GEOLOGY
The Colombian Andes
consist of three
subparallel ranges: the Western or cordillera Occidental; the Central
or Cordillera Central and the Eastern or Cordillera Oriental.
The Western and Central ranges consist primarily of granites and they
are known for gold deposits. The Eastern range consist mostly of
sedimentary units, principally limestones and shales with minor igneous
and metamorphic rocks.
The major emerald deposits are limited to the eastern (Chivor) and
western (Muzo) margins of Cordillera Oriental where there are visible
Cretaceous sediments. The emerald are found in the Villeta formation
that dates from the Lower Cretaceous age. This formation consists
of black carbonaceous shale with minor limestone inclusions. The
Villeta formation is divided in two members: the Cambiado and Emerald
beds. Locally these members are separated by two thin agglomeratic
layers of calcite crystals designated the Cama and the Cernicero. The
emerald-bearing formations lie along the flanks of the eastern branch
and occur mainly in the provinces of Boyacà, Cundinimarca and
Santander or generally N-NE or Bogotà.
The Muzo mine penetrate a series of black pyritiferous argillites
(shales) intercalated with thinbedded limestones and folded into
varying amplitudes as drag folds which have been intensively fractured
and fissured due to compression and torsion forces created during the
formation of the horst of Muzo and Coscuez. The formations are
laterally limited by faults normal to the N-S direction in the contacts
with the Upper and Lower Albian formations. The fractures are filled
with calcite-dolomite and their analogs, and in places include emerald.
These formations are covered from topsoils called "capas buenas" or
"capas esmeraldiferas". They lie above the Cambiado formation, where no
emeralds occur.
Between the emerald-bearing rocks at the top and the Cambiado below
occur several rocks of great significance in suggesting the origin of
the emeralds: Albite rock (albite apparently replaces calcite; druses
-- crusts of tiny crystals lining a rock cavity -- contain splendid
crystals of albite, calcite and dolomite), Cernicero (ash bed) and Cama
(layers of intergrown large and well-formed calcite rhombs).
The Cambiado is the visible bottom formation at Muzo and consist of
limestone alternative layers of thinbedded argillite.
The formations at Chivor are of conformable sediments consisting of
light grey calcareous shales with some lenses of carbonaceous matter.
The top member is hard grey fossiliferous limestone, the lower member
is hard blue thinbedded limestone or calcareous shale.
Vertical to near-vertical fissures provided mineralization channels for
vein fillings while later faulting resulted in some displacements of
earlier formed veins. Large veins, beds, and masses of limonite, and
sometimes hematite, formed from alteration of pyrite and are
conspicuous features.
Above the limonitic beds quartz also permeates cracks and fissures.
Pyrite appeared after faulting, followed by emerald. Ther appear to
have been two periods of mineralization, the first being in-filling of
E-W fissures and the second, the mineralization afterwards.
No emeralds are found in calcite veins of the uppermost formation at
Chivor despite the presence of numerous early Spanish tunnels. The
strata below the ferojinosas were exposed that emerald were discovered.
Beryl appears to have come from a "deep-seated" source and
crystallized at moderate temperature and pressure in the veins as
evidence by lack of vein-wall alteration. The emerald crystal are
cemented by limonite, and these must be extracted with care to avoid
damage to the crystals.