SEDIMENT-DERIVED EUCLASE MINERAL CHARACTERIZATION …

EUCLASE IN THE COLOMBIAN EMERALD DEPOSITS 1

SEDIMENT-DERIVED EUCLASE MINERAL CHARACTERIZATION AND ITS

IMPLICATIONS FOR THE EVOLUTION OF THE COLOMBIAN EMERALD

DEPOSITS

Author: Julián Mauricio Reyes Álvarez

Director: Idael Francisco Blanco Quintero

Co-Director: Edwin Yovanny Alba Vasquez

Universidad de Los Andes


SEDIMENT-DERIVED EUCLASE MINERAL CHARACTERIZATION AND ITS

IMPLICATIONS FOR THE EVOLUTION OF THE COLOMBIAN EMERALD

DEPOSITS

_____________________________

Julián Mauricio Reyes Álvarez

Estudiante

_____________________________

Idael Francisco Blanco Quintero

Director

_____________________________

Edwin Yovanny Alba Vasquez

Co-Director

Universidad de Los Andes

Facultad de Ciencias

Departamento de Geociencias

Septiembre 2016


Author Note

Julián M. Reyes and Idael Blanco, Department of Geosciences, School of Sciences,

Universidad de Los Andes.

This research was presented as an undergraduated thesis project for Julián M. Reyes at

Universidad de Los Andes, Bogotá, Colombia.

This research was supported by the Fondo Educativo Howard T. Corrigan grant from the

Asociación Colombiana de Geólogos y Geofísicos del Petróleo (ACGGP) and Corporación

Geológica Ares.

Questions concerning this article should be written to Julián M. Reyes. E-mail:

[email protected]

mailto:[email protected]


Contents Abstract ............................................................................................................................... 7

Introduction ......................................................................................................................... 8

Conceptual Framework ....................................................................................................... 9

Fluid inclusions (FI) ........................................................................................................ 9

Microthermometry ........................................................................................................ 12

Hydrothermal ore deposit ............................................................................................. 13

Albitization ................................................................................................................... 14

Geological Framework...................................................................................................... 15

Methodology and Analytical techniques ........................................................................... 20

Field work ..................................................................................................................... 20

Laboratory work............................................................................................................ 22

Petrography. .............................................................................................................. 22

Whole Rock Chemistry. ............................................................................................ 22

SEM & X-Ray spectroscopy. .................................................................................... 22

Microthermometry. ................................................................................................... 23

Results ............................................................................................................................... 23

Petrography ................................................................................................................... 23

Whole rock chemistry ................................................................................................... 24

SEM & X-Ray spectroscopy ......................................................................................... 28


Microthermometry ........................................................................................................ 32

Discussion and Conclusion ............................................................................................... 33

Acknowledgements ........................................................................................................... 42

References ......................................................................................................................... 44

Tables ................................................................................................................................ 51


Abstract

Euclase is a precious stone commonly found in igneous or metamorphic rocks formed after

dissolution/reprecipitation of beryl. However, it is found in cretaceous black shales, of the Paja

formation, in the Colombian Emerald Belt. The field occurrence of euclase is in albitite veins,

perpendicular to large fault systems (dip 144°/43°). The mineralization of these veins is

composed of euclase + apatite + albite, but emerald (beryl) coexisting with euclase is scarce and

generally has not been identified in most deposits. To determine the conditions of formation in a

sedimentary environment, fluid and solid inclusions have been investigated by

microthermometric studies, micro-Raman spectroscopy, X-ray spectroscopy and electron

microscopy. Further, the host rock is analyzed to investigate the interaction between the

hydrothermal fluids with the black shales and the effect in the mineralization. Petrography

studied in thin sections show that the main vein deforms the surrounding crystals. The

information gathered is compared to the reported for emerald deposits. The fluid on euclase has a

mean eutectic temperature of -96.5 ºC and a minimum temperature of formation of 338 ºC, with

similar temperatures for apatite and albite, moreover all inclusions have high salinity ~40 % in

weight of NaCl eq. Mineral inclusions inside the main phases (euclase, apatite and albite) are

quartz, plagioclase, phengite, pyrite, phenakite, rutile and diaspore. Whole rock chemistry shows

that the host rock, close to the mineralization, is depleted in most of the elements. The data here

presented shown that the fluids that produce the euclase in the sedimentary deposits of Colombia

has similar temperature and salinity of the emerald; precluding the formation by

dissolution/reprecipitation of the former by the second. Alternatively, the data support the

crystallization of euclase instead emerald by change in the activity of some major components

like Al and/or Si.

Keywords: Euclase, Colombian Emerald Belt, Albitization, Mineralogy.


Introduction

The Colombian Emerald Belt (CEB) is an area with hydrothermal veins of low-T which

crystalizes a variety of gemstones of great quality, such as emerald, parisite, codazzite and

euclase (Kozlowski et al., 1988; Cheilletz et al., 1994; Giuliani et al., 1995, 1999; Banks et al.,

2000). The CEB is located in the center part of the Eastern Cordillera, from the south of

Santander to the north of Cundinamarca departments, mainly in the department of Boyacá. This

zone is divided into two regions, the Western and Eastern Emerald Belts. Each one is subdivided

into mining districts where the most important are Muzo, Coscuez and Peñas Blancas from the

western and Chivor, Gachala to the eastern districts (Maya et al., 2004). Probably the most

important feature of the CEB is the sedimentary origin for these gemstone crystals.

