Direct Measurement of Submicroscopic Gold: Methods and Applications

The ability to recover gold effectively is largely dependent on the nature/carriers of gold and the mineral processing techniques utilized. Gold ores are often classified into two major ore types: free-milling and refractory. Note that many gold deposits contain proportions of both ore types. Free-milling ores refers to those where the majority of gold is liberated through crushing/milling and recoverable by cyanide leaching. Refractory ores are defined as those which yield low gold recoveries, requiring complex and costly chemical pre-treatment prior to cyanidation. Refractoriness is directly controlled by ore mineralogy, generally resulting from the presence of either naturally occurring ‘preg-robbing’ carbonaceous minerals or submicroscopic gold.

Figure courtesy of David Holder: SIMS element distribution maps of a colloform pyrite grain showing Au and As in solid solution. Note the pyrite has been stained using KMnO, which can indicate compositional variations within and between pyrite morphologies.

Submicroscopic gold primarily occurs in solid-solution and as colloidal particles within sulfide and Fe-oxide phases which are impervious to cyanide solution. This gold, which is refractory in nature, is unrecoverable by conventional methods (e.g. gravity concentration/cyanidation) and requires pre-treatment before leaching. The principle carriers of submicroscopic gold are arsenopyrite and pyrite (typically As-rich), although gold can also be hosted within but not limited to marcasite, Cu sulfosalts/sulfides, As sulfides and Fe oxides. Submicroscopic gold has been documented from all gold deposit types (e.g. orogenic, carlin, epithermal) and in some deposits constitutes the primary carrier of gold. Since the occurrence of abundant refractory gold can render a project uneconomic it is critical to fully quantify the concentration and nature of any submicroscopic gold present at an early stage. Gaining an insight into the abundance, carriers and form of submicroscopic gold is therefore vital for optimizing recovery.

The concentration of submicroscopic gold is often inferred indirectly from diagnostic cyanide leach test-work. Although this technique provides an indication of the refractory gold concentration, limitations include: (i) it commonly overestimates the refractory gold content due to incomplete leaching; (ii) it can produce inaccurate and misleading data on the forms/carriers of gold and its mineral associations (e.g. carbonates, sulfides, tellurides); and (iii) it provides no detail on the exact nature and carrier of submicroscopic gold and associated trace-element concentrations of the host phases. Alternatively, the percentage of refractory gold can be quantified by direct measurement of the submicroscopic gold content of sulfides/Fe oxides using high-resolution analytical techniques such as LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) and SIMS (Secondary Ion Mass Spectrometry). This approach relies on the statistical representation of all potential Au carriers but provides greater accuracy and insight into the forms/carriers of refractory gold than is possible with the cyanide leach method. Through directly measuring pre-selected grains the distribution of gold, its major carrier(s) and its relationship with particular trace elements (e.g. As, Cu, Te, Se, Sb) can be fully determined and quantified. Depth profiling on LA-ICP-MS/SIMS also provides detail on the nature of the submicroscopic Au, distinguishing between solid solution and colloidal gold.

Figure courtesy of David Holder – SIMS time resolved depth profiles showing counts per second (CPS) of Au, As, S and Fe during spluttering of pyrite grains. The smooth profile of As and Au shown in first profile indicates lattice-bound or solid-solution within pyrite. The rise in the Au and As in the second profile relates to intersection with a compositional zone. The spikey nature of the third profile indicates Au developed as nanoparticles.

In complex multi-generational ore systems, gold is commonly associated with a particular sulfidation event. Ore microscopy coupled with LA-ICP-MS/SIMS, can identify and characterize these gold-rich sulfide phases, which are often distinct in terms of their grain morphology (e.g. pyrite: coarse, porous, zoned, microcrystalline) and trace element composition. Gaining such a detailed insight into the distribution and association of gold could allow for targeted processing such as selective flotation. Furthermore, understanding the relationship between elements such as As, Te and Sb with Au in sulfides is not only beneficial for mineral processing but also for mineral exploration. Identification of pathfinder elements can aid large-scale geochemical surveys and can be critical for generating potential exploration targets. In addition, quantification of sulfide phase’s trace-element content, may identify high concentrations of environmentally hazardous elements (such as As and Sb). The early identification of these lattice-bound elements is critical as they can require difficult and expensive mitigation.

Ore microscopy coupled with LA-ICP-MS/SIMS analysis is evidently a powerful tool, having direct applications for both processing and mineral exploration. This method not only fully quantifies the percentage of ‘locked up’ gold but also reveals a wealth of information regarding its distribution, preferential associations, form and carriers, which cannot be accurately determined from other techniques. Undertaking such a study therefore enables a fuller understanding of the ore deposit to be gained.

If you have any questions or require further information please contact MinAssist.