Cheilletz, A.

Féraud, G.

Giuliani, Gaston

Rodriguez, C.T.

Horizon / Pleins textes

Source, Transport and Deposition of Metals, Pagel& Leroy (eds) Q 1991 Balkema, Rotterdam. ISBN90 5410 020 6 
40~pAr laser-probe dating of the Colombian emerald deposits: Metallogenic 
implications 
,~,Cheilletz - 'Fluides et Minérarcx' ENSG-CRPG, Vandoeuvre-&-Nancy, France 
G.Féraud - Institut de Géodynamique URA-CNRS, Université de Nice, France 
G.Giuliani - ORSTOM, Département TOA et CRPG, Vandoeuvre-l2s-Nancy, France 
c.1 Rodriguez - Mineralco S.A.. Bogota, Colombia 
ABSTRACT: The emerald mineralizations of the Muzo-Quipama and Coscuez mines, Colombia, 
llave been dated for the first time by 4 o A r P A r  induction and continuous laser-probe on 
srnchronous greenish K-micas following step-heating and spot-fusion procedures. Two distinct 
upper Eocene-lower Oligocene ages have been determined for the two deposits at respectively 
31.5-32.6 Ma for Muzo-Quipama and 35-38 Ma for Coscuez. These ages provide better constraints 
for the genetic model of the mineralization particularly concerning (1) the depth and pressure (1.2 
Kb) of formation and (2) the temperature of the hydrothermal solutions (270 rt 30°C) which gives 
to the Colombian emerald mineralization a low temperature character. 
INTRODUCTION 
The Colombian emeralds are worldwide known 
for their great quality and unique geological 
environment which is constituted by calcite- 
dolomite veins, pockets and brecciated zones 
within lower Cretaceous black-shales 
formations. Hydrofracturing and an intense 
alteration of the wall-rocks favoured the 
development of an epigenetic hydrothermal 
process for the emerald formation. However, 
the lack of precise dating of the deposits 
preclude any accurate P and T estimation of the 
deposition of the beryllium mineralization as a 
preliminary to debate about the possible origin 
of the involved hydrothermal fluids. The 
present work, based on 40ArßgAr dating using 
especially a laser probe step-heating procedure, 
gives a first unambiguous answer to this 
question. 
GEOLOGICAL SETTING AND SAMPLE 
DESCRIPTION 
n e  Colombian emerald deposits constitute two 
mineralized belts located in the Eastern 
Cordillera at about 100 km N-NE of Bogota. 
ne eastern belt is defined by the districts of 
Gachala and Chivor and the western belt, scope 
of this study, is represented by the districts of 
Coscuez, Muzo and La Palma-Yacopi. The 
analysed samples originate from the Muzo- 
Quipama (sample G 65) and the Coscuez 
(sample G 67) mines. For both samples, dating 
was realized using pale green micas deposited 
as 2-3 mm width patches on both walls of 2-5 
cm width. calcite-pyrite-emerald veins and 
considered as cogenetic with the emerald 
mineralization. 
Microscopic examination of the G 67 sample 
shows the successive deposition of the micas 
band with the O01 planes perpendicular to the 
wall followed by a final calcite infilling of the 
vein; by places, calcite and micas are separated 
by aggregates of quartz and albite sometimes 
interlayered within the micas band. The G 65 
sample displays the same microscopic 
organisation but with some chlorite 
(Donbassite) microgeodic rosettes interfacing 
the micas and calcite bands. 
Diffractometric analysis of the micas 
agregates of the two samples showed a uniform 
muscovite 2M1 structure. Microprobe analysis 
revealed an apparent deficit of the interlayered 
charge (C K=O.59-0.76) and apparent excess of 
octaedric cations (E VI=2.01-2.11). These K- 
micas can therefore be considered as 
hydromicas (Brindley, 1980). An additional 
poorly represented Na-bearing phyllosilicate 
373 
ORSTOM Fonds Documentaire 
phase interlayered within the K-hydromicas is 
also revealed by microprobe analysis in the G 
65 sample but remains indistinguishable at a 
microscopic scale. 
40Ar/39Ar DATING RESULTS AND 
DISCUSSION 
40Ar/39Ar analyses were performed (1) on 
bulk samples of micas by the conventional step- 
heating procedure, (2) on single grains of mica 
aggregates, by step-heating procedure and (3) 
10 
%39Ar Released 
Figure 1- Induction (bulk sample) and 
continuous laser-probe step-heating (single 
grain aggregate) age spectra of sample G 65. 
on sections perpendicular to the mica bands by 
spot fusion. The two last kinds of experiments 
were performed with the continuous laser 
probe. 
Bulk samples and single grain aggregates of 
the two samples (Fig. 1 & 2) show similar age 
spectra characterized by increasing ages at low 
temperature, followed by decreasing ages, then 
a more or less flat domain. The sample G 65 
(Fig. 1) gives a plateau-age at 31.5 k 0.3 Ma 
374 
(corresponding to 82 8 of 39Ar released) on a 
single grain aggregate, whereas the 
corresponding bulk sample displays more 
precise concordant ages with an integrated 
value of 32.6 rt 0.1 Ma, over 60 % of 39A, 
released only. The sample G 67 (Fig. 2) does 
not give any plateau-age over a high percentage 
of 39Ar released, but excepted for the low 
temperature domain, it displays no more 
variable ages than for G 65. However, most of 
the apparent ages for this sample range from 35 
to 38 Ma. 