The CEB has been known for the great quality of gems found through all the area. One of

this gemstones is the euclase (AlBeSiO4(OH)) which is a greenish blue, pale blue mineral related

to emerald (Al2Be3Si6O18(Cr,V)) (Rubiano, 1990; Chavez-Gil et al., 1997). Formally euclase is

defined as an orthosilicate of beryllium and aluminum with a radical (OH), crystallizes in the

monoclinic system (Gossner & Mussgnug, 1929; Biscoe & Warren, 1933 in: Vlasov, 1966)

Generally, euclase is found in pegmatites, a high-T environment associated with an intrusion of

igneous body (mostly of acid composition), characterized by the large size of the crystals (bigger

than 2 cm; Gallagher & Hawkes, 1966; Strand, 1953). Euclase normally is considered as formed

in pegmatites after dissolution of beryl and then crystalized in a different position. (Strand, 1953).

However, the genesis of euclase in low-T (sedimentary) systems is not well reported.

Hence, the main objective of this thesis is to determine how the euclase crystals form in a

low-T environment, the conditions (temperature and composition) of the fluid that crystallizes it,

and how does his formation affects the evolution of the emerald deposits. To accomplish the


research, the fluid and solid inclusions of the euclase are going to be studied, such as those found

in related minerals of apatite and calcite. Also geochemistry and petrography of the vein and host

rock is going to be done, combined with Scanning Electron Microscope and Raman

Spectroscopic techniques. The starting materials for the studies are samples collected in the La

Marina mine located in the Western Emerald Belt.

Conceptual Framework

Fluid inclusions (FI)

A fluid inclusion is a small quantity of fluid that got trapped in imperfections of a crystal

while it was crystalizing or as product of the healing of fractures (Bodnar, 2003). Commonly the

size of an inclusion is very low, less than 1mm, and generally between 1 – 10 µm. There is

relation between the size of the inclusions and the quantity of them, which is inversely

proportional, bigger inclusions means less quantity. Further, in standard ambient temperature and

pressure (SATP) the liquid could present different states (solid, liquid or gas).

According to the phases presented in the inclusions they could be classified in FI type I,

II, III, IV y V (Nash & Theodore, 1971) (Figure 1).

Type I: Are liquid FI with a low percentage of gas and no solid as a result of an

undersaturated fluid (<26 % in weight of NaCl eq.) and rich in H2O (liq.). These

inclusions generally homogenized as liquid during heating.


Type II: Are liquid FI with a high percentage of gas and no solid as a result of a

fluid rich in voletile. These inclusions generally homogenized as vapor during

heating.

Type III: Are polyphase FI (liquid, solid and gas), could contain one or more solid

phases, commonly halite and/or silvite. These FI are related to a saturated fluid

with >26 % in weight of NaCl eq. There are two subtypes:

a. Undersaturated FI in which during heating the salt dissolves first and

then the gas.

b. Saturated FI in which during heating the gas dissolves first and then

the salt.

Type IV: Are polyphase FI characterized by the presence of two liquid phases,

minerals and gas. These FI are rich in CO2, from which there is one liquid with

high content of CO2 (liq.) and the other one the common H2O rich liquid. The

overage of CO2 makes the gas with a very high content of CO2 (g.). Also the FI has

the presence of one or more solid phases by a saturated fluid.

Type V: Are triphasic FI with CO2 as liquid and gas, no solid and H2O (liq.). These

FI are the result of an undersaturated and rich in CO2 fluid.


Figure 1. Scheme of the different types of fluid inclusions according to the phases (V-Gas, L-Liquid)

present at standard ambient temperature and pressure (SATP). Taken from Nash & Theodore (1971).

Moreover, the FI could be classified by their genesis into primary, secondary and

pseudosecondary FI (Roedder, 1984; Goldstein, 2003) (Figure 2).

Primary: Those which get stuck in the planes where the crystal is growing and

represent the original fluid, from which the mineral crystalized.

Secondary: Are inclusions that reached the crystal through fractures after the

crystal ended its formation, these inclusions represent a liquid post-crystallization

that shows the evolution of the system.

Pseudosecondary: Liquid that got trapped in a fracture while the crystal was

forming, could represent the original fluid.


Figure 2. Schematic representation of the different types of FI according to their genesis through the

crystal formation. Taken from Bodnar (2003).

Microthermometry

During the determination of the temperatures, the FI change its phases when cooling or

heating happens. These studies provide information about the temperature of formation, density,

pressure, salinity and the composition of the fluid (Poty et al., 1976; Hollister & Crawford, 1981).

In order to know these conditions, the FI are solidified by cooling and start increasing the

temperature, seeing the:

Eutectic temperature or of first fusion to determine the composition of the fluid. It

is when the first particle starts to melt, TR in Figure 3.

Temperature of final fusion in order to know the salinity. It is when all the sample

becomes fluid (liquid or gas), TD in Figure 3.


Critic temperature or of homogenization to determine the temperature of

formation. When all the phases (solid, liquid and gas) consolidated as liquid, TB

in Figure 3.

Figure 3. Diagram that shows the isocore of a triphasic FI since its entrapment conditions Ta until the

standard ambient temperature and pressure (R) and its phases changes (L-liquid, S-solid and V-gas).

Taken from Shepherd et al. (1985).

Hydrothermal ore deposit

Is a natural concentration of particular minerals in a restricted area, over the Earth’s crust

that is economically viable to exploit which is formed by the precipitation of solids from a hot

aqueous solution of sedimentary, igneous or metamorphic source (Skinner, 1997). In order to

concentrate these minerals, the hot water has to pass by interconnections through the host rock,

where the water must dissolve particular elements (Au, Cu, Fe) from the rocks do to its high

temperature (Pirajno, 2010). Then the enriched solution has to move away to free spaces, with

the right chemical and physical conditions, where the minerals can precipitate.


Depending on the environment of formation the hydrothermal deposits are divided in two

groups, magmatic and sedimentary. The epithermal, porphyry, VMS (Volcanogenic massive

sulphide) are the main known magmatic hydrothermal deposits and MVT (Mississippi Valley

Type), SedEx (Sedimentary Exhalative) for sedimentary hydrothermal deposits. The type of

hydrothermal deposit could be stablish with the mineral assemblage and knowing the conditions

of crystallization (pressure and temperature) such as the fluid composition (Pirajno, 2010).

