Simandl, G.J., Paradis, S. and Birkett, T. (1999): Colombia-type Emeralds; in Selected British Columbia Mineral Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl, Z.D. Hora and D.V. Lefebure, Editors, British Columbia Ministry of Energy and Mines, Open File 1999-10.
IDENTIFICATION
SYNONYMS:
Emerald veins, Muzo and Chivor-type emerald deposits. COMMODITY: Emeralds (pale-green and colorless beryl gemstones).
EXAMPLES (British Columbia - Canadian/International): No Colombia-type emerald deposits are known in British Columbia. Chivor, La Mina Glorieta, Las Cruces, El Diamante, El Toro, La Vega de San Juan, Coscuez and Muzo (Colombia).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION:
Colombia-type emerald deposits consist mainly of
carbonate-pyrite-albite quartz veins forming "en échellon" or conjugate
arrays and cementing breccias. So called "stratiform tectonic breccias"
may also contain emeralds. Emeralds are disseminated in the veins as
clusters, single crystals or crystal fragments; however, the best
gemstones are found in cavities. Country rocks are black carbonaceous and
calcareous shales. TECTONIC SETTING: Probably back arc basins (shales deposited in epicontinental marine anoxic environments spatially related to evaporites) subjected to a compressional tectonic environment.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: The deposits are controlled by deep, regional decollements, reverse or thrust faults; hydraulic fracture zones, intersections of faults and by permeable arenite beds interbedded with impermeable black shales.
AGE OF MINERALIZATION: Colombian deposits are hosted by Cretaceous shales. Ar/Ar laser microprobe studies of Cr-V-K-rich mica, believed to be penecontemporaneous with the emerald mineralization, indicate 32 to 38 Ma for Muzo area and 65 Ma for Chivor district. It is not recommended to use these age criteria to constrain the exploration programs outside of Columbia.
HOST/ASSOCIATED ROCKS: Emerald-bearing veins and breccias are hosted mainly by black pyritiferous shale, black carbonaceous shale and slate. Claystone, siltstone, sandstone, limestone, dolomite, conglomerate and evaporites are also associated. Two special lithologies described in close association with the deposits are albitite (metasomatized black shale horizons) and tectonic breccias ("cenicero"). The latter consist of black shale and albitite fragments in a matrix of albite, pyrite and crushed black shale.
DEPOSIT FORM: The metasomatically altered tectonic blocks may be up to 300 metres in width and 50 km in length (Beus, 1979), while individual productive zones are from 1 to 30 metres in thickness. Emeralds are found in en échelon and conjugate veins that are commonly less than 10 centimetres thick, in hydraulic breccia zones and in some cases in cenicero.
TEXTURE/STRUCTURE: Emeralds are found disseminated in veins as clusters, single crystals or crystal fragments, however, the best gemstones are found in cavities. Quartz is cryptocrystalline or forms well developed hexagonal prisms, while calcite is fibrous or rhombohedral. In some cases, emerald may be found in black shale adjacent to the veinlets or cenicero.
ORE MINERALOGY: Emerald; beryl specimens and common beryl.
GANGUE MINERALOGY [Principal and subordinate]: Two vein stages are present and may be superimposed, forming composite veins. A barren stage 1 consisting mainly of fibrous calcite and pyrite and a productive second stage with associated rhombohedral calcite and dolomite, albite or oligoclase, pyrite, ± quartz and minor ± muscovite, ± parisite, ± fluorite, ± barite, ± apatite, ± aragonite, ± limonite and anthracite/graphite-like material. Some pyrite veins also contain emeralds. Cavities within calcite-rich veins contain best emerald mineralization.
Solid inclusions within emerald crystals are reported to be black shale, anthracite/graphite-like material , calcite, dolomite or magnesite (?), barite, pyrite, quartz, albite, goethite and parisite.
ALTERATION MINERALOGY: Albitization, carbonatization, development of allophane by alteration of albite, pervasive pyritization and development of pyrophyllite at contacts between veins and host rocks has also been reported.
WEATHERING: In Columbia the intense weathering and related alteration by meteoric water of stratiform breccias and albitites are believed to be responsible for the formation of native sulfur, kaolinite and gypsum. Albite in places altered to allophane.
