The Muzo emerald deposit

Colombia

Emerald and pyrite (7 x 10.5 centimeters; coll. N. & J.-P. Voilhes). Photo: L.-D. Bayle.

Emerald and pyrite (7 x 10.5 centimeters; coll. N. & J.-P. Voilhes). Photo: L.-D. Bayle.

Geological Period

Paleogene

Main geological interest

Mineralogy
History of geosciences

Location

Muzo, Colombia
05°32’22”N, 074°08’43”W

Emerald and pyrite (7 x 10.5 centimeters; coll. N. & J.-P. Voilhes). Photo: L.-D. Bayle.

Unique for its hydrothermal-sedimentary genesis this site is known since Precolombian times and yielded the finest gem emeralds on Earth.

The historical Muzo mines are the traditional source of the world´s finest and largest green-coloured gem emeralds, including the famed trapiche crystals and parisite-(Ce), found exclusively in the black shales of the lower Cretaceous series from the Eastern Cordillera basin. The deposit, still actively worked, is characterized by compressive structures formed during the Paleogene. Breccias and veins are infilled by carbonates and pyrite with accessory minerals such as emerald. The proposed hydrothermal-sedimentary model involves at 300°C the generation of brines through dissolution of evaporites by hot basinal waters that leached Be but also Cr and V from the black shales.

The Quipama mining zone, a highly productive zone in the 1990s, hosting several mining sites such as Cincho, Las malvinas, Gallinazo, Aguardiente. Photo D. Schwarz.

The Muzo emerald deposits are located on the western flank of the Eastern Cordillera basin, 100 kilometers north of Bogotá. Emerald occurs in black shales intercalated with dolomitic limestones of Valanginian-Hauterivian age, the Rosablanca and Paja Formations. The deposits are linked by tear faults and associated thrusts formed during a Paleogene compressive phase; contemporaneously, circulation of hydrothermal fluids and emerald deposition occurred. All the tectonic contacts are marked by hydrothermal breccias, emerald-bearing banded carbonate veins and dykes, en-echelon sigmoidal tension gashes and drag folds indicating shearing in the roof of the breccia zones (Branquet et al., 1999). The veins are filled by calcite, dolomite and pyrite with accessory quartz, barite, fluorite, bitumen, Cr-muscovite, emerald and parisite-(Ce).The Muzo deposits are famous also for trapiche gem emerald disseminated in black shales (Pignatelli et al., 2015). Detailed geological and geochemical studies have led to a genetic model based on a hydrothermal-sedimentary origin. Key aspects of formation involve hot brines due to interaction of fluids with evaporites, resulting in intense albitization, carbonatization, and pyritization of black shales, and coeval leaching of Be, Cr and V necessary for emerald formation. The thermochemical reduction of sulphates led to precipitation of pyrite, calcite, dolomite, bitumen and finally emerald.

Emerald mines of Muzo were exploited since Precolombian times and rediscovered in the 16th century by Spaniards (Giuliani et al., 2022). Early scientific works refer to the discovery of parisite by Paris (1828-48) and the construction of the first geological maps (e.g., Pogue, 1916). Today a hydrothermal sedimentary model for the genesis of emerald is accepted (Ottaway et al., 1994).

Geological map after Laumonier et al. (1996); only the black shale of unit U3 is emerald-bearing. A corresponding profile of the Tequendama Mine is given in addition.

Branquet, Y. et al. (1999) ‘Emeralds in the Eastern Cordillera of Colombia: Two tectonic settings for one mineralization’, Geology, 27(7), pp. 597–600. Available at: https://doi.org/10.1130/0091-7613(1999)027<0597:EITECO>2.3.CO;2.

Giuliani, G. (2022) Émeraudes, tout un Monde ! Les Éditions du Piat, Saint-Julien-du-Pinet, France.

Laumonier, B. et al. (1996) ‘Mise en évidence d’une tectonique compressive Éocène-Oligocène dans l’Ouest de la Cordillère orientale de Colombie, d’après la structure en duplex des gisements d’émeraude de Muzo et de Coscuez (Evidence for an Eocene-Oligocene compressive tectonics in the western part of Eastern Cordillera of Colombia, from the duplex structure of the Muzo and Coscuez emerald deposits)’, Comptes rendus de l’Académie des sciences, 323, pp. 705–712.

Ottaway, T.L. et al. (1994) ‘Formation of the Muzo hydrothermal emerald deposit in Colombia’, Nature, 369, pp. 552–554. Available at: https://doi.org/10.1038/369552a0.

Pignatelli, I. et al. (2015) ‘Colombian Trapiche Emeralds: Recent Advances in Understanding Their Formation’, Gems & Gemology, pp. 222–259. Available at: https://doi.org/10.5741/GEMS.51.3.222.

Pogue, J. (1916) ‘The emerald deposits of Muzo, Colombia’, Transactions of the American Institute of Mining and Metallurgical Engineers, 55, pp. 810–834.

Gaston Giuliani.
Université Paul Sabatier, GET/IRD and Université de Lorraine. France.