Metallogenic approach of the Orogenic Gold mineralization present at Libano, Tolima - Tesis doc. Juan Sebastian Duran T

METALLOGENIC APPROACH OF THE OROGENIC
GOLD MINERALIZATION PRESENT AT LIBANO,
TOLIMA.
By Juan Sebastian Duran Torres
Thesis dissertation for obtain the degree of:
Geoscientist
PhD. Yamirka Rojas Agramonte
Director
MSc. Juan Carlos Molano Mendoza
Co-Director
UNIVERSIDAD DE LOS ANDES
FACULTAD DE CIENCIAS
DEPARTAMENTO DE GEOCIENCIAS
Bogota, May 2018
____________________________
PhD. Yamirka Rojas Agramonte
____________________________
MSc. Juan Carlos Molano Mendoza
____________________________
Juan Sebastian Duran Torres
To my family…
CONTENTS
1. INTRODUCTION
1
1.1 Orogenic Gold Deposits
…………………………………
2
1.2 Intrusion-related Gold Deposits
…………………………………
4
1.3 Fluid Inclusion studies
…………………………………
5
2. REGIONAL GEOLOGY AND TECTONIC SETTING
6
2.1 Tectonic Framework
…………………………………
6
2.2 Regional Geology
…………………………………
8
3. GEOLOGY OF THE MINING AREA
11
4. METODOLOGY
16
4.1 Field work
…………………………………
16
4.2 Laboratory work
…………………………………
17
4.3 Petrography
…………………………………
17
4.4 Scanning electron microscope
…………………………………
18
4.5 Fluid inclusion studies
…………………………………
18
4.6 Raman analysis
…………………………………
18
5. RESULTS
19
5.1 Petrography
5.1.1 Host Rocks
…………………………………
…………………………………
19
19
5.1.1.1 Quartz-chlorite-sericite schists
……..……..
19
5.1.1.2 Graphitic schists
……..……..
21
5.1.2 Porphyritic intrusives
……………………………
23
5.1.2.1 DH-16
……………………………
23
5.1.2.2 DH-15
……………………………
24
……………………………
25
5.1.3.1 Samples from the first level………….………..
25
5.1.3.2 Samples from the fifth level………………
28
5.1.3.3 Samples from the ninth level ………….…
31
5.1.3.4 Samples from the tenth level……..………
39
5.1.3 Quartz veins
5.1.4 Igneous dykes
……………………………
41
5.1.4.1 Sample 10-ALT
……………………
41
5.1.4.1 Sample DA-1
……………………
41
5.2 Mineral paragenesis
…………………………………
44
5.3 Mineral chemistry
…………………………………
46
5.4 Microthermometry
…………………………………
51
5.5 Raman Spectroscopy
…………………………………
60
6. DISCUSSION
6.1 Mineralizing styles and vein textures
62
…………………………….. 62
6.2 Hydrothermal alteration
……………………………63
6.3 Paragenetic sequence
.………………………….. 65
6.4 Mineralizing fluids
…………………………… 66
6.5 Source of fluids and relationship to magmatism
……..…..…….…. 69
6.6 Comparative table between deposit types
……….………… 70
7. CONCLUSIONS
72
8. RECOMMENDATIONS
73
9. AKWNOWLEDGMENTS
73
10. REFERENCES
74
ANNEXES
77
RESUMEN
La mineralización que ocurre en el área de Libano (Tolima) se caracterizó como un
sistema ramal vetiforme de tipo oro orogénico, mesozonal, hospedado en esquistos
grafitosos y verdes pertenecientes al Complejo Cajamarca. En el depósito, la
alteración hidrotermal se presenta en zonas proximales adyacentes a las vetas de
cuarzo, principalmente con sericita, calcita y cantidades menores de dolomita,
clorita, micas blancas, turmalina y rutilo. Las vetas presentan rumbo preferencial
noreste e inclinaciones al noroeste entre 40° y 70°, son concordantes con la foliación
del hospedante, y presentan espesores variables entre los 0.8 y 3 metros,
alcanzando hasta 6 metros localmente. Mineralógicamente están constituidas por
pirita, esfalerita, esfalerita de cadmio, galena, calcopirita, pirrotina, y en menor
proporción arsenopirita, argento-tenantita, pirargirita, polibasita, plata nativa y
electrum con relaciones variables de Au/Ag, asociados con cuarzo, calcita,
dolomita, scheelita y ferberita. Se distinguieron 4 eventos de mineralización
definidos por las relaciones de corte y texturas entre las vetas y los minerales, en
donde el grado del oro disminuye a medida que aumenta la cantidad de plata en el
sistema. Las vetas son de tipo “crack & seal”, presentan una textura bandeada y se
interpreta que fueron formadas durante múltiples eventos de fracturación, mezcla
y flujo de fluidos. Además, las vetas presentan texturas masivas, brechosas, y
drusiformes. Localmente se pueden observar en las vetas “dilational jogs”, zonas
de “ore-shoots”, pinchamientos y engrosamientos. El área está controlada por el
“Sistema de Fallas La Chucula” como arreglo conjugado tipo Riedel con rumbo
N330°W, interpretado como satélite a la falla de Palestina de primer orden. Las
fases volátiles que caracterizan el fluido mineralizante son H2O – CO2 + CH4 + N2,
con salinidades entre el rango de 1.71-7.86 wt. % NaCleq, temperaturas de
homogenización entre 250° y 320 °C, una densidad de 0.8 g/cc y tendencias de
evolución de fluidos principalmente de tipo mezcla isotermal.
Palabras clave: Oro orogénico, venas tipo crack-seal, CO2, scheelita, esquistos grafitosos.
ABSTRACT
The mineralization that occurs in the area of Libano (Tolima) was characterized as
a vetiform branch system of orogenic gold type, mesozonal, hosted in graphitic
schists and greenschists belonging to the Cajamarca Complex. In the deposit,
hydrothermal alteration occurs in proximal areas adjacent to the quartz veins,
mainly with sericite, calcite and minor amounts of chlorite, white micas,
tourmaline and rutile. The veins have a northeast preferential strike and northwest
dip between 40 ° and 70 °. Moreover, they are concordant with the foliation of the
host rock, and have mainly variable thickness between 0.8 and 3 meters, reaching
up to 6 meters locally. Mineralogically, the veins are constituted by pyrite,
sphalerite, cadmium sphalerite, galena, chalcopyrite, pyrrhotite, and in a lesser
proportions arsenopyrite, argento-tennantite, pirargyrite, polybasite, native silver
and electrum with variable Au / Ag ratios, associated with quartz, calcite,
dolomite, scheelite and ferberite. Four mineralization events were distinguished,
defined by cut off relationships and textures between veins and minerals, where
the degree of gold decreases as the amount of silver in the system increases. The
veins are of “crack & seal" type, have a banded texture and were formed during
multiple fracturing, and fluid flow mixing events. Moreover, veins display
massive, brecciated, and drusiform textures. Locally "dilational jogs", areas of "oreshoots", pinch and swell structures occur in the veins. The area is controlled by the
“La Chucula” Fault System as a Riedel-type conjugate arrangement with N330° W
strike, interpreted as satellite to the Palestine fault of first order. The volatile
phases that characterize the mineralizing fluid are H2O - CO2 + CH4 + N2, with
salinities between the range of 1.71-7.86 wt. % NaCleq, homogenization
temperatures between 250 ° and 320 ° C, a density of 0.8 g / cc and fluid evolution
tendencies mainly of isothermal mixture type.
Key
words:
Orogenic
gold,
crack-seal
veins,
CO2,
graphitic
schists.
1. INTRODUCTION
Near the town of Libano, which is located on the eastern flank of the central
cordillera in the department of Tolima, diverse gold and silver mineralizations
have been reported associated to hydrothermal quartz veins. Such veins comprise
varying amounts of sulfides, carbonates and scheelite. In turn, veins were find
cutting off metamorphic rocks in greenschist facies belonging to the Cajamarca
Complex. These characteristics are similar to deposits described as "orogenic gold"
type (Groves et al., 1998) or intrusion-related gold systems (Hart et al., 2005).
According to Leal-Mejía (2011) these mineral deposits correspond to the mining
district of Santa Isabel-Libano, and are classified as gold mineralizations intrinsic
to igneous intrusive bodies. Castillo (2016), on the other hand, relates them to
orogenic or mesothermal gold deposits due to their structural and mineral
characteristics. So, similarity between characteristics of these deposit types makes
them difficult to classify.
Although Colombia is recognized as a gold producing country per excellence
(Leal-Mejia, 2011), there have not been many studies about gold deposits hosted in
metamorphic terranes belonging to Colombian territory. These studies are
necessary and fundamental for the development of exploration criteria in active
mining regions such, as the Libano mining area. Therefore, the present study
intends to characterize the mineral deposit that occurs in the underground mine
called “Mina el Gran Porvenir del Libano”. Defining different mineralization
events based on mineral paragenesis and fluid inclusion families found in the
quartz veins. Also with the structural characteristics controlling mineralization,
petrography, mineral chemistry, Raman spectroscopy, as well as identifying
mineralization styles and alteration mineral assemblages in host rocks. Based on
the above, this study opens the possibility to give clues about the formation
processes of this gold deposit and the evolution of the ore forming fluid. The
present study will also provide valuable information to the metallogenic map of
Colombia, developed by the Geologic Survey of Colombia, which is actually in
process.
1
The mine is located 5 kilometers to the north of the Libano town and it can be
reach over the road that links the Libano and Convenio municipality in
approximately one and half hour. The topography of the area is characterized by a
steep topographic gradient mountain relief with high to medium slopes, as
consequence of pluvio-erosion. Heights vary in the area from 1500 to 1800 meters
above sea level (see Figure 1).
Figure 1. Location map of the mining area.
1.1 Orogenic gold deposits
Orogenic gold deposits are characterized by having an important structural control
because they were formed by tectonic events, during compresional to
transpressional deformation processes occurred at convergent plate boundaries,
along shear zones (Eilu, 1999). In orogenic gold deposits, the host rocks correspond
mainly to metamorphic rocks in greenschist facies. In addition, they present
diverse quantities of sulfides, such as pyrite, pyrrhotite, arsenopyrite, sphalerite,
chalcopyrite, galena and carbonates, white micas (muscovite, fuschite), scheelite,
tourmaline and high concentrations of gold and silver (Groves et al., 1998). Also,
fineness of gold in this deposit ranges between 780-1000 (Morrison et al., 1990).
2
Hydrothermal alteration processes that took place correspond to sericitization and
carbonatization (Goldfarb, 2010), with mineral assemblages of alteration such as
chlorite, sericite, calcite, tourmaline, dolomite, hematite and rutile in proximal
zones (Eilu, 1999). Mineralizing fluids have a low salinity range between 2- 10 wt%
NaCleq, high CO2 content commonly observed in liquid phase in the system H2O CO2 ± CH4, and homogenization temperatures between 200 and 400°C (Wilkinson
., 2001).
Most gold-bearing veins in metamorphic belts occur as fault-fill shears (i.e. crack &
seal structures) or fractures. Such veins are laminated or banded; locally contain
breccia fragments and elongate ore-shoots in some places (Goldfarb et al., 2005).
Emplacement of these deposits occurs in forearc and arc settings (Figure 3), to
variable depths that range between 2 and 20 kilometers. Thus, depending on
formation depth of the deposit, Groves et al., 1998 classified them as hypozonal,
mesozonal and epizonal orogenic deposits (see Figure 2). Also, these deposits are
spatially and temporally related to transcortical faults and granitic magmatism
could be associated (Groves et al, 1998).
Figure 2. Epigenetic gold deposits in metamorphic terranes (After Goldfarb et al., 2005; Groves et
al., 2003; Groves et al., 1998).
3
1.2 Intrusion-related gold systems
Intrusion-related gold systems (IRGS) are best developed in intrusions emplaced in
the region behind an accretionary orogen (Groves et al., 2003, figure 3), involving
that ore formation is synchronous with granitoid crystallization (Goldfarb et al.,
2005).
