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Short answer: A mineral deposit is a mineral occurrence of sufficient size and grade that it might, under the most favorable of circumstances, be considered to have economic potential (USGS Bulletin 1693). It is not yet an ore deposit: that term is reserved for a deposit that has been tested and shown to be producible at a profit.
Definition and scope
The U.S. Geological Survey’s Bulletin 1693, Mineral Deposit Models, sets out three nested terms that are often used interchangeably in everyday speech but are distinct in geoscience.
- Mineral occurrence: a concentration of a mineral, usually considered in terms of a commodity such as copper, barite or gold, that is considered valuable by someone somewhere or is of scientific or technical interest.
- Mineral deposit: a mineral occurrence of sufficient size and grade that it might, under the most favorable of circumstances, be considered to have economic potential.
- Ore deposit: a mineral deposit that has been tested and is known to be of sufficient size, grade and accessibility to be producible to yield a profit. The bulletin notes that the profit decision may reflect wider economic considerations.
The ladder matters because of how exploration actually proceeds. The bulletin describes field work that begins with mineral occurrences, or clues to their existence, and progresses with further study to mineral deposits and only rarely to ore deposits. Some occurrences are unrecognized deposits; others are simply mineralized localities where ore-forming processes were too weak or incomplete for a deposit to form. Related background is in the CASRAI guide to mineralogy.
How deposits are characterized: models and grade-tonnage plots
USGS defines a mineral deposit model as the systematically arranged information describing the essential attributes of a class of mineral deposits. Models can be empirical (descriptive) or theoretical (genetic), and deposits that share many attributes are characterized as a “type”. A descriptive model has two parts: the geological environment (rock types, textures, age, tectonic setting, associated deposits) and the deposit description itself.
Attributes are considered at two scales: local features observable in the field (mineralogy, zonal patterns, chemical haloes) and regional geologic setting, which has to be interpreted from local studies or inferred from tectonic considerations. Grade-tonnage models plot the cumulative proportion of deposits of a type against grade and tonnage, with tonnage on a logarithmic scale; the bulletin notes that cutoff grades vary by region and operator. The bulletin covers non-fuel minerals, including uranium.
Deposit, resource and reserve: the USGS classification
“Deposit” describes a geological concentration; “resource” and “reserve” are classification terms with their own evidence and economic tests. USGS Circular 831 (1980), Principles of a Resource/Reserve Classification for Minerals, defines them as follows.
| Term | USGS definition or meaning | Source |
|---|---|---|
| Mineral occurrence | A concentration of a mineral that is valuable to someone or of scientific or technical interest | Bulletin 1693 |
| Mineral deposit | An occurrence of sufficient size and grade to have potential economic interest under the most favorable circumstances | Bulletin 1693 |
| Ore deposit | A deposit tested and known to be producible at a profit | Bulletin 1693 |
| Resource | A concentration of naturally occurring solid, liquid or gaseous material in or on the Earth’s crust in such form and amount that economic extraction of a commodity is currently or potentially feasible | Circular 831 |
| Identified resources | Resources whose location, grade, quality and quantity are known or estimated from specific geologic evidence; subdivided into measured, indicated and inferred (demonstrated = measured + indicated) | Circular 831 |
| Reserve base | The in-place demonstrated resource that meets minimum physical and chemical criteria | Circular 831 |
| Reserves | The part of the reserve base that could be economically extracted or produced at the time of determination; extraction facilities need not yet be in place | Circular 831 |
| Undiscovered resources | Hypothetical (similar to known bodies in the same district) or speculative (unrecognized types or settings) | Circular 831 |
Circular 831 also distinguishes marginal reserves, which border on being economically producible, from subeconomic resources, which do not meet the economic criteria. A footnote states that “proved,” “probable” and “possible” are not part of the USGS system.
Where it matters for labs and reporting
Circular 831 sets the guideline that every estimate should show a date and an author and should define the geographic area it covers. Sampling and assay data (for example elemental analysis by techniques such as atomic absorption spectrometry) are among the geological evidence behind resource estimates, but the classification itself is a judgement about geological confidence and economics, not a lab result.
For public reporting, CRIRSCO promotes high standards of reporting of mineral deposit estimates (Mineral Resources and Mineral Reserves) and of exploration. Its International Reporting Template draws on national codes including JORC (Australasia), SAMREC (South Africa), PERC (Europe), CIM (Canada), the SME Guide (USA) and the Certification Code (Chile). Policy context for mineral supply is covered in the Critical Raw Materials Act guide, and obligations after extraction in the Surface Mining Control and Reclamation Act guide. Geophysical methods used in the earth sciences are introduced in what is geophysics.
Common misunderstandings
- A deposit is not automatically ore. Under the USGS definition, ore status requires testing that shows size, grade and accessibility sufficient for profitable production. Field work only rarely progresses that far.
- A resource is not a reserve. Reserves are the economically extractable part of the reserve base at the time of determination; resources also include marginally economic, subeconomic and undiscovered material.
- Terms differ between systems. “Proved” and “probable” are outside the USGS Circular 831 system. Check which system a figure is reported under before comparing numbers.
FAQ
What is the difference between a mineral deposit and an ore deposit?
A mineral deposit has enough size and grade to possibly have economic potential; an ore deposit has been tested and is known to be producible at a profit (USGS Bulletin 1693).
What is a mineral occurrence?
A concentration of a mineral that is considered valuable by someone or is of scientific or technical interest, without any implication of economic size or grade.
What is a mineral deposit model?
The systematically arranged information describing the essential attributes of a class of mineral deposits, either descriptive (empirical) or genetic (theoretical).
Does the USGS use “proved” and “probable” reserves?
No. Circular 831 states those terms are not part of the USGS system, which uses measured, indicated and inferred categories for identified resources.
Sources
- Cox, D.P. and Singer, D.A., eds., Mineral Deposit Models, USGS Bulletin 1693, Introduction: pubs.usgs.gov/bul/b1693 (read 2026-11-08).
- USGS Circular 831 (1980), Principles of a Resource/Reserve Classification for Minerals: pubs.usgs.gov/circ/1980/0831 (read 2026-11-08).
- CRIRSCO: crirsco.com (read 2026-11-08).