ORE CONTROLS: Deep, regional fault systems (reverse or thrust); intersections of faults; breccia zones; permeable arenites interbedded with impermeable shales.
GENETIC MODELS: The hypotheses explaining the origin of these deposits are fast evolving. The most recent studies favor a moderate temperature, hydrothermal-sedimentary model. Compressional tectonics result in formation of decollements that are infiltrated by alkaline fluids, resulting in albitization and carbonatization of shale and mobilization of Be, Al, Si, Cr, V and REE. The alkaline fluids are believed to be derived from the evaporitic layers or salt diapirs. As the regional compression continues, disharmonic folding results in the formation of fluid traps and hydrofracturing. A subsequent decrease in fluid alkalinity or pressure could be the main factor responsible for emerald precipitation. Organic matter is believed to have played the key role in emerald precipitation (Cheilletz and Giuliani, 1996, Ottaway et al., 1994).
ASSOCIATED DEPOSIT TYPES: Spatially associated with disseminated or fracture-related Cu, Pb, Zn, Fe deposits of unknown origin and barite and gypsum (F02) deposits.
COMMENTS: Colombia-type emerald deposits differ from the classical schist-hosted emerald deposits (Q07) in many ways. They are not spatially related to known granite intrusions or pegmatites, they are not hosted by mafic/ultramafic rocks, and are emplaced in non-metamorphosed rocks. Green beryls, where vanadium is the source of colour, are described at Eidsvoll deposit (Norway) where pegmatite cuts bituminous schists. Such deposits may be better classified as pegmatite-hosted.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE:
Black shales within the tectonic blocks are depleted in
REE, Li, Mo, Ba, Zn, V and Cr. The albitized zones contain total REE<40
ppm while unaltered shales have total REE values of 190 ppm. Stream
sediments associated with altered shales have low K/Na ratio. Soils
overlying the deposits may have also low K/Na ratio. GEOPHYSICAL SIGNATURE: Geophysics may be successfully used to localize major faults where outcrops are lacking. The berylometer, has applications in ground exploration.
OTHER EXPLORATION GUIDES: Regional indicators are presence of beryl showings, available sources of Cr and Be and structural controls (decollement, reverse faults, fault intersections). In favourable areas, exploration guides are bleached zones, albitization and pyritization. White metasomatic layers within black shale described as albitites, and stratiform polygenetic breccias consisting of black shale fragments cemented by pyrite, albite and shale flour are closely associated with the mineralization.
ECONOMIC FACTORS
TYPICAL GRADE AND TONNAGE:
Distribution of emeralds within the mineralized zones is
erratic; therefore, pre-production tonnage estimates are difficult to
make. The official grade reported for Colombian deposits is approximately
1 carat/m3. All stones are valued according to size, intensity
of the green colouration and flaws, if present. Tonnages for individual
deposits are unknown; however, Chivor reportedly produced over 500,000
carats between 1921 and 1957. ECONOMIC LIMITATIONS: The earliest developments were by tunneling. To reduce mining costs benching, bulldozing and stripping of mountainsides were introduced. Recently, apparently to reduce environmental pressures, underground developments have been reintroduced at Muzo. Physical and chemical properties of high-quality synthetic emeralds match closely the properties of natural stones. There is currently uncertainty if synthetic emeralds can be distinguished from the high-quality, nearly inclusion-free natural specimens. Recent attempts to form an association of emerald producers may have a similar effect on emerald pricing as the Central Selling Organization has on diamond pricing.
END USES: Highly-valued gemstones.
IMPORTANCE: Currently, world production of natural emeralds is estimated at about $US 1 billion. In 1987 ECONOMINAS reported emerald production of 88,655,110 carats worth US$ 62,910,493. Colombia is the largest producer of natural emeralds by value; most of the gemstones come from the Muzo and Chivor districts. The other major producing countries are Brazil, Zambia, Zimbabwe, Pakistan, Afghanistan, Russia and Madagascar which have schist-hosted emerald deposits (Q07). Brazil is the world’s largest producer of emeralds by weight.
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