Nanoparticle suspensions from carbon-rich fluid make high-grade gold deposits

Gold has been central to human societies and is primarily extracted from mineral deposits formed by circulation of gold-bearing aqueous solutions within the Earth. Orogenic-type gold deposits, responsible for over 75% of global production, typically comprise high-grade gold in quartz veins formed in metamorphic belts, where deeply sourced fluids are focused into faults and veins. These fluids commonly have temperatures of ~250–450 °C, pressures of ~500–1500 bar, low salinity (≤3 wt% NaCl eq.), high CO2 and H2S, and near-neutral pH. Gold is transported as dissolved complexes with maximum solubilities of hundreds of ppb, so prevailing models invoke repeated percolation of large fluid volumes to form extremely high-grade (~10,000 ppm Au) veins. However, many very high-grade veins are narrow, non-laminated, show limited alteration footprints, and lack growth zoning in vein-filling minerals (quartz, pyrite), all of which argue against repeated or large fluid-fluxing events and instead point to single injections of gold-bearing solutions—difficult to reconcile with low Au solubility. Recent work proposed that Au may also be transported as nanoparticle suspensions, which could carry up to ~5,000 times more Au than dissolved species, but direct evidence in orogenic systems remains limited. This study examines exceptionally high-grade, gold-rich quartz vein samples from five deposits spanning varied host lithologies, depths (~1.5 to >5 km), and ages (Archean to Cretaceous), to investigate the nucleation, stabilization, and deposition of Au nanoparticles and their role in forming high-grade orogenic gold deposits.

Prior models for high-grade orogenic gold veins emphasize repeated or large fluid flux due to low Au solubility in aqueous hydrothermal solutions. Observations of narrow, non-laminated, nuggety high-grade veins with limited alteration and lacking growth zoning challenge these models, suggesting single-event vein formation. Previous studies have documented colloidal transport and nanoparticulate Au in epithermal systems and proposed nanoparticle transport in orogenic settings, but direct evidence for Au nanoparticles in orogenic deposits has been sparse. CO2-rich, H2S-bearing fluids with near-neutral pH are characteristic of orogenic systems, and reduced sulfur is considered the dominant ligand complexing Au. CO2 buffering affects pH and Au complex stability, potentially enhancing Au solubility and transport. The literature thus points to a possible role of NP suspensions and silica colloids in metal transport and deposition, but mechanisms for NP nucleation, stabilization, and focused deposition in orogenic veins remained unclear prior to this work.

Sample selection: Five hand samples of gold-rich quartz veins were obtained from five high-grade orogenic gold deposits characterized by abundant visible gold and formed at crustal depths from >5 km to ~1.5 km. Deposits span different host rocks and ages from Archean to Cretaceous (details in Supplementary Information). The studied veins are exceptionally high-grade, nuggety, non-laminated, and lack growth zoning in quartz and pyrite.

Sample preparation: For each sample, ~2 cm × 2 cm pieces containing coarse gold were cut and mounted in one-inch epoxy. To minimize surface Si and C contamination, mounts were mechanically ground (1200 grit, Al2O3 abrasive) and polished in three steps (9, 3, 1 µm diamond). Diamond (crystalline) used for polishing cannot be mistaken for amorphous carbon by the analytical techniques. Mounts were coated with Pt for SEM.

Petrography and site selection: Using a Verios XHR SEM with an Oxford Instruments 80 mm² X-Max SDD EDS detector, areas of interest with inclusions in coarse gold were identified. Coarse gold grains showing intergrowth with quartz were targeted; grains in secondary cross-cutting features were avoided.

FIB-SEM foil extraction: Ultra-thin TEM foils (~10 × 2 µm plan view, ~100 nm thick) were extracted from gold grains using an FEI Helios NanoLab G3 CX dual-beam FIB-SEM. Foils were prepared through steps at 2–30 kV ion energies and 40 pA–21 nA currents. After initial thinning to ~1 µm, foils were lifted out with an in-situ W micromanipulator and welded to PELCO FIB lift-out Cu TEM grids. Final thinning to ~100 nm used low beam currents.