Zonation of ore mineral assemblages outward from the central,
mineralizing intrusive body is common, and there is an increase in structural
control on more distal mineralized veins from the pluton (Hart, 2007). Opposite to
orogenic gold deposits, the intrusion related gold systems are characterized by a
wide range of gold grades (generally low) and mineralization styles. The most
distinctive characteristic of this deposit types are sheeted array of parallel quartz
veins with low sulfide content (Hart et al., 2005). Mineral assemblages are mainly
pyrrhotite, pyrite, loellingite, arsenopyrite, and scheelite, with gold intergrowth
with bismuth or tellurium and locally molybdenum or scheelite. Commonly there
is a lack of copper minerals (Goldfarb et al., 2005).
In addition, there is no relation of gold with the presence of wolfram in scheelite
(Hart, 2007). Fluid inclusions of IRGS consist of low salinity to high salinity fluids
(2 – 40 wt% NaCleq), based in the presence of the one high salinity fluid brine
recognized in the majority of IRGS deposits (Mernagh et al., 2007), variable
amounts of CO2 and homogenization temperatures between 160 to 380° C.
However, fluids that formed Ag-Pb-Zn veins lack of significant CO2 (Hart, 2007).
Magmatism and the associated mineralization is completely post-orogenic, and the
mineralizing fluids are the cooling result of the igneous bodies instead of fluids
coming from the dehydration generated by a metamorphic event (Hart et al., 2005).
Figure 3. Different deposit types according to different tectonic settings. (After Groves et al., 2003).
4
1.3 Fluid inclusion studies
Fluid inclusions trapped within crystals in hydrothermal veins are recognized as
an important tool for describing the nature of mineralizing fluids and processes by
which mineral deposits were formed (Wilkinson, 2001). Often, fluid inclusions are
present in minerals as different phases such as liquid (L), liquid and gas (L+V), and
liquid, gas and solid (L+V+S). Also, fluid inclusions are classified according to the
timing of formation of the inclusion relative to that of the host mineral (Samson, et
al. 2003). So, fluid inclusions are classified as:
-Primary: Inclusions formed during the growth of the host crystal. Hence, they
represent the original fluid from which the host crystal formed.
-Secondary: Inclusions formed within fracture planes after crystal growth is
complete.
- Pseudosecondary: They are formed within fractures at the same growing time of
the host crystal.
Primary and pseudosecondary inclusions are important because they are
representative of the ore forming fluid (Wilkinson, 2001). So, measuring different
temperatures in microthermometric experiments of cooling and heating inclusions
gives important information such as temperature of formation, density, pressure,
salinity and volatile content of the mineralizing fluid (Hollister, 1981). Following
temperatures must be measured and introduced in different fluid inclusions
software that contains empirical equations of state that estimate those fluid
characteristics:
-
Eutectic temperature (Te): When the fluid inclusion is frozen, it corresponds
to the temperature at which the inclusion starts to melt. This temperature is
useful to determine of the fluid composition (i.e. fluid volatile system).
-
Final fusion of ice temperature (Tffh): It corresponds to final fusion
temperature of ice and it is useful to determine salinity in fluid systems with
medium to low content of carbon dioxide (CO2) and methane (CH4).
-
Final fusion of clathrates (Tffc): Clathrates are gas hydrates trapped into ice
cubic structures during cooling experiments (Samson et al. 2003) that play
an important role in defining salinity in CO2 - CH4 gas saturated systems
(i.e. orogenic or mesothermal deposits).
5
-
Homogenization temperature (Th): This temperature is interpreted as
trapping temperature of the inclusion. This happen when all phases inside
inclusion homogenize into same phase (liquid or vapor) in heating
experiments.
2 TECTONIC FRAMEWORK AND REGIONAL GEOLOGY
2.1 TECTONIC FRAMEWORK
The Colombian Andes are arranged in three north-south trending mountain ranges
denominated as Eastern, Central and Western Cordilleras (Blanco-Quintero et al.,
2014). The study area is included into the Central cordillera tectonic domain, which
comprise the Cajamarca Complex and other accreted terranes of oceanic and
continental affinity called Arquía and Quebradagrande complexes. The Central
Cordillera is a tectonic domain composed of a pre-Mesozoic basement
corresponding to a low-pressure metamorphic belt (i.e. Cajamarca Complex),
which is intruded by Mesozoic and Cenozoic plutons related to the subduction of
oceanic lithosphere (Taboada , 2000). Also, the Central Cordillera is limited by the
Otu-Pericos (at east) and Cauca-Almaguer (at west) strike-slip faults (BlancoQuintero et al., 2015).
Moreover, on the western side of the Central Cordillera, San Jeronimo and SilviaPijao faults define the Romeral fault system that separate terranes of oceanic and
continental affinity. Those are strike-slip fault characterized by transpressive
movements and their distribution follow an echelon pattern parallel to the
Garrapatas Fault (Figure 4), that is possibly related to the accretion of segments of
oceanic crust to the west of the cordillera (Taboada, 2000). Study area is influenced
by Palestina Fault which is more than 350 kilometers long on the strike, and
between 50 to 600 meters wide. Strike movement of this fault follows a regional
structural tendency to the north-northeast, and cut off a variety of rocks in the
central cordillera (Rodriguez, 2007). According to Feininger (1970) this fault
formed as response to a single unchanging regional stress system, and his
reactivation produce subordinate structures such as Chapetón-Pericos, Ibagué, El
Bagre, Nus and Otú faults.
6
On the other hand, Leal-Mejia (2011) relates the Palestina Fault to an early middle
Paleozoic suture between Cajamarca and the early Paleozoic continental margin of
northwestern South America. Moreover, the nature and origin of regional-scale
faulting is important for metallogeny of gold deposits because those structures
allow the emplacement of fertile magmas and the circulation of deep-seated
hydrothermal fluids. Thus, subordinate and major faults as the mentioned above
are important for the emplacement of dykes and hydrothermal fluids, which lead
to precipitation of ore forming minerals (Leal-Mejia, 2011).
Figure 4. Fault and suture systems of Colombia. EC: Eastern Cordillera, CC: Central Cordillera,
WC: Western Cordillera. (Modified from Leal-Mejia (2011); after (Cediel et al., 2003))
7
2.2 REGIONAL GEOLOGY
The studied mine is located in a tectonically complex area that includes different
geological units belonging to the Cajamarca Complex. Also, intrusive and
extrusive igneous rocks such as Andesites and the “El Hatillo” stock are present
surrounding the mine. Other metamorphic and sedimentary rocks crop out at few
kilometers of the studied area (Figure 5). Those units are the Tierradentro
amphibolites, Santa Teresa and Honda sedimentary groups.
Figure 5. Regional geologic map (Modified from 207-Honda and 226-Libano geological maps
produced by the Servicio Geologico Colombiano, 1:10000 scale).
CAJAMARCA COMPLEX
The Cajamarca complex comprise the core of the Central Cordillera (Maya and
Gonzales, 1995), it comes out largely to the west of the department of Tolima and
consists mainly of green schists and black schists in greenschist to amphibolite
facies. Micaceous schists, amphibole schists, quartzites and amphibolites are also
found in lesser amounts. All rocks mentioned above are products of the regional
metamorphism from medium to low grade.
8
According to Blanco-Quintero (2014) metabasites and pelitic schists show a strong
penetrative foliation, locally related to mylonitization which suggest intense
dynamic-thermal metamorphism. The protoliths of these rocks was probably a
marine volcano-sedimentary pile formed by lava flows and sediments enriched in
organic material (Nuñez, 2001). An age between Proterozoic and Cretaceous has
been proposed for this complex, but specific age has not yet been determined. A
Permo-Triassic age was proposed for the last important thermal event of the
Complex (Restrepo, et al., 1978; Spikings (2015) after Cochrane (2014)) propose an
age of 236 ± 0.6 Ma through U / Pb dating and 221.8 ± 1 Ma by Ar / Ar, and BlancoQuintero et al., (2014) reported late Jurassic metamorphism ages between 147 and
158 Ma through Ar/Ar dating methods.
TIERRADENTRO AMPHIBOLITES
This complex was described by Barrero and Vesga (1976) as quartz-feldespatic
stripped gneisses with local variations to amphibolites, product of regional
metamorphism that extends on a NE direction on the eastern flank of the Central
Cordillera. Associated with this unit there are also marbles, quartzites and
granulites. These rocks have been dated as Proterozoic using the whole-rock K-Ar
method (1350 ± 270 Ma, after Kroonenberg, 1982; Álvarez, 1981). On the other
hand, the complex is found in fault contact with rocks of the Cajamarca Complex
to the west, and Santa Teresa Group along with the Ibague batholith to the east.
EL HATILLO STOCK
This pluton is found intruding the Cajamarca Complex and is overlapped by
sediments of the Honda Group. It is an equigranular body of dioritic composition
with local variations to hornblende gabbro (Rodríguez, 2007). K/Ar radiometric
dating gives an age of 53 ± 1.8 Ma corresponding to Paleocene-Eocene. The quartz
diorite is commonly found with narrow aureoles of hornfels at the contact with
rocks of the Cajamarca Complex as consequence of contact metamorphism. Also,
some auriferous mineralized veins are found associated with this intrusive body
(Nuñez, 2001).
9
HONDA GROUP
The Honda Group outcrops from south to north within the Tolima department,
along the southern end of the Middle Magdalena Valley. In this area, correspond
to grey and red sandstones and mudstones in the lower part of the group,
including conglomerates and sandstones. Also, in the upper part there are red
mudstones and fine grained sandstones. The thickness of this sedimentary group
ranges between 120 and 1200 meters. Based on Ar/Ar dating, Guerrero (1993) gives
ages of 11.0 to 13.5 Ma.
SANTA TERESA GROUP
According to INGEOMINAS (1976) this sedimentary group is composed of
mudstones, sandstones and conglomerates with presence of metamorphic
boulders. For this unit was proposed a Paleozoic age by Gonzáles et al., 1995.
QUATERNARY ROCKS
These are mainly volcanic deposits that consist of lavas and pyroclastic deposits.
The lavas are Andesites with pyroxene and augite (INGEOMINAS, 1976). Those
deposits probably are product of highly explosive events from the Cerro Bravo,
Cerro Machin, Nevado del Ruiz and Nevado del Tolima volcanoes (Rodríguez,
2007).
10
3. GEOLOGY OF THE MINING AREA
The “Gran Porvenir del Libano” mine is divided in ten levels, where mineralized
quartz-calcite veins occur as branch lodes with banded, brecciated and massive
textures (Figure 6). These veins are cutting graphitic and chloritic schists (Figure
7a) in greenschist facies belonging to the Cajamarca Complex.
Figure 6. Vein textures observed in the field.
Almost three different quartz veins concordant with the host rock foliation (45 to
70° inclination to NW) are recognized within the conjugated system. The first one
is a branch vein of massive quartz, locally brecciated (Figure 7b) containing
euhedral (cubic) pyrite, galena, locally scheelite and carbonates. This vein is up to
3 meters wide and dips 42 ° to NW. The second vein is found cutting the massive
vein (Figure 6) and is of the “crack and seal” type with banded-laminated textures.
11
It is called “veta techo” by locals, and is the principal continuous vein structure
along the deposit with dips ranging between 40-70° to NW and an average
thickness of 1.5 meters, locally up to 6 meters in ore-shoots. Gangue minerals in
this vein are quartz and late calcite accompanied by auriferous sulfides including
pyrite, pyrrhotite, galena, sphalerite and chalcopyrite, which are concentrated
between bands of graphite (Figure 8a). Third vein is another branch that also has
banded texture, similar mineralogy to the previous one, and visible grains of gold
that could be observed near the contact with the host rock (Figure 8b).
Additionally, supergene enrichment of chalcopyrite occurs in some parts of the
mine, where malachite was observed.
Figure 7.a) Contact between chlorite schists (up) and graphite schists (bottom). b) Massive
brecciated vein.
12
Figure 8. a).Crack and seal type quartz-carbonate vein with proximal alteration zone.
Abbreviations: Py = Pyrite, Sph = Sphalerite, Gal = Galena, Cpy = Chalcopyrite, Po = Pyrrhotite, El =
Electrum. b). Quartz vein showing visible electrum near the graphite bands.
