Minerals Hub / Mining & Production / Why one assay can be reported as two different grades
Mining & Production · · 5 min read
Why one assay can be reported as two different grades
A practical decoder for the units and bases behind a reported grade: percent and ppm, oxide and element, TREO and MREO, THM, cut-off, grade × thickness and true width.
Reviewed by Peter Uppal
The short version
A grade is one measurement written on a chosen basis, so the same assay can be reported as an element, as an oxide or, for the rare earths, as a sum of oxides whose membership depends on who defines it. As illustrative arithmetic from CIAAW's 2024 atomic weights, 1.00% Nd₂O₃ is 0.857% Nd: two grades that look different, describing one sample. Comparing two grades means putting them on the same basis first, and because the % sign carries no basis of its own, the basis has to be written beside the number.
One result, two numbers
Take an illustrative sample that assays 1.00% neodymium oxide. Using CIAAW's 2024 standard atomic weight for neodymium, 144.242(3), and the abridged value for oxygen, 15.999 ± 0.001, neodymium makes up, on this publication's arithmetic, 0.8574 of the mass of Nd₂O₃, so the same sample is 0.857% Nd. A second illustrative sample at 2,500 ppm CeO₂, converted with cerium at 140.116(1), is, on the same arithmetic, about 2,035 ppm Ce. Neither figure is any deposit's grade; both show that the basis changes the number without changing the rock.
Percent, ppm and grams per tonne
One percent equals 10,000 parts per million, and a USGS professional paper notes that deposit grades are commonly given in percent or in grams per tonne (g/t), which is equivalent to parts per million. So 1,000 ppm is 0.1%. NIST's guide to SI usage accepts the symbol % for the number 0.01, and adds that because the symbol % represents simply a number, it is not meaningful to attach information to it. It is less welcoming to ppm: "This ambiguity in the names of numbers is one of the reasons why the use of ppm, ppb, ppt, and the like is deprecated."
Why grades are written as oxides
By geochemical convention, concentrations of the major elements in terrestrial rocks are not stated as % of the element but as the % of the corresponding oxide, a convention applied because all the metallic elements are oxidised on earth. These instruments do not measure the oxide itself: neither ICP-MS nor XRF actually measures the concentration of oxygen, and, in that page's example, they do not "see" SiO₂ or CaO, only the Si and Ca atoms. An oxide grade from these instruments is therefore calculated from a measured element.
USGS statistics follow the same habit. Its rare-earth data are in tonnes, rare-earth-oxide (REO) equivalent, unless otherwise specified; its titanium concentrate data are in thousand tonnes of TiO₂ content, unless otherwise specified. Its zirconium chapter uses several bases in one document: reserves in ZrO₂ content, mine production and zircon prices by gross weight, with ZrO₂ content calculated as 65% of gross weight.
Read the basis before you read the number.
Oxide, element and mineral are different quantities
On this publication's arithmetic from the same atomic weights, titanium at 47.867(1) and zirconium at 91.222(3) make up 0.5993 of the mass of TiO₂ and 0.7403 of ZrO₂.
A mineral's formula gives a third number. As idealised arithmetic, the textbook formula of zircon, ZrSiO₄, is about 67.2% ZrO₂ and 49.8% Zr by mass, using silicon at 28.085. That is not the 65% USGS uses as its calculation convention for zirconium concentrate, and neither is a measured composition of any natural sample. On this publication's reading, mineral percentages calculated from chemistry are related to what geochemists call normative mineralogy, an idealised assemblage of minerals in a sample calculated from its bulk geochemical composition, which requires the major oxides as a minimum.
TREO and MREO are sums with changing membership
Geoscience Australia notes that companies usually report REE content as rare earth oxides (REO) and yttrium oxide (Y₂O₃), and that conversion factors must be applied to each individual REO to determine REE metal content. Its review, last updated 19 December 2023, reproduces two company definitions of TREO. One sums fifteen oxides, La₂O₃, CeO₂, Pr₆O₁₁, Nd₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₄O₇, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃ and Y₂O₃; the other is the sum of all rare earth oxides and excludes ZrO₂, HfO₂, Nb₂O₅, Ta₂O₅ and Y₂O₃. Yttrium is in one total and out of the other.
