Columbian emeralds

Colombian emeralds are meta-evaporitic gems formed via the mixing of fault-focused hypersaline basinal solutions released during the subsurface dissolution and alteration of evaporites. Emeralds occur within carbonate-silicate-pyrite veins and breccias hosted in black shales and limestones, tied to occurrences of hydrocarbons/bitumens. Gem fields constitute two zones, the eastern and western emerald zones, some 80 km apart in the Eastern Cordillera of Columbia (see Warren, 2016, Chapter 14 for literature detail).

The western zone crops out in the core of the Villeta anticlinorium (A). The sedimentary series that enclose the emerald deposits (B) are a few hundred meters thick and are composed of, from bottom to top: (1) micritic, largely dolomitic limestones (Rosablanca Formation, Valanginian-Hauterivian); (2) calcareous black shales (Hauterivian); and (3) siliceous black shales that form the base (Hauterivian) of the thick mudstones (Barremian-Aptian) of the Paja Formation. 

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Columbian emerald, Muzo Mine, Columbia

Most of the emeralds are found in hydrothermal breccias or carbonate-pyrite veins developed within both dolomitic limestones and calcareous black shales. Individual deposits are hectometer-sized at most and display numerous folds, thrusts, and tear faults. All the tectonic contacts are marked by centimetre-to metre-thick hydrothermal breccias that are cataclasites with clasts of black shales and albitites (i.e., massively albitized shales) within a carbonate-albite-pyrite cement. These breccias derived from a fluid-rich pulp. Part of the overpressured fluids escaped and triggered intense hydraulic fracturing in surrounding rocks, especially along tear faults. In each deposit, there is evidence of complex deformation that resulted in polyphase duplex structures. For example, in the Coscuez deposit, the ore-hosting N30°E verging folds and thrusts were a response to movement on the sinistral N20°E trending Coscuez tear fault (C).

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In the eastern zone (D), Andean thick-skinned tectonics are responsible for the main deformation observed on regional cross-sections through the Eastern Cordillera and adjacent Llanos foothills. This Andean deformation corresponds mainly to reverse faults (with an overall vergence to the southeast) and folds affecting the Palaeozoic basement and its Cretaceous-Tertiary sedimentary cover. Some of these faults are inverted Early Cretaceous growth faults. The Esmeralda fault likely represents the preserved part of an Early Cretaceous normal fault (D). As the result of Andean thick-skinned tectonics, the Chivor emerald deposit is located on a gently northwest dipping monocline situated on the western flank of a large, N30°E-trending, upright fold, devoid of cleavage.

The enclosing sedimentary series (E) corresponds to the upper part of the Guavio Formation (Berriasian), which unconformably overlies the Palaeozoic basement and is overlain by the shales of the thick (2900 m) Valanginian Macanal Formation. The series hosting the emerald-bearing veins and associated hydrothermal breccias are composed of, from bottom to top: (1) shales and siltstones that are locally massively albitised (lower albitites); (2) a 1–10-m-thick, stratiform brecciated level, primarily made of hydrothermal breccia after a former evaporite level; (3) an albitised and carbonatised sequence (upper albitites) that is white and initially contained anhydrite beds (as evidenced by phantom nodules, chicken-wire, and tepee structures); and (4) bioherms of micritic or shelly limestones grading vertically and laterally into black shales intercalated with calcareous pebbly mudstones and olistostromes.

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Within the brecciated level, disrupted blocks of the hanging wall (albitites, black shales, limestones) and caving structures are evidence for the collapse of the roof. Thus, the dissolution of an evaporitic horizon (probably initially dominated by halite) appears to be a major process controlling the formation of the brecciated level. The stratiform association of evaporites, limestones, albitites, and the brecciated level is consistent across tens of kilometres around the Chivor deposit, owing to its folding by the Andean phase. All the emerald deposits of the Chivor mining district are located within or just above this regional level, and so define a stratigraphic emerald horizon (F, G).

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Emeralds grew hydrothermally during thrusting and folding of black shales in a salt lubricated rauhwacke system. Highly reactive hypersaline brines, generated by the circulation of hot basinal waters passing through and dissolving halokinetic evaporite beds, created chloride-rich brines that passed from thrust planes to drive wall-rock metasomatic alteration. By this process, Be, Cr and V were mobilised, and could then precipitate emeralds in extensional veins and hydraulic breccias (rauhwacke), which are also rich in calcite and pyrite.

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