Scheelite-ferberite-dolomite mineral associations are accompanied by minor
amount of sulfides as “lenses” between graphite bands in the lower part of the
banded principal vein (Figure 9a) Also, drusiform quartz is found in the center of
the scheelite horizon filling cracks or spaces between scheelite and dolomite
(Figure 9b).
Figure 9. a) Scheelite and dolomite within graphite bands at the base of banded vein. b) Drusiform
quartz at the center of the lens filling spaces between scheelite and dolomite
13
Native silver veinlets and silver sulfosalts accompanied by quartz were observed
cutting banded veins forming breccia textures in sulphides (Figure 11a). On the
other hand, hydrothermal alteration occurs in proximal areas adjacent to the
quartz veins mainly with sericite, carbonates, chlorite and white micas (Figure 8a)
leading to an alteration known as carbonation and sericitización, however
alteration haloes are narrow (<10 cm) and localized.
Mineralization is controlled by “La Chucula” conjugate riedel fault system (Jimmy
Torres, personal communication 2017) , but in site only sinistral strike-slip faults
were observed with 330° NW strike and near vertical dip (Figure 10). Locally,
dilational jogs (Figure 11 c) and pinch & swell structures are produced due to the
shear tectonic regime in the zone. Moreover, dykes and silos are present within
(Figure 11c), under and over the veins (Figure 9) locally containing fragments of
mineralization. Those dykes display a pervasive sericitic alteration and
carbonation, and change in color from center to borders could be observed in some
of them (Figure 11 b).
Figure 10. Strike-slip fault from “La Chucula” conjugate fault system.
14
Figure 11. a) Silver and silver sulfosalts cutting sulphides within quartz veinlets. b) Igneous
intrusive dyke with color change from center to borders within banded vein c) Dilational jog
produced due to shear tectonic regime.
15
4. METODOLOGY
4.1 Field Work
Fieldwork was carried out on November 2017 and January 2018 in the Gran
Porvenir S.A mine, from which the corresponding permission was obtained to
access the tunnels. The underground mine has ten levels with north and south
mining fronts. Samples of mineralization, host rocks, and dykes found in the host
rocks and the mineralized veins were taken (Table 1) as well as structural
measurements.
SAMPLE
ROCK TYPE
LEVEL
ELEVATION(m)
P1
Quartz massive vein
One north
1621
VO-1
Mineralization
One south
1630
P-5
Mineralization
Five north
1506
E2
Host Rock
Eight north
-
E3A
Host Rock
Eight north
-
9N
Mineralization
Nine north
1452
9S
Mineralization
Nine south
1450
9S-Ag
Mineralization
Nine south
1448
9R2
Mineralization with Scheelite
Nine north
1450
PV2
Mineralization
Nine north
1450
PV3
Mineralization
Nine north
1450
DA1
Host rock with dyke
Nine north
1450
10N
Mineralization with Scheelite
Ten north
1420-1435
10ALT
Mineralization with dyke
Ten north
1420-1435
10ALT2
Host rock with dyke
Ten north
1420-1425
DH-15
Altered porphyritic rock
-
Depth : 305,9 m
DH-16
Altered porphyritic rock
-
Depth : 47,5 m
Table 1. Table of collected samples.
Samples from the first level, between first and second level, fifth level, nine level
and tenth level are listed above (Table 1). Additionally, five samples (PV-1, PV-2,
PV-3, PV-4 and PV-5) of mineralization were borrowed from Professor Julian A.
Lopez I. from Colombian Geological Service (SGC in Spanish), as complement to
The Metallogenic Map of Colombia. The collection of samples was based on the
properties of rock and mineralization content, with the purpose of covering most
16
of the characteristics of the mineralogy and the relationships between ore minerals,
host rocks and alteration phases.
4.2 Laboratory work
Samples were cut at the sample preparation laboratory of Geoscience department
at Universidad de los Andes into thirteen polished thin sections (Table 2). Those
samples were chosen taking into account the mineral association representativity
and the relationships between gangue and ore phases.
Other samples were cut with the purpose of observing macroscopic features of the
mineralization.
SAMPLE
P1
VO-1
P-5
E2
E3A
9N
9S
9S-Ag
9R2
PV2
DA-1
10ALT
DH-15
DH-16
POLISHED SECTION
P1
VO-1
5A
E2
E3A
9N
9S
9S-Ag
9R2
PV2
DA-1
10-ALT
DH-15
DH-16
ROCK TYPE
Quartz massive vein
Mineralization
Mineralization
Host rock
Host rock
Mineralization
Mineralization
Mineralization
Mineralization with Scheelite
Mineralization
Host rock with alteration
Mineralization with alteration
Altered porphyry
Altered porphyry
LEVEL
1
1
5N
8N
8N
9N
9S
9S
9N
9N
9N
10N
-
Table 2. Polished thin sections prepared.
4.3 Petrography
Optical microscopy studies of 13 polished thin sections (30 μm) in transmitted light
were done in order to determine textural relationships between translucent and
opaque minerals (Table 2). Transmitted light was also used to discriminate
different fluid inclusion families and their characteristics. Otherwise, studies in
reflected light were done to describe ore minerals phases, their relationships, and
mineral paragenesis.
17
4.4 Scanning electron microscopy (SEM)
Semi-quantitative chemical composition analyses (EDS) of different gangue and
ore minerals were realized in twelve carbon-coated polished thin sections
belonging to the mineralized quartz veins. These analyses were carried out using
the scanning electron microscope JEOL, model JSM 6490 LV at the microscopy
center in Universidad de los Andes with a 20kV acceleration voltage and 0.5 to
300.000 X magnification.
4.5 Fluid Inclusion Studies
Once the families of fluid inclusions were identified in the Petrography phase of
the work, quantitative analysis of fluid properties such as density, composition,
pressure and temperature of entrapment were carried out in the Laboratorio de
Microfluidespectral-Caracterización
Litológica
at
Universidad
Nacional
de
Colombia, using a Zeiss Axio Microscope, and a heating plate Linkam 600. This
process is done by cooling and heating pieces at 4°C per minute of crystals or chips
of selected double polished thin sections. Next step consist on determining
eutectic, final fusion salt, final fusion ice, and homogenization temperatures of the
fluid inclusions. Then, the results are processed in fluid inclusion analysis software
such as FLUIDS or CLATHRATES depending on the inclusions type (Baker, 2001).
4.6 Ramman
Ramman spectrum analysis was carried out in order to probe the presence of
volatile species in fluid inclusions, and elucidate the semi-quantitative composition
of them. This work was done in the Laboratorio de MicrofluidespectralCaracterización Litológica (Microscopía y Microtermometría) at Universidad
Nacional de Colombia with a SHAWN Ramman with a diameter of laser of 732 nm
and Laboratorio de Química Inorgánica at Universidad de los Andes with an
Olympus Ramman microscope with a diameter of laser of 732 nm. Then, obtained
Ramman spectrum of different minerals was compared with RUFF International
database using Crystal Sleuth software.
18
5. RESULTS
5.1 Petrography
5.1.1 Host Rocks
5.1.1.1. Quartz-chlorite-sericite schists (E2, E2A)
These samples comprise foliated granolepidoblastic metamorphic rock (S1), with
microfolds also called crenulation folds (Figure 12 e, f), which define a
superimposed foliation (S2). Is composed primary of quartz (33%), chlorite (27%),
white micas (15%), albite (7%), sericite (6%) and epidote (3%). Carbonate (10%)
veinlets are cutting main metamorphic foliation (S1) and the secondary
microfolding foliation (S2) (Figure 12 a, b).
Main mineral associations are: quartz and chlorite-white micas- epidote
(Figure
12. c, e). These associations are separated by bands of quartz with different sizes
(Figure 12d). Moreover, quartz shows deeply contact sutured (grain boundary
migration), mosaic, and wavy/shadowy extinction textures (Figure 12d). Also,
massive aggregates of quartz are present within the foliation (S1).
Alteration phases are present as sericite, calcite (Figure 12 c, d, e) and quartzcarbonate veinlets with euhedral to subhedral grains. However, metamorphic and
hydrothermal chlorite cannot be distinguished. Based on mineral association,
proposed protolith for this rock corresponds to a pelite.
Moreover, quartz textures and microfolding in the rock would correspond to a
high deformation tectonic event, as the development of crenulations are
accompanied by an intense mineral segregation ; with quartz concentrated at hinge
on the microfolds and micas in the flanks (Figure 12e ,Yardley, 1997). Finally,
metamorphic facies of this rock would correspond to greenschist facies from
medium to high temperatures.
19
Figure 12 a) XPL Calcite veinlets cutting metamorphic foliation. b) PPL calcite veinlets cutting
metamorphic flotation S1 and S2. c) XPL White micas intergrown with chlorite accompanied by
quartz and calcite d) XPL Foliated quartz bands of different sizes showing recrystallization textures
accompanied by chlorite, white micas and calcite e) XPL Mineral association of quartz, white micas,
chlorite, clinozoisite and alteration calcite. f) Metamorphic foliation S1 cut by crenulation foliation
S2.
20
5.1.1.2. Graphitic schists (E3A)
This sample shows metamorphic lepidoblastic foliation (Figure 13a). Microfolding
or crenulation foliation (S2) is observed superimposing metamorphic foliation (S1)
(Figure 13 c, f). Additionally, another foliation (S3) recognized as transposition
foliation is cutting the previous ones. This foliation could be recognized because of
orientation of some graphite-white micas bands affecting sulphides mineralization
(Figures 13 c, f). Some foldings within foliation are asymmetric with chevron
structure (Figure 13 e). Graphitic schists are composed of sutured contact quartz
(45%), graphite (30 %), white micas (17%) intergrowth with chlorite (3%), and
calcite (5%). Mineralization of pyrite, chalcopyrite and sphalerite is observed
within microfolds (S2) following the main foliation of the rock (S1) (Figure 13 d, f).
Nonetheless, foliation S3 is cutting off pyrite folds (Figures 13 c, d, and f).
Principal mineral associations are: quartz and graphite - white micas. Same as E2
sample, quartz show deeply sutured contacts, mosaic, bulging, and grain
boundary migration textures (Figure 13 a, c). Also, some wavy and shadowy
extinction quartz was observed.
Alteration phases are present as minor amounts of sericite and calcite. Folded and
intra-foliation pyrite are accompanied by overgrown chalcopyrite on the borders
(Figure 13 b, f). Furthermore, chalcopyrite, sphalerite and hematite are present
disseminated as anhedral crystals within the foliation (Figure 13 b). Finally, based
on mineral association, protolith of this rock would correspond to a metapelitic
rock enriched in organic material.
21
Figure 13. a) XPL Intra-foliation pyrite accompanied by white micas and quartz with
recrystallization textures. b) Reflected light microphotograph (RLM) of intrafoliation pyrite (Py)
accompanied by chalcopyrite (Cpy) and sphalerite (Sph) c) XPL Crenulation foliation (S2) cutted by
foliation S3 d) RML intrafoliation folded pyrite affected by foliation S3 e) PPL Chevron-like folds of
graphite + white micas and quartz. f)
RML intrafoliation folded pyrite accompanied by
chalcopyrite following S1 and S2 foliations cutted by S3 foliation.
22
5.1.2 Igneous Porphyry Dikes
5.1.2.1. DH-16:
This sample corresponds to a drill core within the Cajamarca
Complex near the veins. The section shows a porphyritic texture with euhedral to
subhedral phenocrysts of medium to fine grained plagioclase and hornblende
partially replaced by sericite and calcite ( Figure 14 a,b,c) , surrounded by a
microcrystalline matrix of albite and sericite. Additionally pyrite and sphalerite (<
1%) are present as disseminated grains with subhedral to anhedral morphologies
(Figure 14 d). This section consists of 58% matrix and alteration phases, and 42%
phenocrysts.