MREO varies too. One explorer's 2025 release defines MREO (Magnet Rare Earth Oxides) as Nd₂O₃, Pr₆O₁₁, Dy₂O₃ and Tb₄O₇. Another company's 2024 announcement used two different MREO sets. A TREO or MREO figure means little until its list of oxides is known.
THM depends on the liquid
USGS says heavy minerals are generally defined as dense minerals that have a specific gravity greater than 2.85; another USGS paper says commonly defined as dense minerals with a specific gravity greater than about 2.9. For a heavy-mineral-sands deposit, grade most often refers to average heavy-mineral content, typically reported in weight percent, and Western Australia's survey abbreviates THM as total heavy minerals. One peer-reviewed study measured it by float-sink in bromoform (SG = 2.89), to provide an indication of the THM content. On this publication's reading, a THM figure is a mineral-basis weight fraction that depends on the laboratory threshold used.
Cut-off, grade × thickness and true width
Under US SEC rules, cut-off grade is the grade (i.e., the concentration of metal or mineral in rock) that determines the destination of the material during mining. One software manual describes grade × thickness as the sum of all grades multiplied by the thicknesses for each sample interval within the intercept. As illustrative arithmetic, 12 m at 1.5% is 18 %·m and 4 m at 3% is 12 %·m, so a thinner, higher-grade interval can carry less. On this publication's reading, a grade × thickness inherits its grade's basis: a %TiO₂-metre is not a %Ti-metre.
Length has a basis too. The JORC Code says that where only down-hole lengths are reported and true width is not known, there should be a clear statement to this effect, and that if the geometry of the mineralisation with respect to the drill hole angle is known, its nature should be reported. The Code's wider categories begin with Exploration Results, which include data and information generated by mineral exploration programmes.
Related
- Exploration — how the samples behind a reported grade are taken
- Ore Processing — how a sample's minerals are separated and measured
- Metallurgy — why a recovered product differs from an in-situ grade
- Commodity Prices — the basis a price is quoted on
- Regulation — the reporting codes that govern how a grade is disclosed
Sources
- PRIMARYCIAAW (IUPAC Commission on Isotopic Abundances and Atomic Weights), Standard Atomic Weights 2024, full table. Values used: Ti 47.867(1), Zr 91.222(3), Ce 140.116(1), Nd 144.242(3), Si interval [28.084, 28.086].
- PRIMARYCIAAW, Abridged Standard Atomic Weights 2024. Five-significant-figure values: oxygen 15.999 ± 0.001; silicon 28.085 ± 0.001.
- PRIMARYU.S. Geological Survey, World Trade Center USGS Bulk Chemistry Results, Open-File Report 01-0429 (2001, from the report number; no date printed). Chemistry Figure 1 caption: '1 percent equals 10,000 parts per million.'
- PRIMARYVan Gosen, B.S., and others, 2017, Rare-Earth Elements, chap. O of U.S. Geological Survey Professional Paper 1802. Front matter, p. vi: deposit grades commonly given in percent or grams per metric ton (g/t), equivalent to parts per million; 1 ppm = 0.0001 percent. p. O17: heavy minerals commonly defined as dense minerals with a specific gravity greater than about 2.9.
- PRIMARYNIST Special Publication 811, Guide for the Use of the International System of Units (SI), 2008 edition (Thompson and Taylor), Chapter 7. Section 7.10.2: % acceptable for the number 0.01; not meaningful to attach information to it. Section 7.10.3: use of ppm, ppb, ppt deprecated.
- SECONDARYKorotev, R.L., 'Geochemical analysis reports', Washington University in St. Louis (undated teaching page). Major elements in terrestrial rocks stated as % of the corresponding oxide by convention, because metallic elements are oxidised on earth; ICP-MS and XRF do not measure oxygen and only see the element's atoms. University teaching page, so tagged SECONDARY.
- PRIMARYU.S. Geological Survey, Mineral Commodity Summaries 2026, Rare Earths (February 2026). Header: data in metric tons, rare-earth-oxide (REO) equivalent, unless otherwise specified.