TEM/STEM analyses: Foils were analyzed on an FEI Titan G2 80–200 TEM/STEM with ChemiSTEM at 200 kV. High-resolution TEM, HAADF-STEM imaging, and EDS elemental mapping (Super-X detector, STEM mode, ~1 nm probe, ~0.9 nA probe current) were used to determine structure and composition. Element concentrations were quantified from EDS spectra using the Cliff–Lorimer thin-film method (Bruker Esprit). Crystallinity was assessed via FFT diffractograms. Data on areas selected and quantitative compositions of amorphous phases and nanoparticles are provided in Supplementary Information.

Data availability: Data are in Supplementary Information; raw data under accession https://research-repository.uwa.edu.au/en/datasets/nanoparticle-suspensions-from-carbon-rich-fluid-make-high-grade-g.

• Systematic presence of metal nanoparticles (NPs) in micro-inclusions within coarse gold grains across five high-grade orogenic gold deposits. Metals identified: Au, Au–Ag (electrum), Ag2O, and Cu.
• NPs are preserved within amorphous silica and/or amorphous carbonic phases (C–O–N), and in one case within micro-crystalline carbon.
• Size range of NPs: ~1 to 100 nm, with most <10 nm; many are sub-rounded, locally aggregated, except sub-angular Cu NPs.

Deposit-specific observations:

• Red Lake: A ~3 µm elongated inclusion at foil surface comprises quartz, amorphous carbonic phase, and encapsulated electrum NPs. NPs 2–20 nm (aggregates up to ~20 nm). Measured d-spacings 2.34–2.35 Å (111), consistent with Au, Ag, or Au–Ag alloy. Quantitative EDS on one NP: ~54 at% Au, ~41 at% Ag, ~5 at% O (O attributed to overlap with surrounding amorphous carbonic phase).
• Beta Hunt: ~1.5 µm inclusion with amorphous silica core and thin amorphous carbon rim along gold interface. Numerous rounded metal NPs (3–5 nm) concentrated in the carbonic phase near the Au contact; FFT d = 2.37 Å (111), consistent with Au or Ag; EDS signal too low to distinguish Au/Ag. Additional Au NPs observed in amorphous silica within the inclusion (Supplementary Fig. 7).
• Discovery, foil 1: Elongated inclusion (~1.5 × 0.5 µm) filled by amorphous silica containing rounded Au NPs (5–8 nm). FFT d = 2.34 Å (111), indicating Au.
• Discovery, foil 2: Quartz fragments and coarse gold separated by interstitial amorphous carbonic phase; ~20 NPs within carbonic phase identified as Ag2O by EDS (Ag, O) and FFT d-spacings: 1.94 Å (112), 2.78 Å (111), 3.20 Å (101), cubic Ag2O. Diameters 8–15 nm.
• Callie: Previous report of Au NPs in amorphous silica confirmed; full-inclusion EDS mapping shows one side amorphous silica and the rest amorphous carbonic phase.
• Sixteen to One: ~2 µm inclusion composed of micro-crystalline carbon (20–100 nm grains) with >20 interstitial Cu NPs. FFT d = 2.08 Å (111) indicating Cu. Cu NPs 10–100 nm, sub-angular.

Amorphous phase compositions (EDS, at%):

• Amorphous silica: Si ~38–56 at%, O ~44–62 at%, minor/trace Au <1.5 at%.
• Amorphous carbonic phases: C ~70–93 at%, O ~4–19 at%, N ~1–3 at%, minor/trace Au <1 at%.

Overall implications:

• Demonstrates close association of metal NPs with high-grade orogenic gold mineralization and with amorphous silica/carbonic phases.
• Indicates that Au is not the only metal forming NPs; Ag–Au alloys, Ag2O, and Cu also occur as NPs in these systems.
• Supports a role for NP formation, stabilization, and deposition as essential contributors to efficient, focused high-grade Au deposition.