Those phenocrysts are plagioclase with partial loosing of polysynthetic twinning
(Figure 14 a) (55%), hornblende (35%), fine grained quartz (5%) and orthoclase
(5%) recognizable by its baveno characteristic twin. So, this porphyry rock is
classified as an Andesite according to the Streckeisen (1976) classification for
extrusive rocks. The hydrothermal alteration in this rock, as in DH-15, is strongly
pervasive of the carbonatization-sericite type, accompanied by minor amounts of
sulphides. Presence of pyrite in this porphyritic rock could be explained by the
sulphidization of mafic phenocrysts such as hornblende.
Figure 14. a) XPL Hornblende (Hbl) and plagioclase (Pl) ghosts. b) PPL image of a). c) XPL
Hornblende after intense sericitization and carbonatization. d) Disseminated anhedral pyrite (Py).
23
5.1.2.2. DH-15: This sample occurred as a drill core within the Cajamarca Complex
near the hydrothermal veins. Phenocrysts in this sample are completely replaced
by calcite and sericite, with minor amounts of white micas, and iron oxides (Figure
15). This is due an intense pervasive alteration that is known as carbonatization.
Original texture of the rock has disappeared and only “ghosts” of phenocrysts can
be distinguished. Subhedral quartz is present as a minor phase and calcite veinlets
are cutting the rock (Figure 15 c, d). Also, opaque minerals such as disseminated
euhedral to subhedral grains of pyrite are present (Figure 15 e, f).
Figure 15. a) XPL Subhedral quartz aggregates and pervasive carbonatization and sericitization of
the rock. b) PPL of a). c) XPL Calcite veinlet, sericite and iron oxides as alteration phases in the rock.
d) PPL of c) e) XPL Sericite and white micas accompanied by disseminated subhedral pyrite. f) PPL
image of e).
24
5.1.3 Quartz Veins
5.1.3. 1. Sample from the first level
P1: This sample is a translucent pyramidal quartz that was collected from a banded
quartz-scheelite-sulphides vein (Figure 4) with euhedral cubic pyrite and galena
filling open spaces (Figure 16 a). Almost twenty families of secondary fluid
inclusions with similar characteristics are observed in this section. They can be
distinguished as secondary because are following fracture planes on quartz and
are superimposing between them. However, all inclusions are biphasic of the
liquid-gas type and its morphologies are ovoid, tear-like, and polygonal. Those
inclusions has “double ring” in the bubble as consequence of highly amounts of
CO2 in the parental fluid. In addition, water bubbles are very small in comparison
with CO2 bubble and sometimes are moving as response of the Brownian
movement. Moreover, one family of primary fluid inclusions is recognized by its
size and rectangular shape, following a tridimensional array (Figure 16 b). This
family has the same characteristic of double ring equal as the previous described.
Additionally, the proportion between fluid and gas phase maintains constant
along this family. So a homogeneous entrapment could occur when these
inclusions were formed.
Figure 16. a) Photograph of sample P1. Pyramidal quartz could be seen in the bottom right, also
galena is filling spaces within quartz crystals. b) PPL Primary fluid inclusion family with double
ring.
25
VO1: This sample was collected in another family of hydrothermal quartz veins
known as “El Oasis” which share similar characteristics with “El Porvenir” veins.
Both families of veins are separated a distance of ca. 800 m. This section consists of
quartz with different grain sizes and dynamic recrystallization textures such as
grain boundary migration and bulging. Quartz is accompanied by bands of
graphite and sericite-white micas, sometimes showing stylolitic seams (Figure 17
a).
Furthermore, mineralization of sulfides is confined within graphite bands with
sericitic alteration at both sides (Figure 17 b). Alteration in this sample is of the
sericitic type with minor amounts of rutile within graphite (Figure 17 a, b).
Almost two generations of pyrite are distinguished. The first one is coarse-grained,
subhedral and highly fractured, with interstitial galena-electrum and sphalerite
(Figure 17 d, e). The second one is euhedral, without fractures and is present
within or near graphite bands.
Sphalerite is subhedral and shows an orange red color in transmitted light. It has
emulsion texture of chalcopyrite and contains oriented galena inclusions (Figure 17
c, d). However, sometimes those are intergrown. It is also affecting pyrite borders
(amoeba texture) and is present as anhedral crystals within graphite bands.
Galena is related to gold (electrum), because both are intergrown within pyrite
fractures (Figure 17 e). Also fine grained subhedral crystals are contained within
graphite bands and in contacts with them.
Pyrrothite is present as minor inclusions within coarse grained pyrite.
Moreover, secondary fluid inclusions were found within quartz near graphite
bands, those inclusions are biphasic liquid-gas with double ring following fracture
planes (Figure 17 f). Proportion between gas phase and fluid phase are not
constant within the different families, so a heterogeneous entrapment could have
occurred when the inclusions were formed.
26
Figura 17.
a) XPL Stylolitic seam of graphite with sericite. Also, quartz show dynamic
recrystallization textures. b) XPL Brecciated and fractured pyrite accompanied by sericitic
alteration. c) PPL Orange sphalerite (Sph) with oriented inclusions of galena (Gn) and chalcopyrite
(Cpy) exsolutions. d) Reflected light image (RLI) of galena filling fractures of pyrite and as oriented
inclusions in sphalerite. e) RLI of pale yellow electrum (El) intergrowth with galena filling pyrite
fractures. f) PPL Biphasic fluid inclusions following planes.
27
5.1.3.2. Fifth level samples - 5A: This sample was collected in a rich gold zone
according to the mining geologist Jimmy Torres (personal communication, 2017).
This section shows quartz with dynamic recrystallization textures such as grain
boundary migration, bulging, wavy extinction and deformation lamellae (Figure
18 a).Also, another type of quartz is found with comb texture growing over pyrite
(Figure 18 b). Moreover, continuous graphite bands with sericite “stylolitic seams”
are accompanying mineralization (Figure 18 a, b, e), and euhedral crystals of calcite
are overgrowing these graphite bands (Figure 18 b). However, quartz grain size
decreases near the band of graphite, also showing a major recrystallization
between graphite and calcite. Rutile is present as alteration phase within graphite
bands (Figure 18 c, Figure 19 d). Ore minerals are disseminated as individual or
intergrown crystals.
Almost two generations of pyrite can be observed; the first one is coarse grained,
subhedral, massive habit and with fractures filled with galena (Figure 18 f). The
second one (Figure 18 d and Figure 19 a, b, d) has medium grain size (ca. 200 μm)
and fractures filled with electrum (ca. 100 – 20 μm). Electrum shows high
reflectance and a pale yellow-white color, and this could be caused by the
relationship gold: silver. However, another type of electrum with strong yellow
color but minor grain size is included within pyrite (Figure 19 b). In the borders,
this pyrite has overgrowing electrum, sphalerite and galena (Figure 19 d).
Sphalerite is present as anhedral massive crystals of brown to orange color and
contains pyrite cubic inclusions; it also has emulsion textures of chalcopyrite and
overgrowths of electrum in the borders (Figure 18 e, f). Another sphalerite with
strong red color and without emulsion textures was identified (Figure 19 c). Last
one sphalerite has bigger grain size than the first one. Galena shows typical
polishing “pits”, is subhedral and is found interstitially, overgrowing sphalerite
and pyrite and accompanying electrum (Figure 18 f and Figure 19 d). Additionally,
galena and sphalerite are found as anhedral crystals within graphite bands.
Electrum is found within pyrite and in contact with graphite-sericite stylolitic
seams. As could be seen in figure 19 a, b, d, and figure 18 c, d. Two types of
electrum were identified; type 1 (within pyrite, fine grained) and 2 (free or in
crystal-graphite borders), the second one sometimes present as large size crystals
(100-200 μm).
28
Figure 18. a) XPL Quartz showing dynamic recrystallization textures accompanied by subhedral
cubic pyrite within graphite bands. b) XPL Quartz showing comb texture over pyrite. Also calcite is
overgrowing quartz and pyrite. c) RLI Subhedral cubic pyrite filled with electrum (El) , also free
electrum in contact with graphite (Gr) –sericite (Ser)- rutile (Rt) bands. d) RLI Electrum (El) in
borders and within fractures of pyrite, accompanied by sphalerite and galena. e) PPL Orange
sphalerite with emulsion texture of chalcopyrite. f) RLI Interstitial galena filling fractures of
massive pyrite overgrowth by emulsion-like sphalerite.
29
On the other hand, biphasic secondary fluid inclusions were found near host rock
following planes perpendicular to orientation of quartz veins. They show “double
ring” structure and proportions between gas and liquid seem to be constant
(Figure 19 e). Other family of fluid secondary biphasic fluid inclusions was found
near one free electrum grain of 200 μm (Figure 19 f). This family doesn’t present
constant relation of volume gas, which suggests heterogeneous entrapment.
Inclusions of both families has tear, ovoid and elongated morphologies and sizes
range between 5 and 8 μm.
Figure 19. a) RLI Electrum (El) on pyrite borders. b) RLI Electrum dot within pyrite, also radial
fractures are observed around this electrum grain. c) PPL Iron sphalerite (Sph) including pyrite and
overgrowing quartz. d) Rutile (Rt) + Graphite (Gr) + Sericite (Ser) band accompanied by electrum,
galena and cubic pyrite. e) PPL Biphasic fluid inclusions following planes. f) PPL Biphasic fluid
inclusions following planes near free anhedral electrum crystal.
30
5.1.3.3. Ninth level samples
9S
This section shows quartz with recrystallization textures such as grain boundary
migration, bulging and deformation lamellae. Also, quartz presents wavy
extinction (Figure 20 a, b). Dolomite and calcite are present as an introduction
mineral and brecciate pyrite and quartz (Figure 20 d). Alteration mineral
assemblages are characterized by white micas, sericite, tourmaline and rutile
within altered wallrock (graphite) bands (Figure 20 a, c and d). Ore minerals are
conformed by pyrite, chalcopyrite, sphalerite, galena, pirargyrite and electrum.
Euhedral to subhedral pyrite is found with breccia textures and fractures (Figure
20 c, d), those fractures are filled with chalcopyrite or galena (Figure 20 c, d, e).
This pyrite is also accompanied by alteration mineral phases and contains
sphalerite and galena as unoriented inclusions.
Sphalerite is anhedral, has a red color and is found within pyrite crystals. Another
fine-grained anhedral sphalerite is present surrounding chalcopyrite. Moreover,
subhedral to anhedral galena is intergrowth with electrum or chalcopyrite.
Galena could be seen as unoriented inclusions within pyrite or filling fractures.
Chalcopyrite is coarse grained (1-5 mm), is euhedral to subhedral, and is
accompanied by calcite overgrowing quartz crystals (Figure 20 b).
Pirargyrite displays a grey color and red internal reflections, is found within calcite
near chalcopyrite or within chalcopyrite itself (Figure 20 f). Two different
electrums were distinguished. First one is found within pyrite as yellow anhedral
inclusions (15 um) (Figure 20 d), and the other one displays pale yellow color and
is intergrowth with galena (Figure 20 e).
31
Figure 20. a) XPL Dynamic recrystallization textures on quartz and sericitic alteration
accompanying mineralization of pyrite (Py) and Sphalerite (Sph). b) XPL Calcite (Cal) after
chalcopyrite (Cpy) filling spaces between quartz crystals. c) Reflected light microphotography
(RLM) of brecciated pyrite with chalcopyrite and galena within fractures. d) RLM Strong yellow
electrum (El) and galena as inclusions within Pyrite. Also, sericite (Ser) + graphite (Gr) are present
as alteration phases. e) RLM Pyrrothite and electrum intergrowth with galena as inclusions within
pyrite. f) RLM Brecciated pyrite due to chalcopyrite. Also, pirargyrite (Pyrg) is present within and
outside chalcopyrite.
32
9S-Ag
In this sample two different quartz were identified. The first one, is coarse grained
and fractured. The second one is fine grained, displays intense recrystallization
textures (GMB, bulging) and deformation lamellae (Figure 21 a). Alteration
mineral phases are white micas- sericite within graphite bands. Moreover, ore
minerals in this section are pyrite, sphalerite, galena, chalcopyrite, and silversulfosalts. Pyrite is found as euhedral cubic (10-370 um) aggregates (Figure 21 c)
with unoriented galena inclusions sometimes embedded within sphalerite (Figure
21 f) and as massive (8 mm) brecciated crystals with fractures filled with galena,
chalcopyrite and silver-sulfosalts (Figure 21 b).