Albitization

Is a process in which albite is introduced by a hydrothermal fluid that replaces calcic

plagioclase and/or K-feldspar into nearly pure albite (Bowes, 1989; Kaur et al., 2012), the

substitutions are shown in Figure 4. Albitization occurs in two main environments, in granite

magmatic intrusions and in sandstones product of diagenetic processes. Albitization can be

realized by the lost of strain energy in a single crystal, reductions in Gibbs free energy (Brown &

Parsons, 1989; Hirt et al. 1993; Lee & Parsons 1997).

Figure 4. Reactions carried out during albitization. Taken from Fernandez (1991).

In granites the substitution occurs progressively along fractures in the calcic plagioclase

and K-feldspar crystals between a temperature of 200–300ºC (AlDahan et al., 1987). In these

environments the effect of pressure is very small because the intrusions occur at shallow depths

where the pressure is low and can be linked to a change in temperature (Lee & Parsons, 1997).

𝐾𝐴𝑙𝑆𝑖3𝑂8 (𝐾 − 𝑓𝑒𝑙𝑑𝑒𝑠𝑝𝑎𝑟) + 𝑁𝑎+ → 𝑁𝑎𝐴𝑙𝑆𝑖3𝑂8 (𝐴𝑙𝑏𝑖𝑡𝑒) + 𝐾+

𝐶𝑎𝐴𝑙2𝑆𝑖2𝑂8 (𝐴𝑛𝑜𝑟𝑡ℎ𝑖𝑡𝑒) + 4𝑆𝑖𝑂2 + 2𝑁𝑎+ → 2𝑁𝑎𝐴𝑙𝑆𝑖3𝑂8 (𝐴𝑙𝑏𝑖𝑡𝑒) + 𝐶𝑎2+


Hereby the alteration in granites can be explained with changes in the temperature only. In Shap

granite the albitization occurred due to the presence of a Na-rich fluid rather than the

thermodynamic driving force (Lee & Parsons, 1997).

Albitization in sandstones is caused via a dissolution-reprecipitation process instead of a

solid-state ionic diffusion, these is known based on the number, size and form of the albite

crystals from a single plagioclase (Boles, 1982; Morad, 1986; Saigal et al., 1988; Morad et al.,

1990). The diagenetic albitization occurs at low temperatures, commonly higher than 90ºC

(Morad, 1986), however Saigal et al. (1988) starts the alteration for K-feldsapr at 65ºC with a

maximum at 105ºC. Through modeling Baccar et al. (1993) reaches a temperature of 120-150 ºC

where the reactions happen.

Albitization in K-feldspar increases when the rocks are buried and the temperature is

enhanced (Morad et al., 1990; Baccar et al., 1993). In plagioclase, instead, the albitization

decreases when the temperature increases (Baccar et al., 1993). Further, albitization is affected

by the presence of CO2 in the environment, for low CO2 conditions the alteration occurs at

temperatures similar to 60-100ºC, in the other hand, with higher contents of CO2 the alliteration

continue further than the 100 ºC (Baccar et al., 1993).

Geological Framework

The formation of the Colombian Emerald Belt (CEB) began in the Cretaceous when the

Eastern Cordillera was under the sea level and began to deposit material of high organic matter

into a future sedimentary basin (Etayo et al., 1983). This material buried and gained temperature,

until the sediment consolidated to form mudstone and argillite (Barton & Young, 2002). Later,


different tectonic processes started to deform the area and gave rise to the orogeny of the Eastern

Cordillera (Cheilletz et al., 1994; Cooper et al., 1995; Banks et al., 2000; Taboada et al., 2000),

through this process the sedimentary basin got fractured with inverse faults in NE-SW direction

(Ujueta, 1992).

The faults were used as a vertical conduit for the hot mineralizing (Broanquet, 1999) and

filtrated within the host rocks, which have high percents of organic matter, and dissolved part of

them (Ottoway et al., 1994), this started a period of albitization, where the rocks began to enrich

in Na, Mn, Mg, CO2 and depleted in K, V, Th, U, Y, Mo, Sn, Pb, Zn, Cs and Cr (Beus & Mineev,

1972), elements which were leached by the high salinity water. Then the water got deposited in

fractures across the faults where the crystals of emerald and calcite formed. In order to see the

actual configuration, the rocks had to be elevated due to the Eastern Cordillera orogeny and then

exposed by the erosion the overlying rocks (Carrillo, 2006). The actual configuration of the

Western side of the Eastern Cordillera in found in Figure 5.

In the Western Emerald Belt there are plenty of cretaceous formations that could host

enriched veins. However, only the Paja Formation, Unit K5, is going to be described, because it

is where the La Marina mine is located, also there could be found emerald mines such as La Pita,

Consorcio, Totumos and Polveros. According to the fossil content of this unit it was deposited in

the Hauterivian – Barremian (Forero, 1987; Reyes et al., 2006). Its lithology is predominantly

constituted by clayey to silty shales, with dark colors and a low calcareous content. The rocks are

stratified in laminar habit and frequently present veinlets or gypsum (Morales et al., 1958).

Ingeominas – Geoserch LTDA. (2005) did a geological study transect in the Borbur – Pauna road

with these results shown in Figures 5, 6 and 7.


Figure 5: Regional geological framework (Restrepo-Pace et al, 2004 in Ingeominas – Geoserch

LTDA. 2005).


Figure 6: Stratigraphic column of the transect, found in Figure 7, done by Ingeominas –

Geoserch LTDA. (2005).


Figure 7: Location of the transect done by Ingeominas – Geoserch LTDA. (2005).


