Oil-water interfaces drive gold precipitation via microdroplet chemistry in thermal geological systems

Abstract

Sedimentary basins host high-grade gold mineralization at intersections of auriferous hydrothermal fluids and hydrocarbons. However, the precise mechanism of native gold formation associated with organic matter remains poorly understood. Here, we investigate gold precipitation at oil-water interfaces through in situ thermal experiments using various combinations of oil and HAuCl₄-bearing solutions. Our results reveal that gold particles form spontaneously following the extensive generation and evolution of water microdroplets at oil-water interfaces at temperatures of 140 to 400 °C. We propose that electrons (e^-), released from the conversion of hydroxide ions (OH^-) to hydroxyl radicals (·OH) in water microdroplets, together with H atoms (·H) formed through electron transfer involving H₃O⁺, and spontaneously generated H₂O₂ from·OH recombination, drive the reduction of Au³⁺ to Au⁰. The atomic gold progressively aggregates into Au⁰ clusters and Au nanoparticles (AuNPs), ultimately forming micrometer-scale gold particles and wires. This precipitation process occurs within minutes at temperatures above 350 °C and within hours below 200 °C. The experimentally produced gold particles exhibit textures like those in natural ore deposits. This interfacial microdroplet-induced mechanism provides a unique perspective on native gold formation in hydrocarbon-rich geosystems. Beyond its geological significance, this mechanism offers a potentially simplified approach for gold recovery from electronic waste without the need to introduce complex adsorbents or reducing agents into the waste stream.

Keywords: gold; microdroplet; oil–water.

Fig. 1.

Evolution of water microdroplets and associated formation of hydroxyl radicals (·OH), aqueous electrons (e⁻), hydrogen peroxide (H₂O₂), and gold particles near oil–water interfaces in various systems. (A1−A3) n-C₁₆H₃₄-H₂O (HAuCl₄)-10.8 ppm system heated to 390 °C for 2 h. (B1−B3) n-C₁₆H₃₄-H₂O (HAuCl₄)-10.8 ppm system heated to 150 °C for 25 h. (C1−C3) Oil-CO₂-H₂O (NaCl-HAuCl₄)-10.8 ppm system heated to 396 °C. (D1–D3) Water microdroplets showing enhanced fluorescence compared to bulk water at 140 °C in the n-C₁₆H₃₄-H₂O (PF-1) system. (E1) Electron paramagnetic resonance (EPR) spectra of hydroxyl free radicals (·OH) obtained in the n-C₁₆H₃₄-H₂O and water-only systems, with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin trap. ·OH radicals were detected in the n-C₁₆H₃₄-H₂O system but not in the water-only system. (E2–E4) EPR spectra of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) obtained in water-only (E2), alkane-only (E3), and n-C₁₆-H₃₄-water (E4) systems at different temperatures. The clear signals obtained in the water-only system show that TEMPO remains effective at high temperatures; the progressive reduction of EPR signals in n-C₁₆H₃₄ system with increasing temperature indicates thermal cracking of alkanes to form free radicals that reduce TEMPO; the complete disappearance of TEMPO signals in n-C₁₆H₃₄-H₂O mixtures above 140 °C (E4) indicates that interfacial water microdroplets create a unique environment that enhances generation of free radicals and electrons. (F1–F3) Reduction of silicomolybdic acid to its blue Mo(V) form within microdroplets and subsequently spreading to bulk aqueous phase in the heated (150 to 200 °C) n-C₁₆H₃₄-H₂O system (F1 and F2), but not in the water-only system (F3). (G1–G5) Detection of H₂O₂ using Amplex Red (AR)-natural horseradish peroxidase (HRP): color of mixed solutions of AR + H₂O₂ (G1), AR + H₂O₂ + HRP (G2), AR+ Demo (G3), and AR + Demo + HRP (G4), Demo represents postreaction solution obtained from the n-C₁₆H₃₄-H₂O system subjected to two cycles of heating and cooling (40 to 150 °C); (G5) fluorescence spectra of the G1–G4 samples. The red color development after HRP addition to the AR + Demo mixture verifies the generation of H₂O₂ in the n-C₁₆H₃₄-H₂O system.

Fig. 2.

SEM and TEM images of nano–micrometer gold particles formed in different oil–hot water systems. (A) n-C₁₆H₃₄-H₂O (HAuCl₄)-10.8 ppm system heated to 390 °C for 5 min. (B) n-C₁₆H₃₄-H₂O (HAuCl₄)-10.8 ppm system heated to 400 °C for 2 h. (C and D) n-C₁₆H₃₄-H₂O (HAuCl₄)-10.8 ppm system subjected to two cycles of heating up to 190 °C and cooling to 120 °C. (E and F) EDS image of the gold particles in D for Au (E) and C + O + Si + Au (F). (G) Lattice fringes of AuNPs formed in the n-C₆H₁₄-H₂O (HAuCl₄)-10.8 ppm system heated to 380 °C for 5 min showing d-spacing of approximately 0.23 nm and aggregation of multiple smaller bounded AuNPs to form individual AuNPs. Yellow dotted lines and yellow triangles represent dislocation and lattice distortion of the aggregated AuNPs. An angle of 48° can be identified clearly between two bounded lattice planes. (H) AuNPs formed by three times heating up to 190 °C and two times cooling down to 120 °C in the oil-H₂O (NaCl-HAuCl₄)-10.8 ppm system. (I) AuNPs formed by three times heating up to 190 °C and two times cool down to 120 °C in the C₈F₁₈-H₂O (HAuCl4)-10.8 ppm system. EDS data for the labeled points (ESD1–EDS4) and more EDS mapping images are presented in the supporting information of SI Appendix, Figs. S5–S8.

Fig. 3.

Schematic diagram showing the formation of nano–micrometer Au particles near oil–water interfaces. (A) Formation of aqueous electrons (e⁻), ·H,·OH, and H₂O₂ in water microdroplets. (B) Reduction of Au³⁺ to form native Au⁰ and atom clusters in individual water microdroplet through the action of electrons,·H, and H₂O₂. (C) Coalescence of water microdroplets and assembly of atom clusters to form AuNPs. (D) Formation of micrometer gold particles and gold wires with further coalescence of water microdroplets. (E) TEM image showing relationship between gold grain (au), AuNPs, and pyrobitumen film in a fluid inclusion developed in the gold grain. (F) SEM image showing native gold grains intimately associated with bitumen (bit). (G) BSE‐SEM image showing relationship between gold and carbonaceous matter (CM); CM and gold co-occupy the same structures (red arrows), and CM hosts abundant submicron gold grains (yellow arrows in the Inset). (H) Reflected light image showing uraninite–pyrobitumen–gold association with gold in the desiccation cracks of pyrobitumen and coating the intergranular primary surfaces of pyrobitumen. E and F from gold ore in Witwatersrand Basin (3); G from Dufferin gold deposit, Meguma Terrane, Nova Scotia, Canada (7); H from gold deposit in the Peräpohja Schist Belt in Finland (8). Processes in C and D referred to Fan et al. (48).

Fig. 4.

Schematic model for the formation of high-grade native gold after auriferous hydrothermal fluids entering into hydrocarbon reservoirs. (A) Native gold precipitation occurs as auriferous hydrothermal fluids enter hot hydrocarbon reservoirs, and native gold precipitation does not occur in low-temperature reservoirs if the temperature of hydrothermal fluid is lower than 140 to 150 °C. (B and C) Formation of gold particles with the generation of water microdroplets in hydrocarbons near interfaces between hydrocarbons and auriferous thermal fluids.