Amorphous carbon precipitation in high-grade orogenic gold systems: Implications for carbon speciation in ore fluids

High-grade gold in orogenic deposits commonly occurs as visible gold within quartz‑carbonate veins, precipitated from hydrothermal fluids. In such systems, gold and other metal nanoparticles are found encapsulated within amorphous carbon and/or silica micro-inclusions preserved in gold. The origin and nature of the amorphous carbon phase remain enigmatic despite its close association with high-grade gold mineralization. To investigate the amorphous carbon phase potential origin, we analyzed its molecular composition, focusing on the presence or absence of complex hydrocarbons to reconstruct carbon sourcing, whether biogenic or abiogenic, and its role in the ore-forming process. For this purpose, we investigated samples from three different high-grade orogenic gold deposits (Callie, Northern Territory, Australia; Beta Hunt, Western Australia, and Red Lake, Ontario, Canada) using advanced techniques, including Raman spectroscopy, Micro-Fourier Transform Infrared spectroscopy (micro-FTIR), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), Transmission Electron Microscopy (TEM), and Electron Energy Loss Spectroscopy (EELS). Our results confirm the presence of disordered amorphous carbon lacking complex carbon molecules, suggesting precipitation from CO₂- and CH₄-rich fluids without contribution from complex hydrocarbons in the source fluids. We therefore suggest that the source of amorphous carbon is abiogenic. We further propose that CO₂-rich fluids in orogenic systems play an indirect but crucial role in the formation of high-grade mineralization by causing pH changes and fluid immiscibility as they decompress and cool, which may lead to metal nanoparticle nucleation and the focused deposition of gold. These findings provide insights into the metallogenic processes of orogenic systems, emphasizing the role of CO₂-rich fluids in the formation of high-grade gold mineralization.