Methodology and Analytical techniques

Field work

Structural information and samples were taken in the mine La Marina, the main producer

of euclase in Colombia, close to the municipality of Pauna, Boyacá, in the CEB. The mine is

located at 103km to the North of Bogotá, the capital of Colombia, and 13km to the Nor-east of

the famous Muzo mine. The mine is located next to the Borbur – Pauna road where is seen the

black shales of the Paja Formation along. The coordinates gotten on the entrance of the mine are

N 05º38.070” – W 74º02.731” at 463m over the sea level. The mine was visited on October 2015

and May 2016 for a couple of days each time (Figure 8).

La Marina has close to a 1.5 km of tunnels were the veins of albite can be seen cutting

the host rock. One of those veins mineralize euclase, apatite and albite as their main crystals, and

another one produce emerald, but in low amounts. Occasionally there are found accessory

minerals such as gypsum, pyrophyllite, calcite, oxides or pyrite. Through the entire mine it is

seen how the water infiltrates the host rock and relies on the floor with a reddish color do to the

oxidation of metals (mainly iron) in the deposit. About 40 samples were collected on both visits,

the most representative samples were investigated further through laboratory work. The samples

were taken from the euclase mineralized vein, the host rock, fault mirrors and veins without

mineralization.


Figure 8. (a) La Marina mine entry. (b) Water filtrating through the host rock. The surrounded rock is

weathered. (c) Albite veins cutting the host rock. Veins are parallel between them and perpendicular to

the faulting. (d) Host rock with accessory minerals of pyrite to the upper-right corner and iron oxides in

red. (e) Quartz vein cutting the host rock. The vein is composed of euhedral crystals. (f) Pyrophyllite

crystal found at the border of albite veins.

a b

c d

e f


Laboratory work

Petrography. Polished thin sections (30 μm) from the sedimentary host rock and veinlets

of albite were prepared in the samples preparation laboratory at Universidad de Los Andes.

Sections were studied in the polarized light microscope.

Whole Rock Chemistry. Powdered whole-rock samples were obtained by grinding the

host rock, close the mineralization, in an Agatha mill. Major elements were determined by the

glass beads method; which consist of 0.6g of powder sample diluted in 6 g of Li2B4O7 by a

PHILIPS Magix Pro X-ray fluorescence (XRF) at the Centro de Instrumentación Cientifica at the

Univeridad de Granada in Spain. Trace elements were determined at the Universidad Granada

by ICP-Mass Spectometry (ICP-MS) after HNO3+HF digestion of 0.1000 g of sample powder in

a Teflon-lined vessel at ~180 °C and ~200 P.S.I. for 30 minutes evaporation to dryness, and

subsequent dissolution in 100 ml of 4 vol.% HNO3.

The results are compared with the geochemical data taken in the Paja formation by

Campos Alvarez & Roser (2006). The data is processed and plotted into REE normalized to the

chondrite and North American Shale Composite (NASC). This lead to determine the possible

alteration made to the host rock close the mineralized zone. All the trace elements are normalized

to the marine shales composition of Li (1991), grouped and plotted into transition metals, high

field-strength, alkali and alkaline elements, REE, actinides and heavy metals.

SEM & X-Ray spectroscopy. The scanning electron microscope (SEM) images and

energy-dispersive X-ray spectroscopy (EDS) were obtained for polished-thin section recovered

with graphite in the Centro de Microscopía at Universidad de Los Andes, with the high-

resolution scanning microscope JEOL JSM-6490LV, operated at an accelerating voltage of 20 kV

and 0.5 to 300.000X magnification, with a 3.0 nm spatial resolution.


Semi-quantitative analyses were carried out in one Carbon-coated, polished thin-section

belonging to the mineralized vein by scanning electron microscopy/Backscattered electrons

(SEM/BSE). These observations were carried out using a JEOL, model JSM 6490LV

microscope, equipped with an X-Ray Link Analytical QX at the Universidad de Los Andes.

Analytical conditions were 20 kV acceleration voltage. The following X-ray lines were used:

FeKα, SiKα, AlKα, CaKα and Kβ, PKα to classify the textural and chemical composition of the

deposit. These results are shown in Table 1.

Microthermometry. Quantitative analyses were carried out in euclase, apatite, albite

mineralized doubly polished sections. The sections are cooled and heated in ratios between 1 –

20 ºC/min to determine the eutectic, final fusion ice, final fusion salt, homogenization

temperatures. Studies were done in the Laboratorio de Caracterización Litológica (Microscopia

y Microtermometria) at Universidad Nacional. These observations were carried out in a Zeiss

Axio Scope. A1 microscope and a Linkam 600 platen.

Results

Petrography

The samples from the mineralized vein have coarse grains of euclase, apatite and albite.

The euclase and apatite are single crystals of low quality, with subhedral form, both minerals

contain high amount of micro-fractures following the symmetry planes. Albite crystals are finer

than euclase and apatite having euhedral form and massive habit. There could be distinguish

some minerals inside the coarse grains, however these inclusions are going to be properly

discussed in the SEM and BSE images section. The petrography for fluid inclusions is going to

be develop in the fluid inclusions section.


The host rock samples have fine sediments and high organic content. Due to the size of

the minerals they are very difficult to identify, additionally the presence of organic matter make

the samples look dark, which makes it even harder. However, there could be distinguish albite,

quartz and mica crystals in the host rock. The rock is defined as a shale with high organic content.

Veinlets cutting the host rock are almost pure albite, and it is seen that they deform the

surrounding crystals. In order to know more about the host rock whole rock chemistry was done

to samples close the mineralized vein (Figure 9).

Figure 9. Thin section of host rock sample. Host rock has a dark color do to the size of the crystals and

the high content of organic matter. Some albite crystals and mica in the host rock are observed.