The consistent co-occurrence of metal nanoparticles with amorphous silica and carbonic phases across deposits of varied ages, depths, and host rocks suggests a common set of processes governs NP nucleation, stabilization, transport, and deposition in high-grade orogenic gold systems. The amorphous carbonic phase (C–O–N) mirrors compositions of CO2-dominated carbonic fluid inclusions (with minor CH4 and N2), indicating precipitation of carbon-bearing amorphous material directly from hydrothermal H2O–CO2 fluids in the vein. CO2-rich fluids at mid-crustal conditions (300–450 °C, ~1,000+ bar) buffer pH near-neutral (pH ~6), enhancing Au–HS complex stability and Au solubility, while resisting pH changes from fluid–rock interaction. During ascent, decompression and cooling increase H2CO3 acidity, lowering pH and Au solubility; phase separation (immiscibility) further destabilizes Au complexes. The study proposes that sharp, localized changes (cooling, decompression, flash vaporization) cause simultaneous nucleation of metal NPs and silica colloids, with the latter aggregating to silica gel that deposits within veins. Amorphous carbon precipitates via redox reactions involving CO2/CH4; the presence of Ag2O NPs implies substantial oxidation conditions, and these redox reactions likely affect fH2S, contributing to Au precipitation. Small NP sizes (<10 nm, some as small as 1 nm) suggest limited growth by coalescence/Ostwald ripening, potentially delayed by adsorption of metal NPs onto colloidal silica. Mineralization involves physical aggregation and deposition of NP-bearing silica gel and precipitation of remaining dissolved Au as fluids lose CO2 and H2S during rapid decompression. Subsequent dehydration leads to quartz crystallization; gold NPs expelled from silica lattices amalgamate with existing gold grains, while some NP-bearing amorphous phases become trapped as inclusions within coarse gold, preserving amorphous textures. This mechanism explains formation of high-grade veins in single events without repeated large fluid fluxes and highlights the unique role of CO2-rich fluids in promoting focused Au deposition compared to CO2-poor systems, which are more rock-buffered, form pyrite earlier, and disperse Au over broader zones.

This study provides direct, systematic evidence that metal nanoparticles (Au, Au–Ag, Ag2O, Cu) preserved within amorphous silica and carbonic phases are widespread in high-grade orogenic gold deposits. Their consistent association indicates that nanoparticle nucleation, stabilization (including adsorption onto silica colloids), and deposition, together with silica gel formation and amorphous carbon precipitation during flash vaporization of CO2-rich fluids, are essential processes in focusing and efficiently depositing gold to produce bonanza grades in single-event veins. The proposed model links CO2-rich fluid buffering, Au–HS complex stability, rapid decompression, phase separation, and redox changes to the coeval formation of metal NPs, silica colloids, and amorphous carbon, followed by quartz crystallization and growth of coarse gold. Future research should refine constraints on Au complex stability during these transient processes, evaluate transport and deposition efficiency of silica–Au NP suspensions, and determine whether Au enrichment by NP formation occurs proximal (<100 m) or distal (>100 m) to the final deposition site.

• Spatial locus of Au pre-enrichment remains unresolved: whether nanoparticle formation and enrichment occur proximal (<100 m) or distal (>100 m) to the deposition site is unclear.
• Identification limits for very small NPs: in some cases (e.g., Beta Hunt, 3–5 nm NPs), EDS signals were insufficient to distinguish Au vs. Ag.
• Potential sample preparation artefacts: bubble-like textures in amorphous carbonic phases may reflect primary porosity from fluid inclusions or artefacts from inhomogeneous FIB milling due to mechanical contrasts.
• The study focuses on five deposits and NP-bearing inclusions preserved within coarse gold; while systematic across these, broader generalization would benefit from expanded sampling.
• One sample (Sixteen to One) shows recrystallization of amorphous carbon to micro-crystalline carbon, indicating possible post-depositional modification of carbon phases.