Mass-Balance-Consistent Geological Stock Accounting: A New Approach toward Sustainable Management of Mineral Resources

Global resource extraction raises concerns about environmental pressures and the security of mineral supply. Strategies to address these concerns depend on robust information on natural resource endowments, and on suitable methods to monitor and model their changes over time. However, current mineral resources and reserves reporting and accounting workflows are poorly suited for addressing mineral depletion or answering questions about the long-term sustainable supply. Our integrative review finds that the lack of a robust theoretical concept and framework for mass-balance (MB)-consistent geological stock accounting hinders systematic industry-government data integration, resource governance, and strategy development. We evaluate the existing literature on geological stock accounting, identify shortcomings of current monitoring of mine production, and outline a conceptual framework for MB-consistent system integration based on material flow analysis (MFA). Our synthesis shows that recent developments in Earth observation, geoinformation management, and sustainability reporting act as catalysts that make MB-consistent geological stock accounting increasingly feasible. We propose first steps for its implementation and anticipate that our perspective as “resource realists” will facilitate the integration of geological and anthropogenic material systems, help secure future mineral supply, and support the global sustainability transition.


S1. Materials and methods
Key reviews from the domains of economic geology, industrial ecology, and Earth systems modeling highlight the urgent need for transdisciplinary systems integration.Prior review articles from industrial ecology, for instance, are testament to the growing recognition of material flow analysis (MFA) as a robust tool for quantitative assessments and scenario modeling.In Earth and environmental science, the quantification of natural processes, substance flows, and elemental cycles (e.g.erosion and deposition, ore deposit formation, global carbon cycle) has long been central for understanding the Earth system and its spheres.However, it is still debated how mineral resources can be robustly quantified and monitored.Here, we review how mass-balance (MB) principles may support integration of the anthropogenic and the geological subsystems into physical Earth System models, and how MB-consistent geological stock accounting can facilitate scenario modeling and sustainability analysis.Rather than aiming for a complete and systematic literature review of one specific research domain, our Critical Review article takes a broader approach to provide a transdisciplinary and integrative overview across the relevant fields that together define our topic, as shown in Figure S 1-1.

S1.1. Literature selection
We queried the Web of Science (WOS), Scopus, and Google Scholar with various search term combinations including "(environment* OR sustainab*) AND (mining OR mines OR ore OR (mineral reserves) OR (mineral resources) OR Deposits)", "material flow analysis", "geological stocks", "mineral resource depletion", "multidimensional geomodels", "digital twin", and others to collect relevant publications for an integrative literature review approach 1 that can shed light on key terminology, important concepts, and critical knowledge gaps across different research domains.Moreover, we evaluated the references in individual publications that we considered particularly relevant to find further research to include.Next to scientific articles, we also considered documents from the 'grey literature' domain, including books, industry standards, international agreements, and government reports, as these relate to current public administration and business practices that are often not covered by the formal literature, but nevertheless relevant for physical accounting.Queries of WOS and Scopus produce broadly overlapping but also partially complementary results.We used published workflows 2,3 to combine the two datasets into a more complete collection for further analysis and discussion.

S1.2. Bibliometric analysis of mass-balance-consistent accounting for minerals
For our bibliometric analysis and visualization of results, we used WOS, Scopus, and Bibliometrix. 4Publications in the grey literature domain are not indexed in WOS and Scopus and were thus not part of the Bibliometrix analysis.

S1.3. Historical development of MFA
We used Google Scholar to compile a systematic and broad overview over the total number of publications linking "material flow analysis" with "mineral reserves".The combined query "material flow analysis" -[i.e., NOT/without the term] "mineral reserves" for the period 1970 to 2022 shows strong growth in the adoption of MFA outside of the minerals community, with publications in this group accounting for 99.1% of all publications mentioning MFA (Figure S 1-3).In contrast, publications that mention both terms "material flow analysis" + "mineral reserves" (i.e., the mutually exclusive complement of the query above) only make up 0.9%, or 223 out of a total of 24048 publications.That the annual number of publications mentioning both "material flow analysis" + "mineral reserves" remained stable at around 20-30 per year over the past 10 years indicates that MFA has not been widely adopted by the geological and minerals community, while interest in MFA outside of the geological community has rapidly grown.This highlights the gap that our Critical Review article addresses.For reference, we also plot the trends for "3D geological model" and "mineral resource depletion".
The relative importance of MFA in the field of geosciences versus in the field of environmental sciences and engineering can be compared using WOS to calculate the annual field-or categorynormalized research output. 5,6WOS uses a journal-based classification system where journals are assigned to one or more of around 250 categories. 6We normalize the number of publications for an "All fields" query "material flow analysis" for two mutually exclusive and complementary WOS  Further research on applying MFA in geosciences, including the development of dynamic and spatiotemporally resolved system models, could provide data and methods to evaluate raw material supply alternatives and address sustainability issues.That mining is inevitable and yet faces increasing challenges, makes robust physical monitoring and analytical methods that support planning even more pertinent.

