An analytical gold mine

Gold, frankincense and myrrh. Of these first and most famous Christmas gifts, frankincense and myrrh seem odd bestowments as their desirability is often unknown to modern day secret Santas. Conferment of gold goes unquestioned as its status as a top gift and sought after commodity stretches back before the common era. All that glisters is not gold, but all from kings to criminal kingpins have moved mountains to acquire element 79. According to a recent report from the United Nations Office on Drugs and Crime (UNODC), organised crime groups ‘have embedded themselves in gold supply chains to such an extent that they now pose a serious global threat’. 

While mostly mined in sub-Saharan Africa, south east Asia and Latin America and the Caribbean, gold is often refined at hubs in Europe, eastern and western Asia and northern America. Gold’s complex global processing chains offer various vulnerability points for illegal activity, with each region facing both shared and unique challenges. 

A series of UNODC studies conducted in partnership with Brazilian and US research institutions, focused on the Brazilian Amazon region, found that illegal gold mining accounted for two-thirds of the region’s gold production. The resulting shady shift of wealth to ‘the hands of a few’ is but one impact of illegal gold mining. The studies say there is a ‘crime convergence and connectivity’ as ‘strong links have been documented between illegal mining and other criminal activities, such as human trafficking, human rights violations, financial crimes, drug trafficking, and crimes that affect the environment’. 

Brazil is tackling illegal mining and the havoc it wreaks via policy, diplomacy, widespread law enforcement operations and cutting-edge analytical techniques aimed at tracing gold’s origin. Determining the provenance of gold is a herculean task, in large part because of its complex processing chain.^1,2 The typical refining and smelting phase may mix gold sourced from different locations, rendering this phase a deliberate or unintentional forensic countermeasure. 

Golden opportunity

While tracing the origin of smelted and refined gold has been described as nearly impossible, a recent case study by a team of researchers from the University of Brasília’s Geosciences Institute, the Brazilian Federal Police’s National Institute of Criminalistics, and the Geological Survey of Brazil show the dazzling promise of using a multi-analytical approach to pinpoint gold’s provenance.¹ Researchers focused on illegal gold mining in the Yanomami Indigenous Territory, where the influx of thousands of miners brought violence, disease, malnutrition, contamination of rivers, deforestation and other violations. 

Gold from the Yanomami Indigenous Territory often enters the black market, further obfuscating origins. To bring clarity to this problem, the researchers used x-ray fluorescence (XRF), electron microprobe analysis (EMPA) and scanning electron microscopy (SEM) to study gold samples sourced from the Yanomami Indigenous Territory. SEM was used to document morphological information of gold grains and collect images. XRF and EMPA provided, respectively, semi-quantitative and quantitative analysis of minor and trace elements in gold grains. Such elemental analysis has been used to distinguish suspect material sources and aid in ascertaining provenance. 

Analytical chemistry is the gift that keeps on giving

The researchers examined a variety of samples from select locations in two main geological domains (Northeast Domain and Southwest Domain) spanning the Yanomami Indigenous Territory to ascertain if a unique geochemical signature would allow the origin of a gold sample to be traced, with the samples undergoing partial smelting and reanalysis. The elemental analysis of grains revealed distinct patterns that distinguish between gold deposits of the two main geochemical domains. For example, molybdenum was of ‘significant presence’ in many Northeast Domain deposits while its absence was noted in Southwest Domain deposits. Different silver concentrations were also noted between the two domains. The consistent presence of bismuth in high concentrations ‘even after amalgamation and smelting’ suggests that assessing samples for this element could enable end-to-end gold traceability when gold source deposits have significant bismuth concentrations. 

The researchers also evaluated two gold ingots with unknown origins, seized during Brazilian Federal Police operations. The elemental composition of these ingots strongly suggests they have a common origin, but neither sample showed ‘significant compatibility’ with deposits studied by researchers and likely came from outside the Yanomami Indigenous Territory. Evaluation of gold sponge samples also seized by the Brazilian Federal Police of suspected origins within the Yanomami Indigenous Territory showed their composition was consistent with Northwest Domain deposits. 

Analytical chemistry is the gift that keeps on giving investigators the tools they need to combat crime.