Epithermal gold mineralization in Costa Rica, Cordillera de Tilarán

Transcription

Journal of Geosciences, 56 (2011), 81–104
DOI: 10.3190/jgeosci.090
Original paper
Epithermal gold mineralization in Costa Rica, Cordillera de Tilarán
– exploration geochemistry and genesis of gold deposits
Petr MIXA1*, Petr DOBEŠ1, Vladimír ŽÁČEK1, Petr LUKEŠ1, Enrique M. QUINTANILLA2
Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic; [email protected]
MINAET, Dirección de Geología y Minas, Apdo. 10104, San José, Costa Rica
*
Corresponding author
1
2
Epithermal gold mineralization in quartz veins forms part of a large ore belt extending in the NW–SE direction parallel to the Cordillera de Tilarán, Costa Rica. It is confined to Miocene–Pliocene andesites and basalts of the Aguacate
Group volcanic arc. Gold-bearing quartz veins are related to faults and fractures of steep inclinations, accompanied by
pronounced hydrothermal alteration. The key tectonic zones strike NW–SE but the majority of the ore veins are controlled by local extensional structures of Riedel shear type in the NE–SW, N–S to NNW–SSE directions. The brecciation,
mylonitization and healing of deformed structures suggest that three main pulses of mineralization took place during the
hydrothermal process. The gold is present as electrum (30 and 42 wt. % Ag) tiny inclusions up to 25 µm in size enclosed
in quartz, pyrite and arsenopyrite. The other ore minerals are chalcopyrite, galena, sphalerite and marcasite and less
abundant to scarce acanthite, pyrargyrite, greenockite, covellite, bornite and cassiterite. The principal elements exhibiting significant positive correlations with Au are Ag, Sb, As, Pb and Hg. Fluid inclusions of the H2O type were found in
quartz and sphalerite from several Au-bearing occurrences. Temperatures of homogenization of fluid inclusions from
several quartz generations and sphalerite vary generally between 150 and 290 °C; the salinity of the aqueous solution
was very low, not exceeding 5 wt. % NaCl equiv. The age of the mineralization is estimated in the period between the
intrusion of the Guacimal Pluton and effusions of the discordant volcanic Monteverde Formation, which is barren (i.e.
between c. 6.0 and 2.1 Ma). Geochemical study indicated altogether 14 promising gold-bearing areas in the Montes del
Aguacate and Cordillera de Tilarán, of which four can be recommended for further exploration.
Keywords: gold, epithermal, mineralization, fluid inclusions, alteration, Cordillera de Tilarán, Costa Rica
Received: 14 January 2011; accepted: 7 April 2011; handling editor: M. Štemprok
1.Introduction
A joint project of the Czech Geological Survey (CGS)
and Dirección de Geologia y Minas (DGM), Department
of the Ministry of Environment and Telecommunications of Costa Rica (MINAET) was implemented in
the framework of Czech Foreign Aid in 2006–2009.
The area studied covered three map sheets on a scale
of 1 : 50 000, specifically Miramar, Juntas and Chapernal, located in the NW of Costa Rica in the provinces
of Puntarenas, Guanacaste and Alajuela. Moreover,
the project was intended to survey and subsequently
to compile three basic geological maps on a scale of
1 : 50 000, to study natural hazards in relation to the
geological structure of the area under consideration and
to investigate the mineral potential of the area, including
assessment of the impacts of mining and mineral processing on the local environment. With regard to intense
mining of gold in the past (summarized by Muñoz 1997
and USGS et al. 1987) the chief objective of this part
of the project was the evaluation of the gold potential of
new localities. Nevertheless, appropriate attention was
also paid to other mineral deposits, to industrial rocks
and minerals and, in particular, to construction materials. The reason for inclusion of prospecting works in
the scope of the current project consisted in the mining
operations within the above-mentioned three map sheets
that lasted for almost two hundred years (1815–2007).
The mining of gold confined to quartz veins took place
in several ore districts and hundreds of small mines and
deposits, which were mostly discovered by prospectors. In contrast to these extensive mining operations,
no systematic exploration geochemistry combined with
geological mapping and structural geological investigation on a regional scale have been undertaken so far to
establish the mineral potential of the area. This is rather
unfortunate, as it is essential for land use planning and
economic development of the whole region.
The results of new geochemical exploration using, in
particular, heavy mineral and stream sediment surveys
enabled us to identify and outline a number of localities
and areas that are promising for the presence of economic
gold deposits. Some of them were suggested for followup detailed exploration and investigation.
All the field and laboratory data obtained in course
of the project were, together with individual maps and
www.jgeosci.org
Petr Mixa, Petr Dobeš, Vladimír Žáček, Petr Lukeš, Enrique M. Quintanilla
relevant project outputs including the GIS database, summarized in the Final Report (Kycl et al. 2010) and stored
in the archives of CGS Prague, Czech Republic and DGM
San José, Costa Rica. The present study is focused chiefly
on the definition of prospecting criteria, the investigation
of gold deposits and the likely mode of their origin.
