Time-pressure and temperature constraints on the formation of Colombian emeralds; an 40 Ar/ 39 Ar laser microprobe and fluid inclusion study

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 4oAr/39Ar induction and laser microprobe methods on contemporaneous greenish Cr-V-rich K mica aggregates consisting of muscovite as a dominant phase 5 kaolinite, 5 paragonite, quartz, ? albite, and ? chlorite, pyrite, and calcite. Contamination of the K mica aggregates by wall-rock impurities is eliminated by in situ 40Ar/39Ar 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 39Ar loss d u e to recoil during irradiation of the samples. Internal 39Ar 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 HzO-NaC1-CaClz-KCl-CO,-N, brines (38 wt % NaCl equiv). Constrained by the 40Ar/39Ar 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" to 360°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) a basinal 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. F 364 CHEILLET2 ET AL.

Emeralds from the Delbegetey deposit (Kazakhstan): mineralogical characteristics and fluid-inclusion study

“…This is unusual for emeralds foom other deposits, which normally contain a greater pooportion of liquid phase (Grundmann and Morteani, 1989;Nwe and Grundmann, 1990;Cheilletz et al, 1994;Gavrilenko et al, 2001;Groat et al, 2002;Marshall et al, 2004). According to micoothermometric data, all fluid inclusion assemblages may correspond to the same stage of fluid circulation, responsible for emerald mineralization.…”

Emerald Deposits: A Review and Enhanced Classification

“…Their distribution along both sides of the basin ensures abundant production of high-quality emerald. These deposits are unique because there is no connection with granites in their formation [103,104].…”

Notas sobre el contexto tectonoestratigráfico de formación de las esmeraldas colombianas

“…Los casos documentados son aislados y tampoco indican que las zonas de falla o los contactos mecánicos puedan llegar a ser sitios potenciales de prospección de esmeraldas (la cercanía o lejanía de las minas de esmeraldas respecto a las fallas son casos más bien fortuitos). Tampoco se observaron casos suficientes como para generalizar sobre la presencia de despegues mineralizados, como lo asumen Cheilletz, Féraud, Giuliani y Rodríguez (1994). » Posteriormente, en etapas más avanzadas de la deformación, áreas como la zona mineralizada de Coscuez en el CEOC sufren un acortamiento aún mayor, probablemente contemporáneo con la fase de mayor exhumación, que bascula las capas a estados subverticales y probablemente rota localmente los ejes de pliegues a lo largo de ejes verticales.…”

“…Los estudios de microsonda y de inclusiones fluidas de Cheilletz et al (1994) sirvieron de base para sustentar la hipótesis de que las mineralizaciones se producen antes de los procesos mayores de exhumación de la cordillera Oriental, ya que el inicio de la exhumación puede ser un factor capaz de detener los procesos mineralizantes, dadas las condiciones de presión y temperatura propuestas por los citados investigadores.…”