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.
REFERENCES
Beus, A.A. (1979): Sodium: A Geochemical
Indicator of Emerald Mineralization in the Cordillera Oriental, Colombia;
Journal of Geochemical Exploration, Volume II, pages 195-208.
Cheilletz, A. and Giuliani, G. (1996): The
Genesis of Colombian Emeralds: a Restatement; Mineralium Deposita,
Volume 31, pages 359-364.
Cox, D.P. (1986): Descriptive Model of Emerald
Veins; in Mineral Deposit Models, D.P. Cox and D. Singer, Editors,
United States Geological Survey, Bulletin 1693, page 219.
Escovar, R. (1975): Geologia y Geoquimica de
las Minas de Esmeraldas de Gachalà, Cundinamaraca; Boletin Geologico,
Volume 22(3), pages 116-153.
Giuliani, G., Rodriguez, C.T. and Rueda, F. (1990):
Les Gisements d’émeraude de la Cordillère Orientale de la
Colombie: Nouvelles Données Métallogéniques; Mineralium Deposita,
Volume 25, pages 105-111.
Giuliani, G., Cheilletz, A., Arboleda, C., Carrillo, V.,
Rueda, F. and Baker, J. (1995a): An Evaporitic Origin or the
Parent Brines of Colombian Emeralds: Fluid Inclusion and Sulfur Isotope
Evidence; European Journal of Mineralogy, Volume 7, pages 151-165.
Kazmi, A.H., and Snee., L.W. (1989): Emeralds
of Pakistan. Geology, Gemology and Genesis, in: Kazmi, A.H., and Snee,
L.W., Editors; Geological Survey of Pakistan. Van Nostrand Company,
New York, 269 pages.
Koslowski, A., Metz, P. and Jaramillo, H.A.E. (1988):
Emeralds from Somondoco, Colombia: Chemical Composition, Fluid Inclusion
and Origin; Neues Jarhrbuch für Mineralogie Abhandlungen,
Volume 159, pages 23-49.
Oppenheim, V. (1948): The Muzo Emerald Zone,
Colombia, S. A; Economic Geology, Volume 43, pages 31-38.
Ottaway, T.L., Wicks, F.J., Bryndzia, L.T., Kyser, T.K.
and Spooner, E.T.C. (1994): Formation of the Muzo Hydrothermal
Emerald Deposit in Colombia; Nature, Volume 369, pages 552-554.
Sinkankas, J. and Read, P. (1986): Beryl;
Butterworths Gem Books, USA, 225 pages.
Van Landgham, S.L. (1984): Geology of World Gem
Deposits; Van Nostrand Reinhold Co., Publishers, USA. 406 pages. |
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