Sphalerite is clean and displays a brown pale color (Figure 21 d). It is anhedral and
occurs in borders of massive pyrite also having same size and containing galena
and silver-sulfosalts inclusions.
Galena is subhedral, occurs in borders of euhedral pyrites, sphalerite, and is
intergrowth with silver-sulfosalts and chalcopyrite (Figure 21 b, e).
Silver-sulfosalts are solid solutions between argento-tetrahedrite and argentotennantite (Figure 21 e), and display grey-greenish color with deep red internal
reflections. Those sulfosalts are found filling fractures of brecciated pyrite (Figure
21 b), within pyrite, sphalerite or in contact with them. Also they are accompanied
by chalcopyrite-galena and sizes range between few microns up to 1 mm.
33
Figure 21. a) XPL Quartz showing different grain sizes and recrystallization textures around cubic
aggregates of pyrite. b) Reflected light microphotography (RLM) of massive pyrite fractured and
filled with intergrowths of galena (Gal) and polybasite (Pol). c) Aggregates of euhedral cubic pyrite
overgrown by sphalerite and galena. d) RLM of Pale brown sphalerite accompanied by pyrite and
quartz. e) RLM Polybasite crystal with exsolutions of argento-tennantite (Ag-tn) after cubic pyrite.
f) RLM Aggregates of cubic pyrite embedded within sphalerite (Sph) and galena (Gn).
34
9N
Same as the previous samples, quartz displays dynamic recrystallization textures
such as GMB, bulging, etc. (Figure 22 a). It is accompanied by introduction calcite
and dolomite (Figure 22 b). Alteration mineral assemblages of white-micas,
magnesium chlorite, tourmaline and rutile are present within graphite bands
(Figure a, c, d). Tourmaline could be distinguished because his pyramidal habit
with six faces in c axis. Also, display greenish to brown colors with strong zonation
from center to borders. On the other hand, rutile displays high relief, acicular habit
and yellow to green color (Figure 22 d). Ore minerals comprise pyrite, chalcopyrite
and sphalerite, galena and electrum.
Brecciated and fractured subhedral pyrite is found with skeletal textures due to
dolomite and calcite introduction (Figure 22 b, e) and with intergrowth textures
with chalcopyrite. It also has fractures filled with chalcopyrite (Figure 22 e, f).
Chalcopyrite is subhedral and it occurs in borders of pyrite or filling fractures. It
also is disseminated within dolomite.
Galena occurs as intergrowths with chalcopyrite and as unoriented inclusions
within pyrite. Yellow anhedral electrum within pyrite) and pale yellow dots of
electrum within chalcopyrite (Figure 22 f) were distinguished. Those electrum
grains range in size between 25 um and 63 um.
35
Figure 22. a) XPL Quartz showing dynamic recrystallization textures accompanied by bands of
graphite (Gr) + Sericite (Ser) and White micas (Mca). b) XPL Skeletal pyrite due to introduction of
dolomite (Do). c) PPL Alteration mineral assemblage: White micas (Mca) + sericite (Ser) + graphite
(Gr) + tourmaline (Tur). d) PPL Alteration rutile (Rt) accompanied by graphite (Gr) e) Reflected
light microphotography (RLM) of pyrite with galena (Gn) inclusions and chalcopyrite (Cpy) filling
fractures. f) RLM Chalcopyrite overgrowing pyrite with pale yellow electrum inclusion (El).
36
9R
This sample was taken from the vein that is showed in the figure 9 and shows
brecciated and fractured scheelite (Figure 9a, b and Figure 24 a, e) with wavy
extinction. It is accompanied by overgrowing dolomite, calcite and quartz with
dynamic recrystallization textures (GMB, bulging, etc.) Also veinlets of clean
quartz are cutting the scheelite and contain fragments of scheelite and dolomite
(Figure 24 b). Some calcite twins seem to be deformed. The mineral assemblages of
alteration are characterized by: graphite bands with sericite, white micas and
magnesium chlorites (Figure 24 c). In this sample ore minerals are pyrite,
sphalerite, galena and chalcopyrite.
Two types of pyrite were recognized. The first one (Py 1) is fractured, subhedral
with oriented inclusions of galena and grows around dolomite, calcite and
scheelite (Figure 24 d). The second one is found filling fractures of scheelite
accompanied by galena, dissease sphalerite and calcite (Figure 23 a, Figure 24 e).
Sphalerite has strong red to orange color and display exsolution texture of
chalcopyrite (Figure 23 b, Figure 24 f). Also, it has an overgrowing rim of calcite
(Figure 24 c) while other clean sphalerite is filling spaces between carbonates
(Figure d).
Galena is subhedral to euhedral and is overgrowing pyrite, sphalerite, dolomite,
scheelite and quartz (Figure 24 d) and it is also filling spaces and fractures.
Figure 23. a) XPL Scheelite (Sch) fracture filled with pyrite and calcite (Cal). b) PPL Red to orange
dissease sphalerite (Sph).
37
Figure 24. a) XPL Brecciated scheelite accompanied by calcite (Cal), quartz (Qtz) and late galena
(Gn) b) XPL Quartz veinlet cutting scheelite (Sch) and dolomite (Do) c) XPL Sphalerite (Sph) with
calcite rim, accompanied by
quartz with recrystallization textures and sericitic alteration.
d) Reflected light microphotograph (RLM) of pyrite (Py), sphalerite (Sph) and late galena (Gn)
overgrowing calcite (Cal). e) RLM Pyrite, sphalerite and galena filling fractures within scheelite
(Sch). f) RLM Sphalerite with exsolutions of chalcopyrite (Cpy) and galena inclusions (Gn).
38
5.1.3.4. Tenth level samples
10N
This sample display sutured contact quartz with recrystallization textures (Grain
boundary migration, bulging, etc.) and wavy extinction. On borders of quartz
crystals recrystallization is intense and the size of crystals decreases by a factor of
50 (1.25 mm to 25 um). Quartz is accompanied by brecciated scheelite and ferberite
(Figure 25 a). Calcite is an introduction mineral that remobilizes sulfides (Figure 25
b), wolphramates, and is also filling spaces between quartz crystals (Figure 25 a, b).
Ferberite was distinguished because of his brown color in reflected light, low relief
compared to scheelite, cleavage planes and red internal reflections (Figure d, e).
Ore minerals are pyrite, pyrrothite, arsenopyrite, sphalerite, galena and
chalcopyrite.
Pyrite is present as subhedral cubic aggregates with embayments of sphalerite and
galena, also pyrrothite and arsenopyrite (Figure 25 c) are present in borders or
included. Sizes of pyrite range between 125 um and 1000 um.
Sphalerite is strong red to orange and shows exsolution textures with chalcopyrite
growing in cleavage planes (Figure 25 d, e, f). Its range size is between 125 um and
1.25 mm and has a subhedral morphology. Moreover, sphalerite is reddish at
center and in cleavage planes that contains dots of chalcopyrite, it color turns to
orange (Figure 23 b and Figure 25 f). This happen because chalcopyrite in those
cleavage planes is incorporating iron in his mineral structure.
Galena is crystallized within cleavage planes of other minerals, and as oriented
inclusions in pyrite or sphalerite. Chalcopyrite is present as small anhedral crystals
(5 um) within calcite or filling fractures on pyrite (Figure 25 d).
39
Figure 25. a) XPL Brecciated scheelite (Sch) accompanied by calcite (Cal) b) XPL Pyrite (Py)
subhedral crystals embedded within introduction calcite. Also quartz (Qtz) shows recrystallization
textures. c) Reflected light microphotograph (RLM) of arsenopyrite (Apy) filling empty spaces
within pyrite. d) RLM Reddish brown ferberite (Fb) overgrown by dissease sphalerite (Sph) and
pyrite (Py). e) RLM Ferberite (Fb) overgrown by galena (Gn), accompanied by anhedral pyrite and
dissease sphalerite. f) PPL Red sphalerite accompanied by pyrite. Also another brown sphalerite
with minor grain size is present at left.
40
5.1.4 Igneous dykes
5.1.4.1. 10 ALT
This sample displays the contact between intrusive igneous dyke shown in figure
11 b, and the mineralized vein. In the contact zone, quartz is broken and the
intrusive body is filling fractures on the quartz (Figure 26 a, b). In addition, quartz
within the vein appear with sutured contact and shows deformation lamellae and
recrystallization textures such as the mentioned before. Alteration mineral
assemblages in the quartz vein are composed of graphite bands with tourmaline,
rutile, sericite and white micas intergrown with calcite (Figure 26 c, d, and e). On
the other hand, igneous dyke is completely altered and replaced by carbonates and
sericite (Figure 26 a, b) with some disseminated pyrite and hematite (Figure 26 f).
Ore minerals in this section are scarce and are disseminated cubic pyrite with
galena inclusions (50 um) within graphite bands.
5.1.4.2 DA-1:
This sample shows the contact between one intrusive dyke and graphitic schist
(host rock of mineralization). In the contact zone there is possible to see high
deformation on graphite and white micas of the schist (Figure a, b, c) in addition to
recrystallization of quartz. Also some quartz aggregates with microcrystalline
border, sutured contact and wavy extinction are present within the dyke (Figure 27
d). In those quartz aggregates, disseminated pyrite and chalcopyrite occurs (Figure
27 e) with carbonate rims. Also skeletal pyrite with inclusions of pyrrothite,
chalcopyrite and galena inclusions is dragged into the graphitic schist (Figure 27c,
f).
41
Figure 26. a) XPL Euhedral quartz crystals fractured due to completely altered intrusive igneous
dike b) PPL Graphite (Gr) bands with sericite (Ser) and white micas (Mca). c) XPL Sericitic
alteration accompanied by disseminated pyrite. d) PPL image of c). e) XPL Graphite bands with
sericitic alteration (Mac+ Ser) overgrowth by calcite (Cal) f) Reflected light microphotograph (RLM)
of disseminated euhedral pyrite.
42
Figure 27. a) XPL Recrystallized quartz within the contact of dyke and host rock. b) XPL Contact
between intrusive altered dyke and host rock. c) XPL Graphitic schist with graphite bands (Gr) +
white micas (Mca) and sericite (Ser). Also subhedral cubic pyrite is present inside the graphite
bands. d) XPL Pervasive carbonatization and sericitization of the igneous dyke with quartz
aggregates. e) Reflected light microphograph (RLM) of quartz aggregates with pyrite (Py) and
Chalcopyrite (Cpy) inside the altered dyke. f) RLM Dragged skeletal pyrite with pyrrothite (Po)
and chalcopyrite (Cpy) inclusions into graphitic schist.
43
5.2 Mineral paragenesis
Based on mineral textures and cross-cutting relationships between minerals and
mineralized veins, four mineralization events were recognized. The paragenesis of
the mineralized veins is divided in massive quartz vein and crack & seal veins. So,
mineralization event number 1 corresponds to crystallization of cubic pyrite,
scheelite and carbonates accompanied by milky quartz (Figure 7b). The second,
third and fourth mineralization events occur within banded or laminated quartz
veins (Figure 8, 9, 11) and are described below.
The second mineralization event corresponds to quartz, scheelite, ferberite,
dolomite (Do 1), calcite, pyrite (Py 2) and dissease sphalerite (Sph 1). The third
mineralization event starts with introduction of quartz (3), brecciation and
fracturing of scheelite, ferberite, dolomite and calcite. Also, pyrite (Py 2) is
subhedral, massive, and sometimes displays cubic habits (Figure 20 c) or filling
scheelite fractures (Figure 24 e). As seen in figure 18 f, this pyrite is accompanied
by another cubic subhedral pyrite (Py 3) related to two different electrums. The
first one is within pyrite 2 or 3 (Electrum 1, Figure 19 b and Figure 20 d) and the
other one is present in fractures or borders of pyrite 2 or 3 (Electrum 2, Figure 18 c,
d and Figure 19 a). Sphalerite (Sph 2) overgrows pyrite (Py 2, 3) and often present
emulsion textures of chalcopyrite (Cpy 1, Figure 17 c and Figure 18 f). Galena (Gn
1) is overgrowing previous minerals in the sequence and is filling spaces and
fractures within them (Figure 24 d and Figure 25 d). Pyrrothite is included or
overgrowing pyrite 2, and arsenopyrite is filling spaces of pyrite 2.
