reserves in 2024 stood at 980 million tonnes, three and a half times the 1970 figure. Oil tells the same story. Radetzki and Wårell quote the oil economist Morris Adelman: proved reserves were 51 billion barrels in 1944, 605 billion barrels were produced from 1945 to 1998, and 1,035 billion remained. Adelman's point was that reserves grew because of production, not despite it. Producing fields pay for the drilling that outlines more oil around them, and the oil finally recovered from a field has historically been about six times what was booked at discovery. Why do firms not prove up more? Tilton argues that proving reserves costs money, and ore that will not be mined for three decades is worth little today. Once a company holds 20 to 30 years of output, more drilling rarely pays. Radetzki and Wårell show the result over the long run. Between 1925 and 2000 copper output rose ninefold and oil output 24-fold, yet reserves kept roughly in step, and the reserves-to-production ratio often sat near 30 years. From 1995 to 2018 copper reserves rose from 310 to 830 million tonnes. A reserves figure therefore measures how much exploration and development the industry has so far found worth paying for. It says little about how much will ultimately be produced. Common misreading #### "Only 40 years of copper left" Conway calls this one of the most misread statistics in mining. Reserves are what sits in operating mines and approved projects that could be worked at a profit now. Their size reflects the horizon over which miners plan, not what remains in the Earth. His test: between 2010 and 2020 the world mined 207 million tonnes of copper, yet reserves grew by 240 million tonnes. Additions over the decade were more than twice what was dug out. - USGS Mineral Commodity Summaries 2026, Appendix C [web ↗](https://pubs.usgs.gov/publication/mcs2026) · pdf p. 221 - Radetzki & Wårell (2021), A Handbook of Primary Commodities in the Global Economy · pdf pp. 174-176 - Tilton (2003), On Borrowed Time? · pdf p. 32 - Conway (2023), Material World · pdf pp. 218-219 ### 1.3 National reserve figures are built on different rules No one audits world reserves. The USGS does not measure them; it compiles figures from governments, company reports, academic papers and trade journals. Some national numbers are old estimates carried forward for years because nothing new was reported. Others are old figures minus later production. Listed companies report under codes. Firms on the Australian and New Zealand exchanges must use the JORC Code, written by Australian industry bodies together with the exchange. Firms on the Toronto exchange follow a similar Canadian standard, National Instrument 43-101. Both set minimum standards for public reporting and keep resources (measured, indicated, inferred) separate from reserves (proved and probable). Humphreys explains why this matters: Toronto lists about 1,600 junior explorers and the Australian exchange about 1,000, and their share prices rest on what they report. Government inventories use other conventions. Geoscience Australia reports Economic Demonstrated Resources, which add measured and indicated resources to JORC proved and probable reserves. The USGS reports Australia's reserves as accessible EDR and the gap is wide: Australian copper reserves appear as 100 million tonnes, but JORC-compliant or equivalent reserves are 27 million. For lithium the figures are 8.4 and 5.1 million tonnes. Russia's system, inherited from the Soviet Union, layers categories A, B, C1 and C2 with balance and off-balance reserves, and it is often unclear which is quoted, or whether a figure means ore or metal. A country's share of world reserves on any tile therefore depends partly on how generously it counts, and a world total adds up numbers built on different rules, some of them years out of date. Case #### Bre-X and the reporting rules Humphreys tells how Bre-X, a small Calgary explorer, announced a sensational gold discovery in Borneo in 1995 and kept enlarging it through 1996. In March 1997, shortly after its chief geologist fell from a helicopter near the site, Freeport-McMoRan, which had been assessing the deposit with a view to buying it, reported that it held insignificant amounts of gold. The shares collapsed, dragging junior mining stocks down with them, and Canada introduced NI 43-101 afterwards. A reported resource is a claim until someone independent checks it. - USGS Mineral Commodity Summaries 2026, Appendix C [web ↗](https://pubs.usgs.gov/publication/mcs2026) · pdf p. 224 - USGS Mineral Commodity Summaries 2026, Copper [web ↗](https://pubs.usgs.gov/publication/mcs2026) · pdf p. 77 - USGS Mineral Commodity Summaries 2026, Lithium [web ↗](https://pubs.usgs.gov/publication/mcs2026) · pdf p. 123 - Humphreys (2015), The Remaking of the Mining Industry · pdf pp. 41, 142-143 ### See it in the table [Cu Copper](https://www.materialinsights.org/commodities/copper/)[Au Gold](https://www.materialinsights.org/commodities/gold/)[Li Lithium](https://www.materialinsights.org/commodities/lithium/) ### Check your understanding The copper price rises by half and stays there. No new drilling takes place. What should happen to reported copper reserves? Reserves are the economic part of identified resources at the time of estimation. A higher price moves material across the economic boundary of the McKelvey box without any new geological information. The USGS lists Australian copper reserves at 100 million tonnes, but JORC-compliant reserves are 27 million tonnes. Why? Geoscience Australia's EDR combines JORC proved and probable reserves with measured and indicated resources. The USGS reports the accessible part of that broader figure for Australia, so it is not comparable with company-reported JORC reserves. Adelman said oil reserves grew 'not despite interim production but because of it'. What mechanism did he mean? Reserves are an inventory built by investment. Development drilling around producing fields and better recovery keep adding to what is booked, which is why ultimate recovery has been several times the volume recorded at discovery. [← Module contents](https://www.materialinsights.org/learn/commodities/reserves-stocks-depletion/)[Mark read · Part 2: Reserve life and why it misleads →](https://www.materialinsights.org/learn/commodities/reserves-stocks-depletion/p2/) ## Other sections - [Module overview](https://www.materialinsights.org/learn/commodities/reserves-stocks-depletion/index.md) - [Reserve life and why it misleads](https://www.materialinsights.org/learn/commodities/reserves-stocks-depletion/p2/index.md) - [Depletion: fixed stock or rising cost](https://www.materialinsights.org/learn/commodities/reserves-stocks-depletion/p3/index.md) - [The stock above ground: materials in use and recycling](https://www.materialinsights.org/learn/commodities/reserves-stocks-depletion/p4/index.md) - [Inventories: the buffers between harvest, mine and market](https://www.materialinsights.org/learn/commodities/reserves-stocks-depletion/p5/index.md) - [Key terms, quiz and sources](https://www.materialinsights.org/learn/commodities/reserves-stocks-depletion/review/index.md)