S2. Timeline of resource classification and sustainability efforts
The history of mining dates to at least 43 000 years BP. 7Agricola's seminal "De Re Metallica" 8 published in 1556 and the U.S. Geological Survey (USGS) "Principles of a resource/reserve classification for minerals" 9 of 1980 are among many notable contributions that established the methodology and terminology for how to describe, quantify, and classify mineral deposits and their potential for being mined to produce tradable commodities.Many studies on resource classification are published only in the grey literature domain, for instance by geological surveys, industry associations, and UN bodies.While they are typically not covered by literature reviews and bibliometric analyses, we nevertheless consider them relevant for understanding the concepts and the technical documentation behind reported mineral resources and reserves, mineral statistics, and current mineral resource accounting.To close this gap in the scientific discourse, we explicitly include important grey literature in our Critical Review.The concept of sustainability, in turn, has also seen a significant evolution since Von Carlowitz published "Sylvicultura Oeconomica" 10 in 1713.The report "Limits to Growth" 11 by the Club of Rome in 1972 and ensuing discussions have highlighted the importance of quantitative resource assessments for sustainable development.In recent years, mineral resource governance and sustainable mining have taken a more central role in the international debate.Mining companies face increasing pressure to adopt international reporting and sustainability standards.The timeline in Figure S 2-1 and supporting references in Table S 2-1 illustrate that national and international mineral resource classification standards are iteratively revised and increasingly harmonized, both in response to notable events impacting the mining industry including stock market frauds (e.g.Bre-X in 1997) 12 , wars, 13 and mine disasters, and to accommodate new accounting concepts, domestic and international raw material policies, and converging global sustainability efforts.

S3. Resources and reserves information in financial industry reporting
How much industry information ultimately becomes publicly available for further use varies: many jurisdictions require standardized financial reporting, 41 but information on reserves and resources (arguably the key assets of mining companies) is not part of financial statements and remains offbalance-sheet. 42,43A survey by the International Financial Reporting Standards Foundation found that only 46 of 177 jurisdictions specified the use of a resource classification in local non-financial reporting regulations. 43The supporting technical assessment reports 44 may contain some useful information for physical accounting. 43However, few regulators make the systematic public disclosure of these technical reports mandatory.Moreover, the contained information commonly comes in non-geospatial formats and thus is incompletely georeferenced (e.g., pictures of maps in PDF file format, data tables without geospatial coordinates, non-standardized mineral deposit descriptions in text).Documentation lacks comparability and consistency both between entities in the same industry and across jurisdictions. 43,45In addition, granular public reporting tends to be limited to companies that are listed on the stock markets, often shows only some of their projects, and may not be updated on a regular (e.g., annual) basis.Harmonized and standardized terminology and a common framework for systematic documentation of material stocks and flows including geospatial information may help to address these data gaps, and improve data availability and interoperability.
Moreover, technical documentation that builds on a MB-consistent physical accounting approach can make financial disclosures more transparent, and reported data more useful for resource governance.

economic activity."
The "general principles" of the complementary SEEA Ecosystem Accounting (SEEA EA) 35 further specify: "Ecosystem assets should be mutually exclusive, both conceptually and geographically.Thus, EAs should not overlap, either conceptually or geographically, and any area on the land or the sea floor, or any horizontal depth layer in the ocean, should be occupied by one and only one ecosystem asset." Second, concerning MB-consistency, the SEEA notes that "in theory, mass and energy flows must balance" (a physical accounting constraint), but simultaneously states that "physical asset accounts" can change in response to "additions or reductions in the estimated available stock of a specific deposit or to changes in the categorization […] based on changes in geologic information, technology, resource price or a combination of these factors". 23d third, concerning data resolution, the SEEA notes that "the coverage of individual assets does not extend to the individual elements that are embodied in the various natural and biological resources". 23ven our definition of MB-consistent geological stock accounting, we here show for the three forementioned points that the current data reporting and SEEA asset accounting workflows, which use economic accounting methods and concepts, do not satisfy the 'physical accounting' requirements of MFA: As for the first point (scope), the SEEA asset account balancing may not cover the entire physical space (i.e., "all natural resources and areas of land of an economic territory that may provide resources and space for use in economic activity") and whatever is being balanced is thus not "collectively exhaustive" (cf.MECE-concept).This is illustrated by "discoveries of new stock" 23 , which implicitly expand the spatial 3D system boundary that compounded the initial stock volume to include the new additional stock volume(s).Because the SEEA is not spatially explicit, it is unclear for the outside observer (and data user) that, and how exactly, the spatial system boundary changed.
Moreover, it is inherently difficult to guarantee that reported resource and reserve numbers are "mutually exclusive" (MECE); while definitions across government and industry vary, 31,32,49 it is common that mineral "resources" numbers include "reserves", 31,32,49,50 which makes MB-consistent accounting difficult.
For the second point (MB-consistency), we observe that the SEEA 'reserves stock' is not defined in terms of purely intrinsic 51 physical material properties, but rather is a function of extrinsic socioeconomic valuation.Examples are changes of reported reserves stocks in response to expected market price developments, or "upward reappraisal due to improvements in extraction technology", which violate MB-consistency because the spatiotemporal system boundary is not well constrained.
And for the last point (data resolution), it may be noted that both resource and reserves typically only quantify the content of the commodity of primary economic interest (e.g., copper grade and tonnage).Since neither the entire geological stock volume under consideration (including e.g., overburden), nor the whole-rock mineral/material composition thereof are reported, disclosed resources and reserves are not collectively exhaustive with respect to the material content.
Altogether, we show that economic accounting approaches are poorly suited for monitoring the physical reality and changing spatial characteristics.Our definitions and the outlined conceptual principles for MB-consistent geological stock accounting can thus be understood as inputs to the further development of the SEEA, natural capital accounting, 52 and physical monitoring.

