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 breccia s in black shale-limestone-hosted deposits. They are contained within two z ones, the eastern and western emerald zones, in the Eastern Cordillera. Min eralizing fluids were derived from the interaction of low-salinity fluids w ith primary halite and anhydrite. The source of beryllium is sedimentary an d of sulfide sulfur is evaporitic. The presence of organic matter, as bitum en, in both the black shares and the emerald-bearing hydrothermal veins inv ites discussion of the role played by organic matter in thermochemical sulf ate reduction, the mechanism proposed for H2S production. Organic matter is found in the orebodies in two forms: altered and unaltere d. It is also found in primary fluid inclusions of emeralds. Rock-Eval anal yses (hydrogen index <1; 19 < oxygen index < 61) as well the elemental H/C ratio (hydrogen index <1) confirmed the intense cracking suffered by tie or iginal organic matter. In hydrothermal bitumen, infrared microscopy reveale d the absence of aliphatic and aromatic bands and the disappearance of oxyg en functional groups. The eastern emerald zone contains C-13-depleted carbonates (mean delta(13)C = -11.3 +/- 1.3 parts per thousand, n = 14) with an homogeneous oxygen iso tope composition range (delta(18)O = 17.8 +/- 0.3 parts per thousand). nle western emerald zone contains carbonates with relatively heavy delta(13)C ( mean delta(13)C = 5.1 +/- 0.5 parts per thousand, n = 25) and notable delta (18)O variation (17-23.8 parts per thousand). In both emerald zones, C-13-d epleted calcite points to thermochemical sulfate reduction by oxidation of organic matter, but most delta(13)C values are heavier than predicted by th ermochemical sulfate reduction. A model that mixes heavy CO2 evoked from di ssolution of local limestone and light CO2 evolved by oxidizing organic mat ter explains the observed delta(13)C range of carbonates. Variations in del ta(18)O 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 p recipitation. The oxygen and hydrogen isotope compositions of fluid inclusions and struct ural 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 intrusi ons, which are near the emerald deposits. The calculated fluid delta(18)O i n equilibrium with calcite and dolomite (eastern emerald zone: 7.7 < delta( 18)O(Fluid) < 12.8 parts per thousand; western emerald zone: 8.2 delta(18)O (Fluid) < 17.8 parts per thousand) and emerald (eastern emerald zone: 15.5 < delta(18)O(Fluid) < 17.1 parts per thousand western emerald zone: 17.5 < delta(18)O(Fluid) < 23.6 parts per thousand) are typical of metamorphic wat ers or highly evolved sedimentary formation waters (temperature of formatio n = 300 degrees C), The high delta(18)O H2O values of the minerals suggest that the fluid may have been continuously enriched in delta(18)O owing to e xchange with limestones and black shales. In both emerald zones, the delta D values of carbonates and quartz parental waters (-64 < delta D < -40.7 pa rts per thousand) are similar to those found for halite (-66.4 < delta D < -41.2 parts per thousand), whereas delta D values of emerald, muscovite and albite are -51.4 to -7.1 per mil. The increase in delta D is interpreted as a consequence of mixing of fluids of different isotope composition during the mineralizing event. Two observ ations can be made about the O and H isotope ratios of fluid inclusions and structural waters from salt, muscovite, albite. and emerald. In the easter n emerald zone, they indicate that the fluids from halite and aluminosilica tes are the same and are the result of mixing of a meteoric water (delta(18 )O = -12.7 parts per thousand, delta D = -91.6 parts per thousand) with a f luid representing die previous mix-lire of a water with the Early Cretaceou s evaporite brines. In the western zone, the isotope ratios do not show the covariation seen in the eastern zone. The O-18(H2O) shift found in emerald and muscovite is interpreted to be the result of oxygen isotope exchange r eactions with sedimentary formations during the precipitation of the silica te phase.