For a mining company, the orebody is the business — it is a depleting inventory of metal in the ground that can only be sold once. "Reserves and grade" are the two numbers that describe that inventory: how much economically mineable metal a company controls (reserves/resources, measured in tonnes of ore and contained ounces or pounds) and how rich that ore is (grade, measured in grams per tonne for gold or percent for base metals). The core tension is that these figures are estimates governed by economic assumptions, not physical certainties. A higher gold price can turn waste into ore overnight, and a lower one can erase reserves a company spent years drilling out — so the same hole in the ground can be worth wildly different amounts depending on the price deck and cost assumptions baked into the model.

How it's calculated / classified

Disclosure is governed by codes built on the global CRIRSCO template — NI 43-101 (Canada, using CIM definitions), the JORC Code (Australia), and S-K 1300 (U.S. SEC). All three share the same architecture and split metal-in-the-ground into two families by confidence and economic standing.

Mineral Resources (geological confidence, not yet proven economic):

• Inferred — lowest confidence; grade and continuity assumed but not verified from limited sampling.
• Indicated — medium confidence; sampled densely enough to estimate at a reasonable level of confidence.
• Measured — highest confidence; sufficient sampling to confirm grade and continuity between drill points.

Mineral Reserves (the economically mineable portion of measured/indicated resources, after a study):

• Probable — derived from indicated (or sometimes measured) resources.
• Proven — derived from measured resources only; the highest-confidence category.

Two rules matter enormously for investors. First, inferred resources can never be converted directly into reserves — they must first be upgraded to indicated/measured through more drilling (Wikipedia – Mineral resource classification). Second, the leap from resource to reserve requires applying Modifying Factors — mining, processing, metallurgical, infrastructure, economic, marketing, legal, environmental, social and governmental — and at minimum a Preliminary Feasibility Study (PFS) (Micon International; Cassels). Resources are contained metal in the ground; reserves are adjusted for dilution (waste mixed into ore) and metallurgical recovery (the fraction the mill can actually extract), so reserve tonnes are usually lower-grade and recovered ounces lower still.

Cut-off grade is the linchpin. It is the minimum grade at which a block of rock is profitable to mine and process — material above it is "ore," below it is "gangue" (Wikipedia – Cutoff grade). The cut-off is set by metal price and costs, so it is "constantly changing as the economic environment changes." Lowering the cut-off (e.g. after a price rise) sweeps more tonnes into reserves but drops the average grade — the grade-tonnage trade-off at the heart of mine planning.

How it's used in practice

Analysts use reserves and grade to answer a small set of decisive questions:

• Mine life / depletion = reserves ÷ annual production. A miner with 8–10 years of reserves at current rates has a longer runway than one with 4. Reserves deplete every year, so replacement (via drilling, M&A, or cut-off changes) is a continuous treadmill.
• Grade as a margin proxy. Higher grade means more metal per tonne moved and milled, so cost per ounce/pound falls. Grade is the single biggest driver of all-in sustaining cost (AISC) for two otherwise similar deposits.
• Confidence weighting. Sophisticated investors discount inferred resources heavily and value proven/probable reserves most. A "huge resource" that is mostly inferred is a drilling project, not a mine.
• Sensitivity to the price deck. A reserve declared at a $1,500/oz gold price is conservative if gold trades at $2,000; one declared at $2,000 is fragile. Always read the price and cost assumptions in the technical report's reserve table.
• Grade-tonnage and strip ratio. For open pits, the strip ratio (waste tonnes per ore tonne) governs cost; for the same grade, a deep, high-strip deposit is far less attractive.

Adoption, debate & evidence

Reserve/resource reporting is mandatory and near-universal for listed miners — JORC, NI 43-101 and S-K 1300 are the legal language of the sector, and a "Qualified/Competent Person" must sign the estimate. The framework itself is not seriously contested; the application is where skepticism is warranted.

The most reliable empirical trend is secular grade decline: the world's higher-grade deposits were mined first, so average mined grades fall over time. S&P Global Market Intelligence reported that since 2010, gold and copper head grades have declined roughly 13% and 8% respectively (S&P's figures: gold ~13.4%, copper ~7.6%), with stripping ratios rising as miners chase deeper, leaner ore (S&P Global Market Intelligence). Rough, widely cited economic benchmarks: open-pit gold becomes viable around ~0.3–1 g/t and underground around ~3–5+ g/t; copper mill feed typically runs ~0.25–0.6% Cu (Farmonaut grade guide; MarketCap). Treat these as order-of-magnitude ranges — true economics depend on depth, metallurgy, and location.

The honest caveat: reserves are estimates, and they can be overstated. History includes outright fraud (Bre-X's salted gold samples, which NI 43-101 was substantially created to prevent) and more common, subtler optimism — aggressive price decks, generous recovery assumptions, or thin drilling on "indicated" blocks. Reserve write-downs after price drops are routine, not anomalous.

Strengths & limitations

The codes' great strength is a standardised, audited, comparable vocabulary across borders, signed by an accountable expert. Grade and reserves give a genuinely fundamental view of a miner's value that price action alone cannot.

The limitations: estimates are only as good as the price/cost assumptions and drill density behind them, and the headline number can mislead. The #1 misuse is conflating resources with reserves — quoting a large resource (especially inferred) as if it were economically proven metal. The second is ignoring the price deck, so a reserve looks robust only because it was struck at a conservative price. Both flatter a story stock far more than its underlying rock justifies.

Sources

• Wikipedia – Mineral resource classification — measured/indicated/inferred, proven/probable, the inferred-cannot-convert rule.
• Wikipedia – Cutoff grade — cut-off definition and price/cost dependence.
• Micon International – When is a Reserve a Reserve? and Cassels – NI 43-101 technical terminology — Modifying Factors, PFS prerequisite, dilution/recovery.
• S&P Global Market Intelligence – Gold mine stripping ratios rise, grades continue declining — declining gold (~13.4%) / copper (~7.6%) head grades since 2010, rising strip ratios. Benchmark grade ranges cross-checked against MarketCap.
• Farmonaut – typical copper & gold ore grades and NI 43-101 / JORC guide — code overview and grade benchmarks.

Flags: grade/cut-off benchmark numbers are order-of-magnitude industry ranges, not authoritative thresholds. The grade-decline percentages are confirmed against the S&P Global Market Intelligence primary article (gold ~13.4%, copper ~7.6% since 2010); the full S&P dataset itself is behind a paywall, so the precise sample/methodology was not independently re-derived.