Notes
Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Figure S 1
figure underrepresents the topic of mineral resource classification and accounting, which is largely discussed outside the formal literature.Figure S 1-1(b) emphasizes the importance and the broader context of geological stocks serving as the ultimate source of all mineral raw materials in the anthroposphere.Our 'Mineral Materials Tree' highlights that a robust understanding and massbalance-consistent monitoring of geological stocks and stock changes is critical for strategic materials management.It shows that mass balances may be used to resolve the complex linkages and trade-offs involved in the extraction and use of minerals, and that physical monitoring can be facilitated by digital technologies, knowledge integration, and holistic governance approaches.

Figure S 1
Figure S 1-1: (a; left) Bibliometric analysis of top-ranked author keywords in publications we reviewed, showing the broad thematic coverage of our article.Represented are only keywords from articles that are indexed by WOS or Scopus (149 of 296), which excludes grey literature.(b; right): Table of contents figure (TOC) of our article.The 'Mineral Materials Tree' symbolizes the vital role of geological stocks as hosts of the main mineral resource groups that are mined to sustain the global non-renewable raw materials supply.The physical systems perspective also covers key elements of data-, knowledge-, and systems integration, which are reviewed and synthesized in the integration and implementation sections of our article.

Figure S 1 - 2 :
Figure S 1-2: Evolution of trend topics in author keywords of the surveyed literature, 2011-2021.
Category (WC) combinations (Figure S 1-4): One, "WC=( Environmental OR Engineering NOT Geosciences, Multidisciplinary NOT Geology NOT Mining Mineral Processing NOT Geochemistry)" and two, "WC=(Geosciences, Multidisciplinary OR Geology OR Mining Mineral Processing OR Geochemistry NOT Environmental NOT Engineering)".

Figure S 1 - 4 :
Figure S 1-4: Relative importance of "material flow analysis" (MFA) calculated per WOS category "Environmental, Engineering" and "Geosciences, Geology", normalized for comparison.Labels show total annual publication count per category, not normalized.

Figure S 2 - 1 :
Figure S 2-1: Timeline of international and national mineral resource classification standards alongside global sustainability initiatives and reports, and consequential mining events since 2010.Acronyms: USGS, U.S. Geological Survey; UNHCR, United Nations Human Rights Council; NAEN, National Association of Subsoil Expertise; UN, United Nations; CRIRSCO, Committee for Mineral Reserves International Reporting Standards; PERC, Pan-European Reserves and Resources Reporting Committee; CIM, Canadian Institute of Mining, Metallurgy and Petroleum; SEEA, System of Environmental Economic Accounting; SDGs, Sustainable Development Goals; SME, Society for Mining, Metallurgy & Exploration; NPD, Norwegian Petroleum Directorate; UNECE, United Nations Economic Commission for Europe; UNFC, Framework Classification for Resources; Anthro.Specific., Anthropogenic Specifications; UNEA, United Nations Environment Assembly; GB/T, GuoBiao/TuiJian (Chinese, national standard/recommended); ICMM, International Council on Mining and Metals; EU Corp. Sust.Report.Direct, European Union Corporate Sustainability Reporting Directive; EU Prop.CRM Act, European Union Proposal for a Critical Raw Materials Act.

Table S 2
-1: Overview of literature referenced in Figure S 2-1.*Denotes current edition of the respective standard or guidelines.Abbreviations are explained in the references section.