2.Geological setting
The area covered by the three map sheets of Miramar,
Juntas and Chapernal lies virtually in the axis of a volcanic arc extending above the Central American subduction zone. An important regional transcurrent zone of
sinistral character (Marshall et al. 2000) is believed to
have significantly affected the development of brittle
deformations (faults and fracture systems) in volcanic
formations in the Late Cenozoic times.
The region in which the gold deposits are concentrated
consists of two major geological units, the Aguacate
Group (including Guacimal Pluton) and the Monteverde
Formation. Recent geological studies dealing with the
area of interest and adjacent regions include Alvarado
(2000) and Denyer et al. (2003). A new geological survey
on a scale of 1 : 50 000 (Žáček et al. 2010a, b, c; Kycl et
al. 2010) contributed substantially to better knowledge
of the geological structure of the area.
The Aguacate Group is the main geological unit in the
studied area. It consists of tholeiite basalt to basaltic andesite
lavas accompanied by abundant pyroclastics and breccias of
andesite composition. This group of effusive rocks is of Miocene to Pliocene age (2.1 to 23.0 Ma: Bellon and Tournon
1978; Amos and Rogers 1983). The rocks are hydrothermally altered on a regional scale, having been affected by
carbonatization and silicification (Laguna 1983, 1984).
The Aguacate Group is discordantly overlain by the
Monteverde Formation which, according to radiometric
dating, is Pleistocene in age (2.2–1.0 Ma – Kussmaul
and Spechmann 1982; Alvarado et al. 1992). In contrast
to the older Aguacate Group, the Monteverde Formation
is characterized by the occurrence of more acid, calcalkaline volcanism with dominant andesites and by the
absence of regional hydrothermal alteration.
Volcanites of the Aguacate Group were intruded by
granitoids of the Guacimal Pluton exposed over an area
of c. 15 × 6 km, and elongated in the NW–SE direction.
Gray porphyric biotite granite is the dominant rock type
of the Pluton, whereas more mafic varieties (monzodiorites to gabbros) are much less abundant (Žáček et al. this
volume). The K–Ar dating carried out by Alvarado et al.
(1992) yielded ages of 7.2–3.9 Ma, whereas laser ablation
ICP-MS dating of zircons from two granite samples gave
statistically undistinguishable U–Pb ages of 6.3 ± 0.5 and
6.0 ± 0.4 Ma (Žáček et al. this volume).
82
A large contact metasomatic aureole up to 1.5 km wide
developed around the intrusion; here the volcanites were
converted to hornfelses and epidote schists. Although
the Pluton exposed on the surface is relatively small, the
gravity anomaly interpreted by Ponce and Case (USGS
et al. 1987) indicates a large body of relatively light crust
extending below the Cordillera de Tilarán and Montes del
Aguacate. It is believed to represent a huge hidden granitic intrusion extending in the NW–SE direction over a
distance of c. 100 km, of which the southern edge follows
the line of the towns of Esparza – Pozo Azul – Juntas and
continues as far as Liberia. A similar negative anomaly of
almost identical direction, intensity and size was identified in the SE sector of Costa Rica. It corresponds to the
large granitic mass of Cordillera de Talamanca which,
due to deep erosion level, is exposed on the surface.
Related epithermal gold deposits in Talamanca area are
abundant and well known.
3.Mining history
Numerous ore districts exploited in the past lie in the
area of the three map sheets investigated (see Fig. 1).
Gold deposits in the Cordillera de Tilarán and Montes
del Aguacate were discovered as early as in 1815 by
Nicolás Garcia (1756–1825), a catholic bishop stationed in Costa Rica and Nicaragua. Mining operations
on an industrial scale began in the middle of the 19 th
century. The largest mine Mina Tres Hermanos in the
Abangares area was developed in 1870 and during the
main mining boom, when the shaft reached a depth
of 500 m, as many as 1 500 miners extracted ore of
about 8 g/t Au grade. A system of ropeways several
kilometres long was developed to transport the ore to
the central ore dressing plant in the village of Sierra
near the town of Juntas. Following a catastrophic mining accident, during which as many as 120 miners died,
a decline in mining took place and mining has never
recovered.
Attempts were made in the last few decades to reopen
mines in historical ore districts. The most meaningful
achievement was the development of the Bellavista
open-cast mine and ore dressing plant by a Canadian
mining company. However, after several years of operation, a landslide destroyed the ore dump, including
the ore dressing plant using heap leaching technology
of gold extraction, resulting in complete abandonment
of the mine. Similar attempts lasting for a few years
were made by foreign investors in the mining districts
of Veta Vargas, Recio and Tres Hermanos. Currently,
no legal mining for gold takes place in the area under
consideration. Only illegal hand picking of gold by local miners is under way (Fig. 5). Brief information on
Gold mineralization in Cordillera de Tilarán, Costa Rica
a)
map sheet Juntas
La Luz
Tres Hermanos
Sierra Alta
10°20'N
85°00'
b)
Nicara
gua 84°00'
83°00'
11°00'
Ca
rib
be
an
a
Se
Juntas
10°00'
Guacimal
09°00'
am
á
Santa Rosa
08°00'
studied area
Pan
Pacific Ocean
Veta Vargas
50 km
La Union
Bellavista
Chassoul
Buena Suerte
Miramar
Moncada
84°30'W
85°00'W
Puntarenas
10 km
10°00'N
map sheet Chapernal
map sheet Miramar
Fig. 1 Position of the major ore districts within the zone Cordillera de Tilarán – Montes del Aguacate. Following Schulz et al. (1987).
the most important mining districts is given below (see
also Fig. 1).