The introduction of calcite and dolomite (Do 2) produce brecciation, fracturing,
and remobilization of sulphides, producing skeletal textures in some pyrites (Py 2)
(Figure 22 b). Those carbonates are accompanied by subhedral chalcopyrite (Cpy 2)
often with electrum grains inside it (El 3). Pirargyrite is present within chalcopyrite
2 or carbonates (Figure 20 f). The fourth mineralization event is characterized by
the presence of silver sulfosalts such as Polybasite (Pol), Argento-tennantite (AgTn) intergrown with chalcopyrite (Cpy 3) and galena (Gn 2) with late deposition of
native silver (Figure 11 a). Aggregates of cubic pyrite (Py 5), brown pale sphalerite
without exsolution or dissease texture (Sph 3, Figure 21 d) and galena overgrowing
or filling fractures are accompanying these silver minerals.
44
The quartz of second, third, and fourth event present dynamic recrystallization
textures such as grain boundary migration and bulging. Finally, the fifth event
corresponds to supergenic local enrichment of copper minerals producing
malachite and oxidation of some areas inside the mine due to interaction with
meteoric waters.
Figure 28. Paragenetic sequence of mineralized veins. Each event is separated based on mineralogy
and brecciation/fracturing events (Toothed lines represent brecciation or fracturing events).
45
5.3 Mineral Chemistry
Energy dispersive X-ray spectroscopy (EDS) analysis was performed for the
different minerals that occur in the deposit. Thus, structural chemical formulas of
minerals were computed and different families of ore minerals were identified.
ELECTRUM
Electrum show different gold-silver contents (Figure 29) based on EDS analysis.
Three types of gold were recognized based the composition of individual grains.
The first Type 1 electrum is included as dots and subhedral blebs within pyrite
showing fineness ranging between 730 and 750. Also in SEM images display radial
fractures within pyrite (Figure 30 a).
The second of them includes free electrum, Type 1 (within pyrite - el Oasis vein), in
contact with pyrite or graphite (Figure 18 c, d, Figure 19 a, d, and Figure 30 b)
associated with galena and fineness ranges between 530 and 570. The third one, are
Type 2 electrum grains related to chalcopyrite within fractures (Figure 22 f) and
fineness of this grains range between 417 and 466. Also one electrum grain
associated with galena shows the same gold-silver proportions.
Figure 29. Chemical composition of electrum particles analyzed by EDS in atoms per formula unit
(apfu).
46
Figure 30. a) SEM image of electrum grain displaying radial fractures within pyrite. b) Electrum
accompanied by graphite and rutile (light grey).
SPHALERITE
EDS analysis of sphalerite
show high contents of iron
and zero cadmium content in
host rock sphalerite. Three
populations of sphalerite are
confirmed
analysis
optical
based
(Figure
on
this
31)
and
observations.
High
Cadmium (up to 3.1 wt %)
and low contents of iron are
present
sphalerite
in
pale
brown
without
exsolutions of chalcopyrite.
Figure 31. Chemical composition of sphalerite particles
analyzed by EDS in atoms per formula unit (apfu)calculated based on 1 sulfur atom.
Lower contents of cadmium and high contents of iron are present in dark red
sphalerite or with exsolution (emulsion-like) textures. Finally, as the host rock,
dissease sphalerite does not have cadmium.
47
SULFIDES
Structural chemical formula of pyrite, pyrrhotite, arsenopyrite, galena and
chalcopyrite was calculated showing little or none variation in these minerals
along the deposit.
Rock
Type
Host
Rock
(E2)
Quartz
veins
Mineral
Cpy
Sph
Py
Apy
Py
Po
Cpy
Sph -av
Gn
Iron
0.982
0.13
1.02
1
0.97
0.43
0.99
0.084
0
Zinc/Cadmium Copper
0
0.77 / 0
0
0
0
0
0
0.833 / 0.0183
0
0.94
0
0
0
0
0
0.94
0
0
Lead
0
0
0
0
0
0
0
0
1.085
Arsenic
0
0
0
0.981
0
0
0
0
0
Sulfur
2
1
2
1
2
1
2
1
1
Table 3. Structural chemical formula of sulphides. (Abbreviations: Cpy = Chalcopyrite, Sph-av =
Average-Sphalerite, Py = Pyrite, Apy =Arsenopyrite, Po = Pyrrhotite, Gn = Galena)
Figure 32 a. is a SEM image of arsenopyrite filling cavities within pyrite. Also
intrafoliation pyrite SEM image (Figure 32 b) confirmed that pyrite and
chalcopyrite were formed within foliation of graphite bands.
Figure 32. a) Arsenopyrite filling cavities within pyrite. b) Intrafoliation pyrite in the graphitic
schist, also chalcopyrite is present.
48
SILVER SULFOSALTS
Silver sulfosalts in quartz veins were confirmed after petrographic characteristics
of these minerals using EDS compositions. Complex structural chemical formulas
with silver and other metals denote the presence of pirargyrite and other silver
sulfosalts such as argento-tetrahedrite, argento-tennanite and polybasite (Figure
33) (Table 4).
Mineral Sulfur
Pyrg
3
Pol
11
Pol
11
Ag-Tn
13
Pol
11
Silver Iron
2.71
0
13.72 0.564
13.96
0.30
6.45
1.92
12.55
0.55
Zinc
0
0
0
0
0.61
Copper
0
1.58
1.15
4.1
1.11
Lead
0
0
0
0
0.14
Arsenic
0
0
0
0.94
0.48
Antimony
1.05
1.97
1.8
2.87
1.16
Table 4. Structural chemical formula of sulfosalts. (Abbreviations: Pyrg = Pirargyrite, Pol =
Polybasite, Ag-Tn = Argento-tennantite).
SEM images of this sulfosalts show mineral intergrowth between them and a lot of
heterogeneity between crystals (Figure 33 a). Additionally, those silver related
minerals are filling pyrite fractures (Figure 33 b).
Figure 33. a) Polybasite crystal with argento-tennantite exsolutions. b) Fractures of pyrite filled
with silver sulfosalts.
49
WOLPHRAMATES
Two different wolphramates were recognized based on their structural formulas;
scheelite and ferberite.
Mineral
Scheelite
Ferberite
Ferberite
Scheelite
Scheelite
Calcium
0.85
0
0
0.971
0.941
Iron
0
0.93
1.019
0.061
0
Wolphram
1.051
1.024
0.994
0.991
1.021
Oxygen
4
4
4
4
4
Table 5. Structural chemical formula of wolphramates.
Figure 34. Ferberite (center) filled with galena in cleavage planes, accompanied by pyrite,
sphalerite, chalcopyrite and scheelite.
ALTERATION MINERALS
Structural formula of rutile, tourmaline, and hematite was calculated as follow:
Rutile: Ti0.92O2
Hematite: Fe1.98O3
Tourmaline: Na0.507 (Mg 1.49 Al 0.51) (Al 6.034 Fe 1.372) (BO3)3 (Si5.971 O18) (OH, F)4
50
5.4 Microthermometry
Measurements of only primary fluid inclusion families were realized in three
translucent quartz crystals belonging to different sites of the mine.
Sample
9S-Drusa
9N-Drusa
P1
Mineral
Quartz
Quartz
Quartz
Site in the mine
Nine level-south
Nine level-north
First level
Mineral association
Sulphides + Qtz
Sch + Dol + Qtz
Qtz+Py+Gn
Table 6. Samples selected for microthermometric measurements.
Sample: 9S-Drusa
This sample was collected from a sulphide vein within the 9 level that are showed
in the figure 11 c, the selected fluid inclusion family is primary, all inclusions
present double ring (Liquid H2O + Liquid CO2 + Vapor CO2) and petrographic,
microthermometric and fluid characteristics are listed below.
Figure 35. Localization and characterization map of FIA 1. This family is hosted by translucent
quartz which belongs to crack-seal vein accompanied by sulphides.
51
Inclusion
1
2
3
4
5
6
7
8
9
Morphology Liquid area
(um2)
Tear
327.96
Ovoid
243.03
Ovoid
86.18
Ovoid
128.86
Ovoid
23.97
Elongated
115.57
Irregular
16.36
Tear
179.75
Ovoid
75.16
Vapor area
(um2)
32.38
18.72
14.4
7.38
1.08
3.66
0.71
7.18
3.47
Phases
2L + V
2L + V
2L + V
2L + V
2L + V
2L + V
2L + V
2L + V
2L + V
Relation
V/V+L
0.090
0.072
0.143
0.054
0.043
0.031
0.041
0.038
0.044
Table 7. Petrographic characteristics of fluid inclusion family 1.
Inclusion
1
2
3
4
5
6
7
8
9
T°Eu
T°ffh
T°fcl
-60.8
-60.4
-61.0
-60.7
-61.2
-60.7
-60.4
-60.8
-61.0
-6.1
-5.8
-6.0
-6.6
-7.0
-6.4
-6.3
-6.4
-6.3
8.4
8.6
8.1
8.6
8.4
8.6
8.7
8.4
8.4
T°h CO2
T°h
Homogenization (L or V)
26.1
26.1
26.2
26.6
26.7
26.3
26.5
26.2
26.3
315
320
313
318
315
313
317
314
308
Liquid CO2
Liquid CO2
Liquid CO2
Liquid H2O
Liquid CO2
Liquid CO2
Liquid H2O
Liquid CO2
Liquid CO2
Table 8. Microthermometric measurements of fluid inclusion family 1.
Inclusion
Density (g/cc)
Salinity NaCl %wt
1
2
3
4
5
6
7
8
9
0.847
0.876
0.829
0.888
0.895
0.913
0.906
0.909
0.913
2.74
2.36
3.31
2.36
2.74
2.36
2.17
2.74
2.74
Pressure vapor
phase (Mpa)
6.60
6.60
6.61
6.67
6.69
6.63
6.66
6.61
6.63
Table 9. Fluid characteristics for each inclusion within first family.
52
Sample: 9N-Drusa
This sample was collected from the vein that is showed in the figure 9. Measured
fluid inclusions were formed at same time as host crystal because are elongated in
the growth direction of quartz. Moreover, measured crystal was found filling
spaces between scheelite in the central part of the vein. Thus, that crystal must
have formed after the scheelite and dolomite. Same as the previous sample all
inclusions present double ring (Liquid H2O + Liquid CO2 + Vapor CO2) and
petrographic, microthermometric and fluid characteristics are listed below.
Figure 36. Localization and characterization map of FIA 2. This family is hosted by translucent
pyramidal quartz formed within a druse between scheelite and dolomite.
53
Inclusion
1
2
3
4
5
6
Morphology Liquid area
(um2)
Irregular
173.633
Elongated
350.642
Elongated
157.505
Elongated
200.521
Elongated
130.687
Elongated
351.161
Vapor area
(um2)
15.815
32.076
31.634
50.931
8.23
45.509
Phases
Relation V/L
2L + V
2L + V
2L + V
2L + V
2L + V
2L + V
0.083
0.085
0.123
0.152
0.059
0.096
Table 10. Petrographic characteristics of fluid inclusion family 1.
Inclusion
1
2
3
4
5
6
T°Eu
T°ffh
T°fcl
-59.9
-59.8
-59.9
-60.8
-59.9
-59.8
-4.4
-2.8
-2.5
-2.7
-2.5
-2.9
6.4
6.6
5.6
5.7
6.6
6.6
T°h CO2
T°h
Homogenization (L or V)
24.1
24.3
24.2
24.1
24.2
24.1
253.8
253.6
253.8
253.6
253.7
253.8
Liquid H2O
Liquid H2O
Liquid H2O
Liquid H2O
Liquid H2O
Liquid H2O
Table 11. Microthermometric measurements of fluid inclusion family 2.