Plagioclase vein cutting through and deforming the host rock, seen in the left part of the figure.

Whole rock chemistry

For a shale sample of the Paja formation, the major elements are similar to the

distribution of the NASC, except in Ca where is depleted and P where is enriched. The host rock


samples, close to the mineralization, have a shale composition for the silicon, titanium and

aluminum oxides. Nevertheless, for iron, magnesium, calcium, potassium and phosphorus oxides

the samples show an impoverishment. Due to the lack of Fe, Mg, Ca, K and P in the samples

there is a high enrichment in Na (Figure 10) (Table 1, in tables section).

Figure 10. Host rock major elements patterns representative of the study area. Normalized to the North

American Shale Composite (NASC) of Condie, K.C. (1993) and compered to Paja formation sample

PES14 (Campos Alvarez & Roser, 2006).

The mean composition of the REE in the lower Paja formation Group 1 (Campos Alvarez

& Roser, 2006) has a flat behavior around 1 normalized to the NASC. However, the pattern for

the host rock samples close to the mineralization is similar to the anomalous patterns found by

Campos Alvarez & Roser (2006). The samples are depleted in all REE, nevertheless the mean

ratio for HREE is lower than for the LREE (Figure 11).


Figure 11. Host rock Rare Earth Elements (REE) patterns representative of the study area. (a)

Normalized to the chondrite of McDonough & Sun (1995) and compered to the mean composition of the

lower Paja formation Group 1 (Campos Alvarez & Roser, 2006). (b) Normalized to the North American

Shale Composite (NASC) of Condie, K.C. (1993) and compared to Paja formation samples with

Anomalous patterns (Campos Alvarez & Roser, 2006).

The trace elements were divided into the groups of transition metals, high field-strength,

alkali and alkaline elements, REE, actinides and heavy metals. Seen as an average the host rock

samples are depleted the most in alkali and alkaline elements such as in REE and yttrium, when

compared to the marine shales (Li, 1991). The only group that has a flat pattern around 1

normalized to the marine shales are the high field-strength elements. Transition metals, actinides

and heavy metals are lightly depleted, with the lowest values found in Pb and Ni (Figure 12)

(Table 2, in tables section).

a b


Figure 12. Host rock trace elements patterns representative of the study area normalized to marine shales

Li (1991). (a) Transition metals. (b) High-field strength elements. (c) Alkali, alkaline-earth elements. (d)

REEs. (e) Actinides and heavy metals.

a b

c d

e


SEM & X-Ray spectroscopy

The theoretically euclase composition is BeAlSiO4(OH), nonetheless the studies show

that the composition for the sedimentary euclase has a little less Al and Si than what its expected.

The minerals spotted inside euclase are diaspore, plagioclase and phenakite. In this case the

plagioclase registered is an oligoclase (Table 3). For the apatite, the sample was measure in

different positions with similar results, so there was not found any significant zonation (Table 4a).

Figure 13. BSE Images for mineralized vein of euclase, apatite and albite. (a) Contact between albite and

apatite. There is albite in apatite. Albite diffuses in apatite at the bottom of the figure. (b) Albite with

inclusions of mica (gray elongated crystals), pyrite (White rounded crystals at the bottom) and rutile

(White disseminated crystals). (c) Pyrite within albite. Pyrite showing framboidal texture. (d) Host rock

with a vein cutting through.

a b

c d


Table 3.

EDS results for a euclase crystal and mineral inclusions in the mineralized vein in a%.

Note: With the respective balanced formula. In quartz the aluminum is attributed to the surrounding

euclase. In plagioclase part of the aluminum, silicon and oxygen is contributed to euclase.

Plagioclase crystals found in the vein were analyzed showing that they are pure albite.

Not as in apatite that has some Ca or euclase as seen before. Within the vein there are minerals of

pyrite, quartz, phengite, rutile and iron. The phengite crystals composition change the most

notable aspect is the presence of magnesium. When the magnesium enters the structure all the

other elements reduce their contribution to the measured chemical formula.

Mineral O Na Al Si K Ca Formula

Euclase 74.30 − 12.55 13.15 − − BeAl0.84Si0.88O4(OH)

Euclase 73.50 − 13.14 13.36 − − BeAl0.89Si0.91O4(OH)

Diaspore 64.58 − 35.42 − − − Al1.10O(OH)

Diaspore 63.66 − 36.34 − − − Al1.14O(OH)

Phenakite 78.44 − 1.31 20.25 − − Be2Si1.08O4

Plagioclase 66.98 3.09 12.44 15.92 1.01 0.56 (Na0.60K0.20Ca0.11)(Al1.57Si2.21)O8


Host rock was measured showing the main elements belonging to the combination of

minerals in the sedimentary rock. The elements reflected in the analysis were O, Na, Mg, Al, Si,

K and Ca, Cl was spotted in a veinlet cutting the host rock. Some relationships between the

crystals are shown in figure 13, where it is seen how albite diffuses in apatite, and there is a relict

of apatite in albite. Pyrite and phengite are inside albitite vein, pyrite showing framboidal texture.

Also there is evidence of the vein seen cutting the host rock.

Table 4.

EDS results for a apatite crystal in the mineralized vein in a%.

Mineral O F P Ca Formula Location

Apatite 58.51 6.03 14.32 21.15 Ca4.34(P0.98O4)3F1.24 Center

Apatite 60.64 5.99 13.45 19.93 Ca3.94(P0.89O4)3F1.19 Center

Apatite 58.08 7.88 13.60 20.45 Ca4.23(P0.94O4)3F1.63 Border

Note: With the respective balanced formula. Measurements with the position in the apatite crystals.


Table 5.

