A Review of Research Progress on the Development of Global Gold Ore Reference Materials and the Nugget Effect

BRIEF REPORTAs the benchmark for measurement traceability, reference materials play an irreplaceable role in fields such as geological analysis, resource exploration, and trade settlement[1-2]. The current status and research progress in the development of gold ore reference materials worldwide is systematically reviewed in this paper, differences between domestic (Chinese) and international reference material candidates in terms of geogenic types, certified parameter categories, and content gradients are compared, and the development history of China’s reference material development technologies are summarized. Meanwhile, issues existing in China’s reference material development are identified, including limited types, restricted certified elements, and insufficient coverage of low-grade ores. Furthermore, an in-depth analysis of technical challenges in each link of gold ore processing is conducted, pointing out that the “nugget effect” is a key bottleneck restricting the diversity of reference material types, and demonstrating that laser particle size analysis is not applicable for processing particle size inspection. Finally, specific suggestions for China’s reference material development are proposed, aiming to promptly meet the new prospecting models of “increasing reserves and production” under the New Round of Prospecting Breakthrough Strategic Action—namely “transforming a single ore deposit into multiple ones, small deposits into large ones, and inactive deposits into active ones”[7-8]—and ensure the accuracy and reliability of analytical data.　　1. Global progress in gold ore reference material development　　Internationally, 106 gold ore certified reference materials (CRMs) have been developed, encompassing six major genetic categories of gold deposits and spanning diverse rock/ore types such as greenstone, granite, volcanic rock, metamorphic rock, and quartz conglomerate. The certified elements include co-associated elements such as Au, Ag, Cu, Mo, Sb, and Al[9], furnishing a robust traceability foundation for foreign testing institutions to conduct testing, beneficiation, metallurgy, and trade activities related to various gold ore types.　　China has developed 74 gold ore CRMs, yet these exhibit limited genetic diversity, primarily covering quartz vein-type and altered rock-type ores. Notably, there is a critical scarcity of CRMs for complex co-associated ores bearing elements such as antimony (Sb), tungsten (W), and molybdenum (Mo)—which interfere with gold testing—leading to an absence of traceability support for associated testing procedures. Additionally, CRMs covering the content range between the cut-off grade and commercial grade are severely insufficient. For national first-class gold ore CRMs, the scope of certified elements remains narrow: 14 CRMs are certified solely for Au (accounting for 48%), 9 for “Au + Ag” (31%), 4 for “Au + As” (13%), and 2 for multi-element combinations such as “Au + Te” (7%). This configuration fails to keep pace with the current “a single ore deposit into multiple ones” development paradigm.　　2. Technical challenges in gold ore reference material development　　China’s existing specifications for reference material (RM) preparation and certification delineate general requirements for key technologies in RM development, including homogeneity inspection, stability verification, certification methodologies, and data processing. While these specifications are applicable to the development of RMs for fine-grained, easily processed gold ores and other mineral species, they are not suited for difficult-to-process gold ores—primarily due to the “nugget effect”[21-22].　　2.1 Homogeneity constraints imposed by sample processing technology　　The “nugget effect” poses challenges to the uniform processing of gold ores. Gold in ores typically does not occur in a “uniformly dispersed” state; instead, it exhibits localized enrichment or isolated distribution, which directly restricts the homogeneity of gold ore RM candidates.　　2.2 Inapplicability of laser particle size analysis for particle size inspection　　Laser particle size analyzers infer particle size by leveraging the scattered light intensity distribution of particles, operating under the assumption that particles are ideal spheres. This inherent limitation renders the method unsuitable for inspecting the processed particle size of gold ore RM candidates; sieve analysis is therefore recommended as the preferred alternative[27].　　2.3 Uncertainty in certified values　　The accuracy of characteristic values is influenced by certification methodologies, necessitating the adoption of multiple complementary methods for collaborative certification. The standard uncertainty arising from the instability of major-element gold is negligible, whereas the standard uncertainties stemming from the measurement process and sample homogeneity constitute the primary sources of uncertainty for major-element gold RMs. The combined uncertainty is calculated by integrating three components: the uncertainty of the mean measurement results from multiple laboratories, the uncertainty among different certification methodologies, and the uncertainty associated with sample homogeneity. The expansion factor is determined using the t-test parameter under a 95% confidence level, and the expanded combined uncertainty serves as the evaluated uncertainty for the certified value of major-element gold RMs[29-30].　　3. Conclusions　　(1) Unaddressed processing technology bottlenecks. The absence of breakthroughs in gold ore processing technology, combined with challenges in ore processing and failure to meet homogeneity inspection requirements, hinders the development of RMs for gold ores of diverse genetic types. (2) Insufficient RM type coverage. Discrepancies in type and matrix between RMs and tested samples can result in RM “out-of-control” issues and undermine the correct traceability of test data. (3) Irrational content gradient configuration. Advancements in technologies related to gold exploration, mining, smelting, and comprehensive utilization have led to a continuous reduction in the cut-off grade and commercial grade of gold ores. Nevertheless, China’s existing national gold ore RMs suffer from a critical shortage of samples in the low-content commercial grade range (only 12% in the 0.2–1 g/g range), failing to align with the domestic and international demand for “low-grade and large-scale” gold prospecting. (4) Limited scope of certified elements. With the extensive adoption of modern multi-element simultaneous testing instruments, technical methodologies for the simultaneous determination of gold and co-associated elements have been established as national/industrial standards. However, most existing gold ore RMs only include certifications for Au and Ag, which fail to meet the requirements of these new technologies and methodologies.　　4. Future perspectives　　4.1 Develop targeted CRMs to align with new prospecting models　　Capitalizing on the New Round of Prospecting Breakthrough Strategic Action, gold ore samples representing typical genetic types from national key metallogenic belts should be employed as RM candidates to develop low-content and multi-type gold ore RMs. A series of gold ore RMs covering eight major genetic types of gold deposits—with simultaneous certification of gold and its co-associated elements[29]—should be established. This will address the RM “out-of-control” issue arising from the matrix mismatch between RMs and actual samples and adapt to the new prospecting models of “a single ore deposit into multiple ones, small deposits into large ones, and inactive deposits into active ones” for reserve growth and production enhancement under the new round of prospecting.　　4.2 Strengthen research on processing methodologies and international collaboration　　The distribution characteristics and ductility of gold in ores present challenges to uniform sample processing, yet this issue has received limited research attention, restricting the variety of gold ore RMs. It is advisable to enhance the R&D of equipment tailored to gold ore sample processing (e.g., new-type disc mills and Raymond mills) and advance research on processing methodologies. Efforts should be directed toward overcoming challenges in processing complex gold ores, including the difficult dissociation of sulfide-encapsulated gold, marble-like soft rocks, and gold co-associated with Sb, W, and Mo, to enrich the types of China’s gold ore RMs.　　Optimize the integration of wet chemical analysis and fire assay techniques to enhance the accuracy of RM certification. Engage in the development of international standards to enhance the global influence of China’s gold ore RMs.