Inclusion
1
2
3
4
5
6
Density
Salinity %wt
0.79
0.93
0.69
0.689
0.79
0.92
6.43
6.07
7.86
7.68
6.07
6.07
Pressure
(Mpa)
6.30
6.33
6.31
6.30
6.32
6.30
Table 12. Fluid characteristics for each inclusion within second family.
54
Sample: P1
This sample was collected in the first level ore-shoot within the banded texture
vein (Figure 6). This vein comprise sulfides (pyrite, galena, sphalerite) and it was a
rich gold zone in the past (Torres, Personal communication 2017). Same as the
previous samples, all inclusions within the following three families present
“double ring” (Liquid H2O + Liquid CO2 + Vapor CO2) and petrographic,
microthermometric and fluid characteristics are listed below.
Figure 37. Localization and characterization map of FIA 3. This family is hosted by translucent
pyramidal quartz formed within a crack-seal vein accompanied by sulphides.
55
Inclusion
1
2
3
4
5
Morphology Liquid area
(um2)
Ovoid
101.788
Ovoid
299.091
Horn
448.319
Horn
201.000
Horn
244.211
Vapor area
(um2)
8.942
31.031
26.046
11.000
13.470
Phases
2L + V
2L + V
2L + V
2L + V
2L + V
Relation V/L
0.081
0.094
0.055
0.052
0.052
Table 13. Petrographic characteristics of fluid inclusion family 3.
Inclusion
1
2
3
4
5
T°Eu
T°ffh
T°fcl
-61.3
-61.3
-61.2
-61.3
-61.7
-5.5
-5.6
-5.5
-5.7
-7.7
8.2
8.8
6.8
8.3
8.7
T°h CO2
T°h
Homogenization (L or V)
27.6
27.5
29.6
29.5
26.6
281.4
281.9
279.9
281.5
281.8
Liquid CO2
Liquid CO2
Liquid H2O
Liquid CO2
Liquid CO2
Table 14. Microthermometric measurements of fluid inclusion family 3.
Inclusion
1
2
3
4
5
Density
Salinity %wt
0.84
0.90
0.91
0.79
0.87
3.11
2.02
5.70
2.93
2.17
Pressure
(Mpa)
6.83
6.81
7.15
7.13
6.67
Table 15. This is the fluid characteristics of each inclusion within family 3.
Inclusion
1
2
3
4
5
6
Morphology Liquid area
(um2)
Irregular
69.71
Ovoid
166.4
Triangular
357.96
Ovoid
194.06
Irregular
554.78
Ovoid
39.95
Vapor area
(um2)
4
15.5
17.52
13.32
30.92
2.0
Phases
Relation V/L
2L + V
2L + V
2L + V
2L + V
2L + V
2L + V
0.054
0.085
0.046
0.064
0.053
0.048
Table 16. Petrographic characteristics of fluid inclusion family 4.
56
Figure 38. Localization and characterization map of FIA 4. This family is hosted by translucent
pyramidal quartz formed within a crack-seal vein accompanied by sulphides.
Inclusion
1
2
3
4
5
6
T°Eu
-61.1
-61.2
-61.2
-61.0
-61.2
-61.3
T°ffh
-5.5
-6.6
-5.4
-5.3
-6.2
-6.3
T°fcl
7.5
8.4
8.5
8.3
8.2
8.7
T°h CO2
29.7
31.0
29.2
30.9
27.8
27.2
T°h
282
281.2
280
263
282
296.2
Homogenization (L or V)
Liquid CO2
Liquid CO2
Liquid H2O
Liquid CO2
Liquid H2O
Liquid H2O
Table 17. Microthermometric measurements of fluid inclusion family 4.
Inclusion
1
2
3
4
5
6
Density
Salinity %wt
Pressure (Mpa)
0.87
0.88
0.87
0.85
0.86
0.80
4.42
2.73
2.55
2.93
3.11
2.17
7.16
7.38
7.08
7.36
6.86
6.76
Table 18. Fluid characteristics for each inclusion.
57
Figure 39. Localization and characterization map of FIA 5. This family is hosted by translucent
pyramidal quartz formed within a crack-seal vein accompanied by sulphides.
Inclusion
1
2
3
4
5
Morphology Liquid area
(um2)
Ovoid
55.5
Ovoid
46.13
Ovoid
67.40
Rectangular
236.58
Ovoid
200.97
Vapor area
(um2)
3.10
3.42
4.67
26.31
15.03
Phases
Relation V/L
2L + V
2L + V
2L + V
2L + V
2L + V
0.053
0.070
0.065
0.100
0.070
Table 19. Petrographic characteristics of fluid inclusion family 5.
58
Inclusion
1
2
3
4
5
T°Eu
T°ffh
T°fcl
-60.5
-60.5
-60.5
-60.8
-60.6
-6.6
-5.5
-6.5
-5.6
-6.6
8.7
8.5
8.6
9.2
8.6
T°h CO2
28.4
28.1
28.2
28.4
28.2
T°h
300
308
311
299.3
305
Homogenization (L or V)
Liquid CO2
Liquid CO2
Liquid H2O
Liquid CO2
Liquid CO2
Table 20. Microthermometric measurements of fluid inclusion family 5.
Inclusion
Density
Salinity %wt
1
2
3
4
5
0.87
0.83
0.85
0.80
0.83
2.17
2.55
2.36
1.21
2.36
Pressure
(Mpa)
6.95
6.90
6.92
6.95
6.92
Table 21. Fluid characteristics for each inclusion within family 5.
Based on the decrease of eutectic temperature in all fluid inclusions; CO2 and
methane CH4 could be present as volatile phases in the fluid system. Salinity is
low, ranging between 1.21 – 7.86 wt percent NaCl, and the bulk fluid density is
around 0.8 g/cc. Moreover, V/L relation is not constant in all fluid inclusion
families showing a heterogeneous trapping because of mixing of fluids (Wilkinson,
2001).
Homogenization temperatures are consistent between 250 °C and 320 °C. Also,
bulk fluid composition ranges of inclusions were calculated and the amounts of
substance fractions that compose them are:
H2O between 0.66 and 0.92, CO2
between 0.03 and 0.32, and NaCl between 0.0054 and 0.045. This means that the
average of fluid inclusions are made up of 82.36 % water and 15.78 % CO2 with
minor amounts of dissolved salt (NaCl).
59
5.5 Raman spectroscopy
Raman spectroscopy of fluid inclusions of the first and second families was done in
order to confirm the presence of volatiles in the system also having into account
the decrease of eutectic temperature in all analyzed inclusions.
Inclusion – FIA 1
Figure 40. Ramman spectrum of volatile species within one FIA 1 inclusion.
Four raman vibrations (cm-1) were identified in the spectrum for the FIA 1
inclusion. The first two peaks correspond to “Fermi doublet”: 1285 - 1388 cm-1,, and
confirms the presence of CO2. Next peak of 2330 cm-1 corresponds to Nitrogen, and
the last peak (2917 cm-1) corresponds to the main vibration of methane (Frezzotti et
al., 2012).
Inclusion – FIA 2
Figure 41. Ramman spectrum of volatile species within one FIA 2 inclusion.
60
In this case only three vibrations were recognized. The first two correspond to the
Fermi doublet of CO2, and the third vibration corresponds to methane. Also, this
raman spectrum has lower resolution than the previous one, which can be
explained by the orientation of the inclusion and his depth within the crystal.
61
6. DISCUSSION
6.1 Mineralization styles and vein textures
The mineralization that occurs in the “El Gran Porvenir del Libano” mine is
emplaced in metamorphic rocks corresponding to greenschist facies, similar to
those deposits known as “Orogenic gold deposits” (Groves, 1998; Goldfarb et al.,
2005; Eliu, 1999). Mineralized bodies within the deposit display massive,
brecciated and banded textures (Figure 42). Banded texture is typical of the crack &
seal quartz vein type. Thus, indicating brittle-ductile regimes in a shear zone
characterized by hydraulic fracturing during multiple fluid flow events (Goldfarb,
et al. 2005). This means that multiple mineralization events could be evidenced in
the “lamination” or bedding-parallel arrays of those veins. On the other hand,
breccias are typical of brittle conditions and reflect cataclastic deformation of host
rocks.
Figure 42. Schematic diagram of vein textures interpreted in the field, based on Ore shoot
mineralization present in the first level of the mine (Figure 6).
62
Structural control on the mineralization occurs in subordinate third order oblique
faults (i.e. “Sistema de fallas la Chucula” with NW strike) related to NE trending
movement of the first order structure; Palestina fault. According to Goldfarb, et al.
(2005) first order structures are needed for carrying great volumes of auriferous
fluids. While second to third order structures provide favorable environments for
deposition of auriferous veins in areas of jogs, changes in strike or bifurcations of
faults. So, it suggests that mineralization in the Libano area could be strongly
influenced by Palestina fault and other fault systems of second to third order.
6.2 Hydrothermal alteration
Alteration mineral assemblages found in the deposit (Figure 43) are present in the
proximal zone within alteration haloes up to 10 centimeters. Present minerals on
the intermediate zone stands for alteration of non-adjacent host rocks, dykes and
porphyritic intrusive bodies. Alteration mineralogy is similar of those described by
Eliu (1999) for orogenic gold-lode deposits in upper greenschist facies (Figure 44).
Figure 43. Paragenetic alteration sequence of the mineralization in the Libano area.
63
Figure 44. Schematic paragenetic sequence around orogenic lode-gold deposits in upper
greenschist facies. Black represents the common case and green indicates a less common
occurrence. (Taken from Eliu, 1999).
In this case, the main alteration processes are carbonatization and sericitization of
host rocks with calcite, sericite, white micas, tourmaline and rutile as key alteration
minerals. These are two of the main alteration processes that took place in the
formation of orogenic gold deposits, together with silicification and sulfidation
(Goldfarb, 2010).
64
6.3 Paragenetic sequence
Four mineralization events with a supergenic enrichment of copper minerals were
recognized in the mineralized veins and local zones of the mine. The first event
correspond to introduction of great volumes of quartz and cubic pyrite with minor
amounts of scheelite and carbonates. The second event starts with introduction of
quartz as response of hydraulic fracturing process that builds up crack & seal
veins, which could produce brecciation of the first massive vein due to high
pressure fluctuations during fluid injection. Then, precipitation of wolphramates is
followed by carbonates and non-auriferous sulfides such as pyrite and sphalerite
that occur in the second event. This also could be corroborated because scheelite
and carbonate pulses are located in the lower part of the veins. Also, since the
crystallization of fracture-fill veins occur from outer to center, the mineralization
deposited on the outer part is considered to be older than those deposited in
central parts.
The third event corresponds to the auriferous event, represented by pyrite,
sphalerite, galena, arsenopyrite, pyrrothite and chalcopyrite. During this event
multiple remobilizations of electrum and auriferous sulphides could have occur
because of remobilization effects of calcite and dolomite. Within this event, gold
grade is decreasing because of the increasing of silver content into the veins. Thus,
three different electrums were recognized by his color, mineral association and
EDS compositions. The decrease in gold fineness is anomalous for the orogenic
gold deposit type where ranges are between 780 and 1000 (Morrison et al., 1990).
So an intrusion related gold system will explain better the variations of gold grades
along the deposit.
The fourth event is represented by silver sulfosalts (Pirargyrite, Polybasite, AgTennantite) and native silver accompanied by quartz, pyrite, cadmium-rich
sphalerite and galena. Those atypical mineral assemblages of silver sulfosalts in
orogenic gold deposits could be explained by interaction of metamorphic fluids
with surface water influx (Wilkinson (2001). Finally, supergenic enrichment of
copper minerals leads to formation of malachite. Moreover, oxidation zones in the
mine lead to formation of iron oxides.
65
6.4 Mineralizing fluids
According to microthermometric and Raman measurements, the mineralizing fluid
system is formed by H2O – CO2 + CH4 + N2 volatile species. Additionally, “double
ring” CO2 inclusions, low salinities (ranging from 1.21 – 7.86 % wt NaCl) and
homogenization temperatures between 250 °C and 320 °C are consistent with those
reported for orogenic gold deposits (Groves, 1998; Goldfarb, 2005; Wilkinson, 2001;
Goldfarb, 2010).
