Invisible Gold and Platinum Group Elements in Sulphide Minerals: A Detailed Exploration of Their Occurrence and Impact on Gold Deposits

The mining industry is increasingly encountering challenges as the supply of easily accessible, high-grade gold deposits diminishes. Many of today’s economically viable deposits contain precious metals in forms that are difficult to detect and extract. Among these, “invisible” gold and platinum group elements (PGEs) are particularly problematic. These metals are not present as free particles visible to the naked eye or under standard microscopy. Instead, they reside within common sulphide minerals such as pyrite and arsenopyrite, either as nanoscale particles or chemically integrated into the mineral’s crystal lattice.

This hidden nature complicates resource estimation and metallurgical recovery, often resulting in underappreciated reserves and lower-than-expected extraction efficiencies. Recent scientific investigations have advanced our understanding of how these metals are hosted and incorporated, revealing complex chemical and structural relationships involving arsenic and other trace elements.

This report gathers insights from several key studies to provide a comprehensive overview of invisible gold and PGEs in sulfide minerals. It is intended to inform geologists, mineralogists, and mining professionals about the intricacies of these elusive metals and their significance for exploration and processing.

Defining Invisible Gold and Its Forms

Invisible gold is gold that does not appear as distinct grains but exists in two principal forms:

Nanoparticles: Extremely fine particles, often smaller than 50 nanometers, that evade detection by conventional microscopy.

Chemically bound gold: Gold atoms that substitute for other elements within the crystal structure of sulphide minerals, forming solid solutions or complex chemical bonds.

Because these forms resist standard extraction methods like gravity separation and cyanidation, they are often referred to as refractory gold. This refractory nature increases the complexity and cost of processing ores containing significant amounts of invisible gold.

Primary Mineral Hosts

Pyrite (FeS₂)

Pyrite is the most abundant sulphide mineral in many gold deposits and serves as a major host for invisible gold. Its capacity to incorporate gold is closely linked to its chemical composition, especially arsenic content. Arsenian pyrite, which contains elevated levels of arsenic, tends to host more invisible gold compared to arsenic-poor varieties.

Arsenopyrite (FeAsS) and Löllingite (FeAs₂)

Arsenopyrite and löllingite are iron sulpharsenide minerals notable for their ability to accommodate significant quantities of invisible gold and PGEs. Their crystal structures allow gold atoms to replace iron in octahedral coordination sites, often resulting in higher concentrations of invisible precious metals than found in pyrite.

<h3″>Platinum Group Elements (PGEs) </h3″>

PGEs such as palladium, platinum, rhodium, and others frequently accompany invisible gold in sulphide minerals. These metals share similar refractory characteristics, occurring as tiny inclusions or chemically substituted atoms within mineral lattices. Their recovery is economically important but technically challenging.

Mechanisms of Incorporation

Role of Arsenic

Advanced spectroscopic analyses have demonstrated that arsenic plays a crucial role in gold incorporation within sulphide minerals. In arsenian pyrite and arsenopyrite, gold substitutes for iron as Au(II), coordinated by arsenic and sulphur atoms in octahedral sites. This substitution forms stable solid solutions, explaining the elevated invisible gold contents in arsenic-rich sulphides.

Conversely, arsenic-poor pyrite tends to host gold primarily as Au(I) species adsorbed on mineral surfaces, resulting in lower gold concentrations.

Timing and Distribution

Invisible gold is introduced at different stages of pyrite growth, reflecting complex geological histories:

Core incorporation: Gold present in the earliest-formed pyrite zones.

Internal zoning: Gold concentrated in intermediate bands formed during ongoing mineral growth.

Rim enrichment: Late-stage arsenian rims enriched in gold, significantly enhancing overall metal content.

These growth-related variations influence both exploration targeting and processing strategies.

Nano-Inclusions and Tellurium Associations

In some deposits, invisible gold occurs as nano- to micro-scale inclusions of gold-silver-telluride minerals within pyrite grain boundaries or fractures. These tellurides are refractory and contribute to the difficulty of extracting gold efficiently.

Geochemical Patterns and Trace Elements

Gold and arsenic often show strong positive correlations, particularly in orogenic and Carlin-type deposits.

Tellurium and silver are commonly associated with gold in epithermal and some orogenic systems.

The ratio of nickel to cobalt in pyrite can indicate its propensity to host invisible gold, with nickel-rich pyrite generally containing more gold.

Variations in silver-to-gold ratios reflect differences in mineral associations and deposit types.

Implications for Exploration and Processing

Exploration

Traditional assay methods may fail to capture the full extent of invisible gold and PGEs, leading to underestimation of resources. Employing high-resolution techniques such as laser ablation ICP-MS and synchrotron-based spectroscopy enables better quantification and spatial mapping of these metals.

Understanding the timing of gold introduction and its association with arsenic and tellurium helps refine exploration models and guides sampling strategies.

Metallurgical Recovery

Invisible gold and PGEs resist conventional extraction, often requiring advanced processing methods such as:

Pressure oxidation,
Bio-oxidation,
Emerging Ultra-High Temperature Pyrometallurgical technologies such as PAD(TM) by IPRI.

Tailoring processing flow sheets to the mineralogical and chemical nature of invisible metals is essential to improve recovery rates and project economics.

Case Studies

Natalkinskoe Deposit: Invisible gold and PGEs are hosted as nanoscale inclusions and solid solutions in arsenopyrite and pyrite, with arsenic facilitating incorporation.

Global Orogenic and Sediment-Hosted Deposits: Multiple pyrite generations with varying gold distributions; distinct Au-As and Au-Te associations define deposit characteristics.

Arsenic-Driven Pump Mechanism: Atomic-scale studies reveal Au(II) substitution in arsenian sulfides as the fundamental invisible gold incorporation process.

Conclusion

Invisible gold and PGEs locked within arsenian sulfide minerals represent a significant and often overlooked component of many gold deposits worldwide. Advances in analytical techniques and theoretical modeling have clarified the mechanisms of their incorporation and distribution, underscoring the pivotal role of arsenic and tellurium.

Addressing the challenges posed by these hidden metals demands improved characterisation, innovative processing technologies, and integrated exploration strategies. Doing so will be critical to unlocking the full potential of refractory gold resources in the future.

References

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