Abstract

Two Colombian deposits belonging to the western emerald belt of the Eastern Cordillera, namely Coscuez and Quipama-Muzo, hosted in Lower Cretaceous black shales, have been dated for the first time by 40 Ar/ 39 Ar induction and laser microprobe methods on contemporaneous greenish Cr-V-rich K mica aggregates consisting of muscovite as a dominant phase + or - kaolinitc, + or - paragonitc, + or - quartz, + or - albite, and + or - chlorite, pyrite, and calcite. Contamination of the K mica aggregates by wall-rock impurities is eliminated by in situ 40 Ar/ 39 Ar laser spot analysis. Two distinct plateau and spot fusion ages of 35 to 38 Ma and 31.5 to 32.6 Ma were obtained for the Coscuez and Quipama samples, respectively, i.e., a late Eocene to early Oligocene age. Concordant conventional K-Ar ages show that in spite of the small size of these micas, they did not suffer significant 39 Ar loss due to recoil during irradiation of the samples. Internal 39 Ar recoil may explain the slight disturbances observed on the age spectra. Microthermometry, Raman spectroscopy, and SEM experiments performed on fluid inclusions trapped in emerald crystals from the Coscuez deposit show that the hydrothermal fluids are complex H 2 O-NaCl-CaCl 2 -KCl-CO 2 -N 2 brines (38 wt % NaCl equiv). Constrained by the 40 Ar/ 39 Ar age determinations, the Eastern Cordillera subsiding model, and the assumption of lithostatic confining pressures, isochoric extrapolations lead to a pressure-temperature estimate of 1.06 to 1.12 kbars and 290 degrees to 360 degrees C for the emerald deposition. The P-T evaluation is in agreement with the paragenesis accompanying the emerald deposition.A moderate-temperature epigenetic hydrothermal-sedimentary model is proposed for Colombian emerald genesis. This model makes use of the following points: (1) the depth of hydrothermal circulation within the Lower Cretaceous series, (2) abasinal origin of mineralizing fluids as inferred from oxygen and carbon isotope data, (3) an evaporitic source for the NaCl sulfate-rich brines trapped within emerald crystals, as deduced from sulfur isotope data, (4) a likely source of the metallic components, and particularly Be, Cr, and V, being the black shale reservoir, and (5) the timing of hydrothermal circulation and emerald vein formation in relation to Eastern Cordillera tectonic evolution--in particular, the strong shortening episode beginning during the Eocene.

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