Temperature °C - Salinity wt%
Homogenization temperature (°C)
800
700
600
500
FIA 1
400
FIA 2
300
FIA 3
200
FIA 4
100
FIA 5
0
0
10
20
30
40
50
60
70
80
Salinity (wt% NaCl equivalent)
Figure 45. Homogenization temperature °C - salinity diagram illustrating ranges for inclusions
from different deposit types (Modified from Wilkinson, 2001).
Figure 45 shows that all fluid inclusions studied in this work correspond to
orogenic gold deposits (“Lode Au”) or epithermal systems. However, epithermal
deposits lack of significant amounts of CO2 as volatile phases within the system
(Wilkinson, 2001). Compare to intrusion related gold systems; results show
similarity between low salinities and homogenization temperatures ranging from
180 to 360 °C. Nonetheless, mineralized veins in Libano contain silver, zinc and
lead, and according to Hart (2007); Ag-Zn-Pb intrusion related veins lack
significant amounts of CO2. This is the opposite of what was found in this study
(Figure 46).
66
As seen in Figure 46, fluid fraction of CO2 present within fluid inclusions is
relatively high, and suggests that the deposit might correspond to an orogenic gold
deposit.
Figure 46. Fluid fraction diagram to discriminate between porphyry, epithermal and orogenic gold
deposits. (Modified after Goldfarb, 2010).
Fluid trends displayed in figure 47 present isothermal mixing, also corroborated by
the relationship V/L which remains not constant for all fluid inclusion families.
Isothermal mixing trend is consistent with the hydraulic fracturing process that
makes up the crack and seal veins. Also, mixing of fluids is one of the processes
that take place in the deposition of metals and gold within orogenic systems
(Wilkinson, 2001; Goldfarb, 2010). Additionally, it is possible to see a “Surface
fluid dilution” trend which means interaction with superficial fluids, and would
help to explain the presence of complex silver sulfosalts in the paragenesis.
67
Trends of Fluid Evolution
Homogenization Temperature °C
425
400
375
350
FIA 1
325
FIA 2
300
FIA 3
275
FIA 4
250
FIA 5
225
200
0
2
4
6
8
10
12
Salinity (wt% NaCl equivalent)
Figure 47. Fluid evolution trends of studied fluid inclusion families (Modified from Wilkinson,
2001).
Frequency vs Homogenization temperature °C
10
Frequency
8
6
FIA 5
4
FIA 4
2
FIA 3
0
FIA 2
205215225
235
245 255 265
FIA 1
275 285 295
305 315 325
335 345 355
365
Homogenization temperature °C
Figure 48. Frequency versus homogenization temperatures of fluid inclusion families measured on
“El Gran Porvenir del Libano “veins.
68
Finally, a comparison between homogenization temperatures versus frequency of
fluid inclusions families within the veins of “El Gran Porvenir” mine (Figure 48)
relative to “Las animas- Santa Isabel mining district” veins (Figure 49) shows that
peaks are highly correlated between 250 and 320 °C . It is important to mention
that the Santa Isabel mining district is located 25 kilometers away from the “El
Gran Porvenir” mine. The mineralizations in Santa Isabel are reported as orogenic
related to secondary and third order structures associated to the first order
Chapeton-Pericos- Palestina faults (Correa, 2012).
Figure 49. Frequency versus homogenization temperatures of fluid inclusion families measured on
“Las Animas “– Santa Isabel mining area veins (Taken from Acevedo, 2010).
6.5 Source of fluids and relationship to magmatism
Based on petrographic observations of host rocks, intrusive dykes and porphyritic
rocks; Metamorphic fluids could play an important role in deposition of gold,
sulfides and wolphramates. The presence of intrafoliation pyrite overgrown by
chalcopyrite and sphalerite, within folds of the graphitic schists, is a strong
argument to propose a metamorphic fluid enriched in base metals and gold, that
could have produced the mineralization. On the other hand, some dykes are
carrying pieces of the quartz veins with mineralization suggesting postmineralization emplacement of the dykes. Porphyritic rocks also contain minor
amounts of sulfides and are completely altered by carbonatization and
sericitization processes. However, the influence of deep-seated magmatic fluids in
the development of the deposit is not discarded.
69
Comparative table between deposit types and “El Gran Porvenir” deposit.
Intrusion related gold
system
Orogenic Gold
Mine “El Gran
Porvenir”
Tectonic
Framework
Plutonic suites
emplaced during
subduction, collision
and obduction of
outboard terranes.
Compressional to
transpressional
deformation processes in
convergent plate
boundaries and shear
zones
Transpressional
shear zone.
Host rocks
Commonly plutons ,
sedimentary or metasedimentary rocks
Deformed metamorphic
rocks
Gangue
minerals
Quartz, mica,
scheelite, K-feldespar
Quartz, calcite, dolomite,
scheelite.
Alteration
minerals
Sericite, carbonates,
pyrite, Biotite, Kfeldspar, chlorite
Calcite, dolomite, ankerite,
chlorite, sericite, fuchsite,
tourmaline, rutile
Au-Bi-Te±W, As,
Sb,Ag-Zn-Pb
Py, Apy, Po, Mo, BiSulfosalts
Distal : Sph, Gn
Au-Ag, Zn, Pb, Cu, As,
As,Sb ,W, Hg, Bi
Metal
signatures
Ore
minerals
Vein
textures
Gold
fineness
Fluid
system
Salinity
& Th°C
Py, Apy, Po, Tth, Stb, Cpy,
Gn, Sph, Mo
Graphitic schists and
chloritic schists in
greenschist facies.
Quartz, calcite,
dolomite, scheelite,
ferberite.
Sericite+ carbonates,
rutile, tourmaline,
white micas, chlorite,
hematite, pyrite.
Au-Ag, Zn, Pb, Cu,
As-Sb, W
Py, Apy, Po, Cpy,
Gn, Sph, Pol, Ag-tn,
Pyrg
Arrays of sheeted
quartz veins
Breccias, Laminated crackseal veins, Oblique
extension
Massive, Breccias,
crack & seal veins.
Variable
780-1000
460 - 780
H2O – CO2
H2O – CO2 + CH4 + N2
2 – 40 wt. % NaCl
160-380°C
2 – 10 wt. % NaCl equiv.
200 – 400 ° C
H2O – CO2 + CH4 +
N2
1.21 – 7.86 wt. %
NaCl
250 – 320 ° C
Table 22. Table comparing between the different characteristics of intrusion related gold systems,
orogenic gold deposits, and the “El Gran Porvenir del Libano “deposit. (References taken from:
(Groves, 1998; Goldfarb et al, 2005; Goldfarb, 2010; Hart, 2005; Eilu, 1999; Hart, 2007).
70
Metalogenic model approach of the deposit
Figure 47. Metalogenic model of the hydrothermal quartz veins occurring in the “El Gran Porvenir
del Libano” mine. Black lines within veins correspond to faults. Red lines correspond to intrusive
dykes present near or within veins.
Based on the above, mineralization of hydrothermal gold-silver veins that occurs
in the Libano areas correspond to an orogenic gold deposit formed at mesozonal
depths due to vein textures, fluids characteristics, and metal signatures. Therefore,
the influence of metamorphic fluids with different events of mixing during
multiple fluid flow events leads to formation of banded – crack & seal veins.
However, the influence of meteoric or magmatic fluids and their interaction with
metamorphic fluids could have played an important role in the development of the
complex mineralogy evidenced in the deposit. Figure 47 shows the hypothetical
metalogenic model approach for the studied mineralized veins.
71
7. CONCLUSIONS
Based on the mineralizing fluid system characterized by H2O – CO2 + CH4 + N2
volatile phases, low salinities ranging from 1.21 to 7.86 wt. % NaCl , and high
amounts of CO2 that are represented by “double ring” fluid inclusions; the
mineralization that occurs in the “El Gran Porvenir del Libano” mine correspond
to
an
orogenic
gold
deposit.
Additionally,
estimated
homogenization
temperatures, fluid evolution patterns, ore mineralogy, alteration assemblages,
vein textures and tectonic setting are consistent with this deposit type.
Mineralization is emplaced at mesozonal depths hosted by graphitic schists and
chloritic schists in greenschist facies. The crack & seal mineralized veins formed
during multiple fluid flow mixing events with interaction of metamorphic and
probably meteoric fluids. Also, the mineralization is controlled by the “Chucula”
third order fault system related to the Palestina first order fault zone.
Alteration is characterized by a development of proximal alteration zones adjacent
to veins with sericite, calcite, white micas, chlorite, tourmaline, and rutile.
Additionally, sericitization and carbonatization are the main alteration processes
that took place on the host rocks, dykes and porphyritic rocks.
Four mineralization events were recognized. The first one is characterized by the
injection of great volumes of quartz, cubic pyrite, scheelite and carbonates. The
second event represents the beginning of the formation of banded veins with
crystallization of wolphramates, carbonates, pyrite and sphalerite. The third
mineralization event represents the late deposition of gold accompanied by
sulphides such as pyrite, sphalerite, galena, chalcopyrite, arsenopyrite and
pyrrhotite. The fourth mineralization event is characterized by native silver and
complex silver sulfosalts accompanied by pyrite, cadmium-rich sphalerite and
galena. Finally, supergenic enrichment of copper minerals locally occurs in the
deposit.
72
8. RECOMMENDATIONS
In order to complement this work, dating of mineralization and intrusive igneous
bodies such as dykes and porphyries are needed to elucidate genetic relationships
between mineralization and igneous rocks. Additionally, detailed fluid inclusion
analysis of the different mineralization events could be required for defining
temperature and pressure paths for the development of the mineral deposit.
Additionally, stable isotope analysis like Oxygen and Deuterium would be another
approach for distinguishing the source of the mineralizing fluids.
9. AKWNOLEDGMENTS
I want to acknowledge to all of those who were directly or indirectly involved in
this project. Particularly to professors Julián Andrés López, Yamirka Rojas, Juan
Carlos Molano, Andrés Rodriguez, Adriana Delgado, Alba Marina Suarez, and
Ana Ibis Despaigne, for their immense support and patience during the
development of this work. Moreover, Nicolas Bedoya, Sergio Silva, Jimmy Torres
and specially Valentina Carmona since without them this work would have never
been possible. To my family…mom, dad, brothers, aunts and grandparents, as you
give me strength to continue. Thanks to, Ruben Gaitán, Ivette Cucunubo and
Leonardo Santacruz. Also, thanks to my friends. I appreciate you all very much
and once again thanks for everything!
73
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77
ANNEXES
1. Electrum SEM composition
Sample
5A_1
5A_2
9S_1
9S_2
OA
5B_1
5B_2
5B_3
5B_4
5B_5
9N_1
9N_2
9N_3
9N_4
Au wt%
75.18
73.86
45.13
73.11
55.26
57.23
53.85
55.84
54.04
52.76
53.57
56.33
42.54
41.65
Ag wt%
24.82
26.14
54.87
26.89
42.37
42.77
46.15
44.16
45.96
47.24
46.43
40.57
53.38
58.35
Table 23. Electrum wt% composition.
2. Sphalerite SEM composition
Sample
Host
10N_1
10N_2
5B_1
5B_2
9B_1
9N_1
9N_2
9R_1
9R_2
9Ag_1
9Ag_2
9Ag_3
9Ag_4
Fe wt%
8.21
7.42
3.57
6.83
5.5
5.49
8.77
6.97
3.75
6.44
2.61
2.15
2.36
3.05
Zn wt%
55.93
58.24
62.65
58.05
55.94
57.23
54.55
54.25
60.32
56.05
60.67
61.25
61.29
62.38
Cd wt%
2.76
3.05
1.44
1.52
1.79
3.06
2.19
2.79
2.96
2.24
3.05
S wt%
35.86
34.34
33.79
32.36
35.51
35.84
35.16
36.98
32.87
35.32
33.93
33.64
34.11
33
Table 24. Sphalerite wt% composition.
78