Copper in hydrothermal fluids

Copper mobility in hydrothermal fluids is controlled by how Cu(I) binds, hydrates, and interacts with other ions as fluids evolve from dense brines to low-density vapours. Using ab initio molecular dynamics, in-situ spectroscopy, and free-energy calculations, we identified the dominant Cu species under geologically relevant conditions and quantified how fluid density influences hydration and charge neutralisation. For the first time, we computed stability constants (log K) for neutral NaCuCl₂(aq) and mixed-ligand species such as CuCl(HS)⁻, improving predictive models for ore formation and mineral processing.

Highlights

• Linear CuCl₂⁻ dominates in dense brines; CuCl₃²⁻ is weak; CuCl₂⁻ persists to high T
  In NaCl brines from subcritical to high‑temperature supercritical conditions, Cu(I) occurs predominantly as a distorted linear CuCl₂⁻ (Cu–Cl ≈ 2.11–2.14 Å; Cl–Cu–Cl ≈ 153–165°). CuCl₃²⁻ is minor at 25 °C and becomes thermodynamically unfavourable by 327 °C, consistent with XAS and MD. This confirms that charged linear complexes remain the core Cu carriers in high‑density fluids.
• CuCl(HS)⁻: log K ≈ 11.47 @ 327 °C, wide predominance field
  Distance‑constrained MD and thermodynamic integration yield log K ≈ 11.47 for CuCl(HS)⁻ formation at 327 °C (validated along two independent reaction paths). Activity–activity maps show a broad field where mixed‑ligand CuCl(HS)⁻ dominates, reflecting HS⁻ > Cl⁻ > H₂S ligand preference and explaining copper mobility in S‑bearing chloride fluids.
• NaCuCl₂(aq) becomes prevalent as density falls
  As fluid density decreases (e.g., ρ ≈ 0.29 g cm⁻³ at 1000 °C, 1500 bar), CuCl₂⁻ associates with Na⁺ in the outer sphere to form neutral NaCuCl₂(aq) and transient larger clusters. Na⁺ is weakly bound (outer‑sphere), with residence times < 15 ps and rapid exchange—explaining why EXAFS rarely detects the second shell. Neutral species therefore increase in abundance in low‑density vapours.
• Hydration ∝ density (downward linear with ρ) for Cu⁺/Na⁺/Cl⁻
  MD shows hydration numbers drop linearly with decreasing density for Cu⁺, Na⁺ and Cl⁻ (R² ≈ 0.98–0.99), mechanistically linking solvent density to complex stability. The entropy gain from releasing hydration waters favours ion association and neutrality as density falls, offering a molecular basis for density‑dependent log K behaviour used in geochemical models.

 

Geological implications

• Phase separation & vapour transport. During phase separation of magmatic–hydrothermal fluids, the liquid keeps CuCl₂⁻, but the vapour can still carry significant Cu via neutral NaCuCl₂(aq), especially in alkali‑rich systems. This explains metal partitioning into vapour observed in porphyry settings and supports vapour‑mediated pre‑enrichment of shallow ore zones.
• Mixed‑ligand robustness in S–Cl fluids. In many ore‑forming systems both HS⁻ and Cl⁻ are present. The strong stability and wide predominance of CuCl(HS)⁻ mean Cu stays mobile across intermediate S/Cl regimes, resisting premature precipitation. Fluid mixing, pH shifts, or oxidation state changes that reduce HS⁻ availability will tip speciation back toward CuCl₂⁻, sharpening solubility gradients and triggering Cu deposition with sulphides (e.g., chalcopyrite).
• Density as a master control. Because hydration drops linearly with density, neutralisation (outer‑sphere Na⁺ pairing) becomes favoured as fluids ascend, expand, or boil. Models should explicitly include density‑dependent hydration rather than fixed hydration numbers from simple slopes—this reduces error in predicted Cu solubilities along ascent paths and during boiling.
• Triggers for deposition. Loss of alkalis (consumed by feldspar alteration) or increase in density (cooling, compression) will undo neutrality, pushing back to charged CuCl₂⁻, decreasing mobility. Likewise, sulfur consumption (sulfide precipitation) collapses the mixed‑ligand field; both processes create sharp solubility drops that concentrate copper in veins and breccias.

 

Tags: #Copper #Exploration #AIMD #XAS #Thermodynamics

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