- PRIMARYU.S. Geological Survey, Mineral Commodity Summaries 2026, Titanium Mineral Concentrates (February 2026). Header: data in thousand metric tons, titanium dioxide (TiO₂) content, unless otherwise specified.
- PRIMARYU.S. Geological Survey, Mineral Commodity Summaries 2026, Zirconium and Hafnium (February 2026). Reserves header (ZrO₂ content); mine production header (gross weight); zircon price line (gross weight); footnote 1: 'Calculated ZrO₂ content as 65% of gross weight.'
- PRIMARYBuckle, Williams, Nathwani and Hughes, 'WebNORM: a web application for calculating normative mineralogy', Frontiers in Earth Science, 2023. Introduction: normative mineralogy as an idealised assemblage calculated from bulk geochemical composition; section 2.1: requires the major oxides as a minimum.
- PRIMARYGeoscience Australia, Australian Resource Reviews: Rare Earth Elements (page last updated 19 December 2023). Introductory paragraph: companies usually report REO and Y₂O₃; GA reports its inventory as REO+Y₂O₃; conversion factors applied to each individual REO. Footnote 124 (Mount Weld) and footnote 9 (Dubbo): two company TREO definitions, with and without Y₂O₃, as reported by GA.
- PRIMARYVictory Metals Ltd (ASX:VTM), North Stanmore updated Mineral Resource Estimate release, 11 August 2025. Definition of MREO (Magnet Rare Earth Oxides) as Nd₂O₃, Pr₆O₁₁, Dy₂O₃ and Tb₄O₇. A record of one company's usage; no grades are taken from it.
- PRIMARYViridis Mining and Minerals Ltd, Colossus maiden Mineral Resource Estimate, ASX announcement, 4 June 2024. Two MREO sets defined in one announcement (p. 18 and p. 4). A record of one company's usage; no grades or thresholds are taken from it.
- PRIMARYVan Gosen, B.S., Fey, D.L., Shah, A.K., Verplanck, P.L., and Hoefen, T.M., 2014, Deposit model for heavy-mineral sands in coastal environments, U.S. Geological Survey Scientific Investigations Report 2010-5070-L. p. 3 and p. 5: heavy minerals generally defined as dense minerals with a specific gravity greater than 2.85; grade most often refers to average heavy-mineral content, typically in weight percent.
- PRIMARYGeological Survey of Western Australia, Titanium Investment Opportunities (February 2026). Resource table abbreviations: 'HM, heavy minerals; THM, total heavy minerals'.
- PRIMARYRahman and others, 'Distribution, Separation and Characterisation of Valuable Heavy Minerals from the Brahmaputra River Basin', Minerals 11(7), 786, 2021. Methods: dense media separation in bromoform (SG = 2.89) to provide an indication of THM content; results: the procedure recovered all minerals with SG > 2.89.
- PRIMARYU.S. eCFR, 17 CFR 229.1300 (Regulation S-K, subpart 1300, definitions), up to date as of 8 October 2026. Definition of cut-off grade.
- SECONDARYRockWare, RockWorks17 help, 'Computing Grade x Thickness Statistics for P-Data Grade Values'. Grade × thickness as the sum of grades multiplied by thicknesses for each sample interval in an intercept. Software documentation, so tagged SECONDARY.
- PRIMARYJORC, Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves, 2012 Edition. Clause 18 (p. 11): Exploration Results. Table 1, p. 29, 'Relationship between mineralisation widths and intercept lengths': statement where only down-hole lengths are reported; geometry relative to drill-hole angle.
- ANALYSISANALYSIS: oxide-to-element factors and the worked conversions in this article are this publication's arithmetic from CIAAW 2024 atomic weights (oxygen at the abridged 15.999); the compositions of ideal zircon and ilmenite are idealised arithmetic from the same weights, not natural compositions; the grade × thickness examples are illustrative arithmetic; the readings that grade × thickness and THM inherit their basis or procedure, and the relation of mineral percentages calculated from chemistry to normative mineralogy, are this publication's framing.Non-public document · no public URL
