Hydrothermal catalytic conversion and metastable equilibrium of organic compounds in the Jinding Zn/Pb ore deposit

Hydrothermal organic reactions in the presence of mineral catalysts have been well established by laboratory experiments. However, little is documented about hydrothermal catalytic conversion (HTCC) and the metastable existence of complex molecules in continental hydrothermal systems. Five freshly mined ores containing oil and solid bitumen and associated unmineralised host sediment were sampled from the Beichang mine, Jinding Zn/Pb ore deposit, Lanping, near the IndoEurasian collisional margin, southeastern Tibetan Plateau. Here we show evidence for a wide range of mineral (ZnS/PbS) catalysis of polycyclic aromatic compounds in the natural subsurface. An interpretation of hydrothermal alteration (non-biodegradation) was made for the unresolved complex mixture. From unmineralised host sediment through oils to the solid bitumen, a stepwise interaction of organic compounds with metal bearing hydrothermal fluids was demonstrated by the transformation of polycyclic aromatic hydrocarbons (PAHs) and phenyl/sulfur/oxygen-containing compounds that are thought to be generated through HTCC. Thermodynamically unstable phenyl derivatives (ortho- or a-substituted) resulted from hydrothermal alteration (ring opening) of the middle and naphthenic (low energy) rings of peri-condensed parent PAHs, as supported by the dominance of 1-phenyldibenzothiophene, 1-phenyldibenzofuran, triphenyleno[1,12-bcd]thiophene, triphenyleno[1,12-bcd]furan, 9-phenylphenanthrene, 1,1':2',1'':2'',1'''-quaterphenyl, and the corresponding thiophenes or furans over other isomers. Heteroatom addition to phenyl-derivatives generates abundant heterocyclic compounds, which can undergo further dehydrocyclisation. In the presence of ZnS/PbS, the abundance of three compound classes (unsubstituted PAHs, aryl-aryl linked phenyl-derivatives, and S/O atom-containing compounds) is mainly hydrothermally controlled, rather than thermodynamically controlled. Various phenyl/sulfur/oxygen-containing compounds are the metastable equilibrium products of a series of complex redox reactions under specific hydrothermal conditions. We hypothesise that the ubiquitous triphenyleno[1,12bcd]furan is an important marker for hydrothermal organic-inorganic interactions in the Zn/Pb deposit. The relative abun dance of 9-phenylphenanthrene provides a measure of the degree of hydrothermal alteration and HTCC. Our data is consistent with a close interaction of ascending metalliferous hydrothermal fluids with indigenous organic matter/compounds. Hydrothermal fluids, in the presence of sphalerite (and/or galena), are highly effective for activation of the carbon-carbon bonds in PAHs. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Laboratory experiments have established hydrothermal organic reactions in the presence of mineral catalysts, but little is known about hydrothermal catalytic conversion (HTCC) and the existence of complex molecules in continental hydrothermal systems. Evidence from natural subsurface samples shows a wide range of mineral catalysis of polycyclic aromatic compounds, with a stepwise interaction of organic compounds with metal bearing hydrothermal fluids. This study suggests that various phenyl/sulfur/oxygen-containing compounds are metastable equilibrium products of complex redox reactions under specific hydrothermal conditions.