EDS results for a mineralized sample with albite crystals and host rock in a%.

a) Mineral O Na Al Si Ca Formula Location

Albite 65.37 6.57 6.86 21.00 0.19 (Na0.8Ca0.02)(Al0.84Si2.57)O8 Inside apatite

Albite 65.15 6.32 6.85 21.68 − Na0.78(Al0.84Si2.66)O8 Near apatite

Albite 65.67 6.27 6.69 21.37 − Na0.76(Al0.81Si2.6)O8 Near apatite

Albite 66.20 6.42 6.28 21.10 − Na0.78(Al0.76Si2.55)O8 Afar apatite

Albite 65.28 6.52 6.50 21.70 − Na0.8(Al0.8Si2.66)O8 Afar apatite

b) Mineral O Mg P Al Si S K Ca Ti Fe Formula

Quartz 65.79 − 0.39 33.52 − − 0.30 − − Si1.02O2

Phengite 63.12 − − 13.98 16.48 − 5.40 − − 1.02 K1.03(Fe0.19Al2.66)Si3.13O10(OH)2

Phengite 68.56 0.64 − 11.27 14.42 − 4.24 − − 0.87 K0.74(Fe0.15Mg0.11Al1.97)Si2.52O10(OH)2

Phengite 66.82 0.59 − 11.98 15.27 − 4.56 − − 0.79 K0.82(Fe0.14Mg0.11Al2.15)Si2.74O10(OH)2

Rutile 69.38 − − − − − − − 30.62 − Ti0.88O2

Iron − − − 1.50 − − − − 3.25 95.25 Fe

Pyrite − − − 1.42 − 69.00 − − − 28.68 Fe0.83S2

Note: With the respective balanced formula. (a) Albite crystals found in different positions with measures at different locations. (b) Minerals found

inside albite. (c) Measures at the host rock and a veinlet cutting through.

c) Mineral O Na Mg Al Si Cl K Ca

Host rock 64.87 7.04 1.80 3.09 20.98 − 0.35 1.87

Host rock 66.84 6.76 1.70 2.80 19.87 − 0.32 1.71

Veinlet 66.64 7.33 1.85 1.52 20.42 − 0.38 1.86

Veinlet 69.88 6.10 1.64 1.70 18.35 0.30 0.33 1.71


Microthermometry

The eutectic temperature for the mineralized vein is between -102 to -93 ˚C with the three

principal minerals with similar values. For the final fusion of ice two groups could be

distinguish, apatite and albite report temperatures around -18 ˚C, while euclase temperatures

ranges from -16 to -10 ˚C. The same happens with the homogenization of the fluid, where apatite

and albite homogenize to gas at higher temperatures than euclase to liquid.

The final fusion of the salt cannot be interpreted as before, that is why the inclusions

were separated in their type, triphasic or biphasic. Biphasic inclusions always need less

temperature to dissolve the salt but they do not represent the fluid which formed the mineralized

vein. The three minerals have similar final fusion of the salt with temperatures between 321 –

364 ˚C. The final homogenization temperature is the maximum between the homogenization of

the salt or the fluid.


Table 6.

Microthermometry results for euclase, apatite, albite.

Mineral Te (˚C) Tffi (˚C) Thf (˚C) Fh Th (˚C)

Euclase -98 − -95 -15 − -13* 338 − 362 Liquid 338 − 364

Apatite -102 − -95 -17.2 − -17.0 372 − 411 Gas 375 − 411**

Albite -94 − -93 -19 − -17 373 − 416 Gas 374 − 416

Mineral Type Tffs (˚C)

Euclase Triphasic 333 − 364

Biphasic 0 − 10

Apatite Triphasic 332 − 343**

Biphasic 13 − 14

Albite Triphasic 321 − 338

Biphasic 12 − 17

Note: Te = Eutectic temperature, Tffi = Final fusion temperature of the ice, Thf = Hominization

temperature of the ice, Fh = Fluid homegenizated to, Th, homogenization temperature, Tffs = Final fusion

temperature of the salt. *The final fusion of the ice is higher, ~7 ˚C, in the biphasic fluid inclusions than

in triphasic inclusions. **There was a inclusion omitted which did not homogenized the salt in the

maximum temperature of the platen, 600 ˚C.

Discussion and Conclusion

Albitite vein has coarse and equigranular grain sizes indicating precipitation in a fluid-

filled space (Valenta et al., 1994). The massive texture in the vein suggest a higher rate opening

than of filling, typical in deformations in low temperature (Misik, 1971; Dunne & Hancock,

1994). The veins do not present notable deformation, however veinlets in thin section show

partial folding suggesting that veining occurred during folding and faulting (Valenta et al., 1994).

The veins are parallel and there are no fractures in all directions indicating a biaxial stress.


The structural information gathered at the mine is dominated by a fault with a dip of

144°/43°. This fault follows the regional style of the region, a system of inverse faults with

vergence to the west and orientation of N45°E (Reyes, 2006). On the other hand, the veins are

aligned in the 330°/54°direction, almost perfectly perpendicular to the fault. Further, there is

folding present in hand samples and in thin section, and Reyes (2006) reported fold systems with

the same direction of the faulting. For this reason, the structural location of the euclase is similar

to the proposed by Lopes (1995) for emerald, resumed on figure 14.

Figure 14. Model of the location of albitite veins hosting euclase. Based in a compressive environment,

with trust faults and foldings associated to the ramp. Redrawn from Lopes (1995).

The host rock sample close to the mineralization has a different pattern than the

representative sample of the lower Paja formation suggesting that the shales have been altered.

Further, the elements also shows that the rocks have been modified, evidenced as the behavior of

High Field-Strengh Elements (HFSE) is flat compared to the marine shales, but Large-Ion

Lithophile Elements (LILE) are depleted in host rock samples. As LILE are mostly fluid-mobile


and HFSE immobile (Pandarinath, 2007), a hydrothermal alteration will modify their content

removing LILE and maintaining HFSE as seen in these samples.

