Sulfate Reduction by Organic Matter in Colombian Emerald Deposits: Chemical and Stable Isotope (C, O, H) Evidence

Colombian emeralds occur within carbonate-silicate-pyrite veins and breccias in black shale-limestone– hosted deposits. They are contained within two zones, the eastern and western emerald zones, in the Eastern Cordillera. Mineralizing fluids were derived from the interaction of low-salinity fluids with primary halite and anhydrite. The source of beryllium is sedimentary and of sulfide sulfur is evaporitic. The presence of organic matter, as bitumen, in both the black shales and the emerald-bearing hydrothermal veins invites discussion of the role played by organic matter in thermochemical sulfate reduction, the mechanism proposed for H₂S production.

Organic matter is found in the orebodies in two forms: altered and unaltered. It is also found in primary fluid inclusions of emeralds. Rock-Eval analyses (hydrogen index <1; 19 < oxygen index < 61) as well the elemental H/C ratio (hydrogen index <1) confirmed the intense cracking suffered by the original organic matter. In hydrothermal bitumen, infrared microscopy revealed the absence of aliphatic and aromatic bands and the disappearance of oxygen functional groups.

The eastern emerald zone contains ¹³C-depleted carbonates (mean δ¹³C = –11.3 ± 1.3‰, n = 14) with an homogeneous oxygen isotope composition range (δ¹⁸O = 17.8 ± 0.3‰). The western emerald zone contains carbonates with relatively heavy δ¹³C (mean δ¹³C = –5.1 ± 0.5‰, n = 25) and notable δ¹⁸O variation (17–23.8‰). In both emerald zones, ¹³C-depleted calcite points to thermochemical sulfate reduction by oxidation of organic matter, but most δ¹³C values are heavier than predicted by thermochemical sulfate reduction. A model that mixes heavy CO₂ evolved from dissolution of local limestone and light CO₂evolved by oxidizing organic matter explains the observed δ¹³C range of carbonates. Variations in δ¹⁸O of carbonate also reflect the local heterogeneity of the host rocks. Redox reactions produced HCO₃^– and H₂S; these compounds then reacted with the predominant cations (Fe²⁺ and Ca²⁺ extracted from the host black shale and limestones by the hydrothermal fluid) and induced calcite and pyrite precipitation.

The oxygen and hydrogen isotope compositions of fluid inclusions and structural waters of the gangue minerals and emeralds constrain the origin of the mineralizing fluids. The composition of these fluids was compared with the isotope composition of fluids in the Zipaquira halite-bearing salt intrusions, which are near the emerald deposits. The calculated fluid δ¹⁸O in equilibrium with calcite and dolomite (eastern emerald zone: 7.7 < δ¹⁸O_Fluid < 12.8‰; western emerald zone: 8.2 < δ¹⁸O _Fluid < 17.8‰) and emerald (eastern emerald zone: 15.5 < δ¹⁸O_Fluid < 17.1‰; western emerald zone: 17.5 < δ¹⁸O_Fluid < 23.6‰) are typical of metamorphic waters or highly evolved sedimentary formation waters (temperature of formation = 300°C). The high δ¹⁸O H₂O values of the minerals suggest that the fluid may have been continuously enriched in δ¹⁸O owing to exchange with limestones and black shales. In both emerald zones, the δD values of carbonates and quartz parental waters (–64 < δD < –40.7‰) are similar to those found for halite (–66.4 < δD < –41.2‰), whereas δD values of emerald, muscovite and albite are –51.4 to –7.1 per mil.

The increase in δD is interpreted as a consequence of mixing of fluids of different isotope composition during the mineralizing event. Two observations can be made about the O and H isotope ratios of fluid inclusions and structural waters from salt, muscovite, albite, and emerald. In the eastern emerald zone, they indicate that the fluids from halite and aluminosilicates are the same and are the result of mixing of a meteoric water (δ¹⁸O = –12.7‰, δD = –91.6‰) with a fluid representing the previous mixture of a water with the Early Cretaceous evaporite brines. In the western zone, the isotope ratios do not show the covariation seen in the eastern zone. The ¹⁸O_H2O shift found in emerald and muscovite is interpreted to be the result of oxygen isotope exchange reactions with sedimentary formations during the precipitation of the silicate phase.