TIME-PRESSURE AND TEMPERATURE CONSTRAINTS ON THE FORMATION OF COLOMBIAN EMERALDS - AN AR-40 AR-39 LASER MICROPROBE AND FLUID INCLUSION STUDY/

Two Colombian deposits belonging to the western emerald belt of the Ea stern Cordillera, namely Coscuez and Quipama-Muzo, hosted in Lower Cre taceous black shales, have been dated for the first time by Ar-40/Ar-3 9 induction and laser microprobe methods on contemporaneous greenish C r-V-rich K mica aggregates consisting of muscovite as a dominant phase +/- kaolinite, +/- paragonite, +/- quartz, +/- albite, and +/- chlori te, pyrite, and calcite. Contamination of the K mica aggregates by wal l-rock impurities is eliminated by in Situ Ar-40/Ar-39 laser spot anal ysis. 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, respec tively, i.e., a late Eocene to early Oligocene age. Concordant convent ional K-Ar ages show that in spite of the small size of these micas, t hey did not suffer significant Ar-39 loss due to recoil during irradia tion of the samples. Internal Ar-39 recoil may explain the slight dist urbances observed on the age spectra. Microthermometry, Raman spectros copy, and SEM experiments performed on fluid inclusions trapped in eme rald crystals from the Coscuez deposit show that the hydrothermal flui ds are complex H2O-NaCl-CaCl2-KCl-CO2-N2 brines (38 wt % NaCl equiv). Constrained by the Ar-40/Ar-39 age determinations, the Eastern Cordill era subsiding model, and the assumption of lithostatic confining press ures, isochoric extrapolations lead to a pressure-temperature estimate of 1.06 to 1.12 kbars and 290-degrees to 360-degrees-C for the emeral d 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 gene sis. This model makes use of the following points: (1) the depth of hy drothermal circulation within the Lower Cretaceous series, (2) a basin al origin of mineralizing fluids as inferred from oxygen and carbon is otope 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 hydrot hermal circulation and emerald vein formation in relation to Eastern C ordillera tectonic evolution--in particular, the strong shortening epi sode beginning during the Eocene.