The evidence of an alteration, the high amount of Na2O and the impoverishment of K2O

and CaO manifest an episode of albitization. Similar to alterations found in emerald mines (Beus,

1966; Beus, 1979). The host rock sample close to the mineralization is also depleted in MgO,

P2O5, Fe2O3, REE. These elements can be easily associated to minerals in the albitite vein.

Dolomite - calcite captures Mg and Ca; Phengite K and Mg; siderite and pyrite Fe; apatite Ca

and P; parisite - codazzite Ca and REE (Carrillo, 2006). The elements dissolved from the host

rock later precipitated in the mineralized vein as huge crystals of minerals mentioned above.

Crystals in the mineralized vein do not show any zonation indicating a unique stage of

crystal growth where the formation conditions did not change. The composition of the

mineralizing fluid stayed constant, such as the pressure and temperature. There were also

identified accessory minerals of the vein such as quartz, rutile, pyrite, phengite, diaspore and iron.

Crystal relations show that albite precipitated after apatite.

The plagioclase shows a variation on the calcium-sodium proportion depending on its

location. Plagioclase inside euclase has a proportion of 85% Na – 15% Ca, belonging to

oligoclase member. On the other hand, plagioclase inside apatite has a proportion of 97% Na –

3% Ca, belonging to albite member. Additionally, the composition of the albitite vein reports a

100% of Na in its structure.

In agreement with the development of the crystals of euclase, apatite and albite,

combined with their relations, the sequence of formation is in the order mentioned. With this in

mind, as the system evolved the composition of plagioclase changed. In other words, that

albitization re-worked the geochemistry of the deposit through time. Moreover, according to


microthermometric studies the temperature of formation, proportional to the temperature of

homogenization, was higher as the system developed.

With the values registered for the temperature of dissolution of the salt, such as the form

of the crystals, the solid phase of the inclusions corresponds to halite (NaCl). With this

information the salinity of the mineralized fluid can be known with the solubility of halite at each

temperature with the results shown in table 7. Unlike homogenization temperatures, salinity in

the mineralized fluid does not have any clear relation with the evolution of the system. Primary,

or triphasic, fluid inclusions in the main minerals have a similar salinity around 41 wt% of NaCl

eq.

Table 7.

Final fusion temperature of the salt.

Mineral Type Tffs (˚C) Salinity (wt%)

Euclase Triphasic 333 − 364 40.8 − 43.8

Biphasic 0 − 10 26.2 − 26.3

Apatite Triphasic 332 − 343* 40.7 − 41.7*

Biphasic 13 − 14 26.3 − 26.4

Albite Triphasic 321 − 338 39.9 − 41.4

Biphasic 12 − 17 26.3 − 26.4

Note: Final fusion temperature of the salt (Tffs) in euclase, apatite and albite for triphasic and biphasic

fluid inclusions with their respective salinity. *There was an inclusion omitted which did not homogenize

the salt in the maximum temperature of the platen, 600 ˚C. Meaning a salinity higher than 70 wt% of

NaCl eq.

The minimum temperatures of formation are around 338 ˚C for euclase, 375 ˚C for apatite and

374 ˚C for albite. In order to define an upper limit for the temperature of formation the BeO-


Al2O3-SiO2-H2O system under conditions of saturated water was analyzed. The minerals found

belonging to this system are phenakite, quartz, pyrophyllite, diaspore and euclase. The mineral

which crystalizes at the lowest temperature is pyrophyllite. At low pressures pyrophyllite

precipitates around 370 ˚C (Figure 16).

Table 8.

Names, abbreviations, and formulas.

Name Abbr. Formula Name Abbr. Formula

Aluminum silicate Als Al2SiO5 Diaspore Di AlO(OH)

Andalusite And Al2SiO5 Euclase Eu BeAlSiO4(OH)

Behoite Bh Be(OH)2 Gibbsite Gb Al(OH)3

Bertrandite Bt Be4Si2O7(OH)2 Kaolinite Ka Al2Si2O5(OH)4

Beryl Be Be3Al2Si6O18.nH2O Kyanite Kya Al2SiO5

Beryllite Bl Be3SiO4(OH)2.H2O Phenakite Ph Be2SiO4

Bromellite Br BeO Pyrophyllite Py Al2Si4O10(OH)2

Chrysoberyl Ch BeAl2O4 Quartz Qz SiO2

Corundum Co Al2O3 Sillimanite Sil Al2SiO5

Note: Names, abbreviations, and formulas of some phases of geologic interest in the BeO-Al2O3-SiO2-

H2O system. Taken from Barton (1986).

Figure 15. Minerals in the BeO-Al2O3-SiO2-H2O system projected from H2O. Also shown are generalized

fields for some of the major types natural of occurrences. Taken from Barton & Young (2002).


Figure 16. Pressure-temperature projection of phase relationships in the BeO-Al2O3-SiO2-H2O (BASH)

system. Redrawn from Barton (1986). Abbreviation from table 8.

As the temperature for the system cannot higher than 400 ˚C, fluid inclusions with high

salinity >70 wt.% of NaCl eq. that did not homogenized up to 600 ˚C are the product of healing.

A fluid inclusion got divided in two trapping the crystal of salt in a lower volume and lowering

the salinity in the other inclusion as counterpart (Figure 17). The quartz + euclase relationship is

common greisen environments, where the temperature of formation is low. With a minimum

temperature of formation being 338 ˚C, the quartz and phenakite should have an alternative

origin from the dissolution-precipitation of phases of the BASH system.


Figure 17. Healing of a fluid inclusion in order to increase the salinity of a single section.