10 3 
.o 
3- =s - 30 
10 
W39Ar Released 
Figure 2- Induction (bulk sample) and 
continuous laser-probe step-heating (single 
grain aggregate) age spectra of sample G 67 
Spot fusion analyses, performed on sections 
perpendicular to the mica bands of the two 
samples gave concordant ages (at the 20 level) 
with (1) their weighted mean and (2) the 
integrated ages of step heating experiments, 
excepted for one G 65 spot fusion age. No 
correlation between ages and distances to the 
outer rim of the mica band could be detected, 
in spite of variable 37Arca/39Ar~ ratios (Fig. 
1 & 2) related to minute calcite crystals which 
may occur between the individual lamellae of 
K-micas. 
In the G 67 section perpendicular to the mica 
band, three spot fusion analyses were 
performed on the quartz-albite-calcite 
3ssemblage adjacent to the mica band. The ages, 
ranging from 52.2 f 7.4 to 150 f 15 Ma, can 
probably be explained by excess argon. 
Owing to the fact that a majority of ages 
obtained on the mica bands by spot fusion 
analyses are concordant, we interpret the 
heterogeneities in the age spectra as probably 
not due to a mixture of different mineral phases 
with different 40ArPAr ratios, but rather to 
heterogeneities in the argon distribution at a 
significantly lower scale than that of the spot 
fusion (fused spot diameter 2 50 pm). Recoil 
of 39Ar during irradiation of the samples is a 
good candidate to explain these heterogeneities, 
particularly because the thickness of the 
individual micas crystals (1 to 3 pm) is not 
very high compared to the accepted 39Ar recoil 
distance (0.08 pm; Turner and Cadogan, 1974). 
Nevertheless, as it was already noticed by Bray 
et al. (1987) who analysed similar minerals by 
the 40ArßgAr method, no noticeable net loss of 
39Ar due to recoil may occur when the samples 
are left as aggregates. This may be due to the 
f x t  that 39Ar that recoiled from one individual 
mica would have had a high probability to be 
implanted within a neighbouring crystal. In our 
case, minor amounts of calcite, quartz, albite, 
chlorite or Na-bearing phyllosilicate may be 
intercalated within the K-micas aggregates. It is 
therefore likely that some 39Ar may have 
recoiled from K-micas to neighbouring grains 
Of different nature. For instance, the humped 
shape around the 690°C step of the G 65 bulk 
sample age spectrum may be due to a deficit in 
39Ar, in the micas (K-rich phase), whereas the 
low ages displayed at lower temperature may 
onginate from a gain of 39Ar in K-poor phases. 
Let us notice that the integrated age of these 
domains of the age spectrum (steps 400- 
720°C) is 33.1 f 0.2 Ma, nearly similar to 
that of the plateau-age i.e. 32.6 f 0.1 Ma. 
This is in agreement with a lack of total 39Ar 
loSS in the analysed samples. 
Therefore, it is likely that the ages of 
31.5-32.6 Ma (sample G 65) and 35-38 Ma 
G 67) represent reasonable estimates of 
the age of the K-micas cogenetic with the 
emerald deposition. 
METALLOGENIC IMPLICATIONS 
These fist 40ArPAr dating of the Co€ombian 
emerald deposits give a representative late 
Eocene-lower Oligocene age following the 
stratigraphic scale of Odin and Odin (1990). 
This result rules out the hypothesis of Ulloa 
(1980) attributing the circulation of the 
emerald mineralizing fluids to the thermal 
effect provoked by the basic magmatism which 
intruded the lower Cretaceous series at 115-118 
Ma (Fabre et Delaloye, 1983). On the other 
hand, these Paleogene ages are in agreement 
with the Escovar hypothesis (1979) of a tertiary 
age for the emerald mineralization correlated 
to the emplacement of supposed Cenozoic-aged 
rhyolitic intrusives. It can also be noticed that 
these ages correspond to a major compressive 
tectonic phase affecting the whole Andean- 
Carribean domain during the middle-late 
Eocene (Beck, 1986). This important 
shortening episode affecting the Eastern 
Cordillera may have created deep decolement 
levels, reverse faulting allowing further 
circulations of hydrothermal connate basinal or 
metamorphic fluids throughout the whole 
stratigraphic column. Following this model, the 
hydrothermal fluids can originate either from 
the cretaceous or older underlying rocks of the 
Eastern Cordillera, or from the overthrusted 
subsiding foreland Llanos basin. 
Finally, the 40ArJ39Ar upper Eocene-lower 
Oligocene age for the emerald mineralization 
gives rise to an estimation of a 4500m of 
overburding at that time using Hebrard (1985) 
stratigraphic reconstitution. i.e. a lithostatic 
pressure evaluation of 1.2 Kb. This permits to 
constrain the preliminary fluid inclusions 
results (Giuliani et al., 1990) at 270 +_ 30°C as 
an effective trapping temperature for the 
hydrothermal solutions. Therefore, the 
Colombian emerald mineralizations appear to 
have developped at relatively low temperature 
conditions as early proposed by Beus and 
Mineev (1972). In that sense, they constitute an 
original type totally distinct from the classic 
high temperature pegmatite-biotite schists- 
emerald association (Schwarz, 1987; Giuliani et 
al., 1990). 
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