Similar temperatures are found in the Chivor mine in the Eastern Emerald Belt, reported

for euclase (Chavez-Gil, 1997) and aquamarine (Romero, 2003). If we consider the first works

done to emerald formations the temperature of formation was from 130 to 330 ˚C (Barton &

Young, 2002). So the temperature of formation of beryl is similar or lower than euclase (Figure

18), different for igneous or metamorphic environments. The lowering of the temperatures is the

one which allows the dissolution of beryl and precipitation euclase as could be seen in figure 16.

Our data confirm that euclase in the Colombian Emerald Belt (CEB) is not the product of

dissolution of beryl but precipitation directly from a fluid. Evidence of it is the temperatures of

formation of beryl and euclase, the size of the crystals in the CEB are the biggest ones found and

the possibility to have euclase and beryl close. Size of the crystals suggest that euclase formed

with space, time and a high amount of Be, Al, Si, O, H, conditions that cannot be reached when

euclase is formed by the dissolution of beryl. Samples with euclase-beryl close are not found in

igneous or metamorphic environments because there should be big changes in temperature in a

couple of centimeters, conditions that are impossible to accomplish.


Figure 18. Salinity and temperature of the mineralizing fluids for different types of deposits. The

formation of euclase in the Colombian Emerald Belt is in blue. Edited from Barton & Young (2002).

The mechanics that allows euclase precipitate in the CEB are the changes in the activity

of alumina. As euclase requires a relatively high chemical potential of alumina (Barton, 1986),

compared to emerald and phenakite, when the alumina activity increases at a same temperature

and pressure the euclase would precipitate (Figure 19). The evidences for the theory are the

presence of phenakite, the presence of samples with emerald close to euclase, similar

temperatures of formation of emerald and euclase, the alumina activity in the host rock and the

presence of albite.

Phenakite crystalizes at < 300 ºC at low pressures, so it is not possible to associated it

origin with the euclase that formed at higher temperatures. However, if the alumina activity


decrease in a given condition the phenakite would precipitate. Generally, as emerald dissolves to

form euclase at low temperatures, in order to have both in a very short distance the activity of

alumina should change in particular points as temperature is the same. The activity of alumina

increases if the fluid assimilates part of the host rock that has high alumina activity. Rocks near

the mineralization has less alumina activity than none altered shale (Table 9), so the mineralized

vein augmented its alumina activity. Albite crystals suggest that there is a high activity of SiO2 so

the conditions to have beryl, euclase and phenakite are very similar, and could be explained with

small anomalies in the alumina activity.

Figure 19. Beryllium mineral stabilities as a function of silica and alumina activities at ~250 ºC - ~1 kb.

(Barton & Young, 2002)

Table 9.

Alumina activities.

Sample MM1A MM2A PES14

Log aAl2O3 -0.89 -0.89 -0.86

Note: Host rock samples close to the mineralization have less alumina activity than none altered shale.


Acknowledgements

I will like to thank all the persons who supported me through the realization of my thesis

project. Starting with my family who always were available when I needed help and doing it

with the best disposition.

Also to the Universidad de los Andes for lending me the equipment and laboratories

necessary for qualitative and quantitative analysis specially to the geosciences department. To the

plant of teachers who solved any concerns I had. To laboratory technicians for the excellent work

environment they fostered in each space.

To alba gems for the acquisition of samples, the planning of field work and for lending its

professionals to solve any inconvenience or doubt that arose from the project.

To Fernando and his son, Javier, for allowing us to enter La Marina mine. And the

employees who guided us along the tunnels, helped to gather samples and showed us high

quality gems.

To the Universidad Nacional for letting me use the equipment for fluid inclusion analysis,

and my companions of the Geologia Economica Aplicada course at this university who suggest

improvements to the research.

To the Asociación Colombiana de Geólogos y Geofísicos del Petróleo (ACGGP) and

Corporación Geológica Ares, for supporting young researches, contributing to the geological

knowledge of the Colombian territory and supporting this research with the Fondo Educativo

Howard T. Corrigan grant.

To Gmas consultores for advising for this research, making available all their knowledge

and tools in favor of the project.


To all my friends and mates for their constructive criticism and recommendations,

specially to Nicolas Zuluaga and Kevin Henao for accompanying me to the field and gather

information and photos of the mine.

A special mention for the persons who were fundamental for the development of this

research my brother Jose Reyes, the director Ideal Blanco, the one who had the initial idea of the

project Yovanny Alba, Jesús Ruiz, Juan Carlos Molano, José María Jaramillo and Holman

Gonzales.


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Tables

Table 1

Major elements

Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total without LOI

MM-1A 64.81 0.98 20.38 0.42 0.01 0.05 0.14 10.31 0.68 0.02 2.04 97.80

MM-2A 63.99 1.19 20.43 0.34 0.00 0.07 0.15 9.91 0.88 0.02 2.77 96.98

Table 2

Trace elements

Sample Li Rb Cs Be Sr Ba Sc V Cr Co Ni Cu Zn

MM-1A 2.108 11.391 0.565 1.315 66.354 6.96 5.769 133.519 64.887 4.236 3.8 43.425 28.004

MM-2A 2.712 18.535 0.804 1.472 3.217 9.6 4.928 189.064 96.62 8.102 3.839 6.299 4.579

Sample Ga Y Nb Ta Zr Hf Mo Sn Tl Pb U Th La

MM-1A 16.107 0.414 18.571 1.544 97.658 2.235 7.106 3.794 0.161 1.793 0.932 0.987 2.287

MM-2A 18.106 1.019 23.738 1.861 180.96 4.57 13.211 3.637 0.19 0.919 1.168 0.641 1.005

Sample Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

MM-1A 4.304 0.748 2.932 0.463 0.093 0.23 0.026 0.094 0.02 0.051 0.01 0.072 0.015

MM-2A 1.926 0.294 1.126 0.202 0.045 0.147 0.025 0.159 0.044 0.141 0.027 0.189 0.037

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