3.1.Bellavista
Until 2007, Bellavista (NE of the city of Miramar) was
the only and so far the last gold mine in operation on the
territory of Costa Rica. It was abandoned following a
catastrophic landslide triggered by torrential rains. The
ore zone consists of stockwork and quartz veins inclined
at angles of 30–90° to the Falla Liz fault line trending
N–S. The main strikes of local quartz veins run NE, less
frequently NNE. The host rocks are volcaniclastic breccias and basaltic lavas of the Aguacate Group overlain by
sterile andesite lava flows and lahars of the Monteverde
Formation, which were removed as overburden. The gold
occurs both in quartz veins and stockwork with max.
grade 50 g/t and also in altered wall rocks with a grade
of 0.5–1 g/t. Recalculation of ore reserves adjusted to
a workable grade of 1.54 g/t (Alán 1990) gave 13.6 Mt
of ore reserves. Underground drilling penetrated dykes
of diorite to granodiorite which probably belong to the
Guacimal Pluton.
3.2.Mina Chassoul (Prospecto Corinto)
The Mina Chassoul is located on the Miamar map sheet. The
deposit was exploited in the past but its old stopes were reopened in 2008. The mine was developed on the Veta Cajeta
and Veta Virgilio veins trending N–S to NE–SW. The veins
are intensely affected by tectonics and filled with mylonite
(Fig. 6). The average grade in the Cajeto vein corresponds
to 9.67 g/t Au, locally exceeding 100 g/t Au. The average
grade in the Virgilio vein corresponds to 7.34 g/t Au.
3.3.Mina Moncada
Occasionally recently exploited gold mine (on the SE
edge of the Miramar map sheet) with ore dressing plant
using cyanide leaching technology. The mine was developed in a max. 1.6 m thick, N–S trending quartz–carbonate vein, accompanied by numerous stringers. Local ore
is uncommonly rich in base-metal sulphides (chalcopyrite, sphalerite and galena) with an average grade of 8.91
g/t Ag, locally exceeding 100 g/t Ag. The steep vein was
extracted by a system of headings. The ore was transported by ropeway to the ore dressing plant.
83
Fig. 2 A scheme of economically
most important ore veins in the
Abangares mining district (LLM
– La Luz Mine, Rec – El Recio
Mine, RGB – casa de Rigoberto). Following Castillo (1997)
and Kycl et al. (2010).
3.4.Abangares mining district
The Abangares mining district (Juntas map sheet) used
to be the most important mining centre in the whole
region (see also Fig. 2). The ore is confined to numerous thick veins running N–S to NNE–SSW with steep
inclinations (60–85° to NW). The ore district is divided,
from the west to the east, into sub-districts: La Luz, Tres
Hermanos and El Recio located to the west, whereas San
Martin, Sierra Alta, Boston and Gongolona are situated
in the east.
3.4.1.La Luz Mine
This mine was developed on the parallel La Pita and
La Olga veins oriented 10° NNE having a thickness of
1.2–2 m with average grade 12.5–37 g/t Au (OEA 1978).
An ore dressing plant was erected in place of historical
stopes, using cyanide leaching technology, and processing material extracted by illegal mining and dumped near
the plant.
3.4.2.El Recio Mine
El Recio Mine (known also as Silencio) is located about
3 km N of the city of Juntas. It exploits sub-vertical veins
of El Silencio (which splits southwards into the Guayacán
and Santa Anna veins), El Recio and Villalobos running
in general N–S. El Recio vein dips 80° to the west with
a thickness of as much as 5 m. The grade is estimated
to have ranged between 7.8 and 15.6 g/t Au. The veins
84
were exploited by a system of levels, raises and winzes
about 30 m apart down to a drainage tunnel, about 150 m
below the surface.
3.4.3.Tres Hermanos Mine
This mine was developed on a namesake vein and also on
the neighbouring Limón, Pedernal, Palo Negro, Nispero
and Balsa veins. The Tres Hermanos vein was the major
object of exploitation in the past. It is 3 m thick and runs
20–30° NNE, steeply dipping. In its northern part, it was
exploited to a depth of 200 m, whereas its southern section was mined to a depth of 150 m. Data on historical
production indicate the grade to have been around 16 g/t
Au. The gold is rich in silver (electrum) accompanied by
sphalerite, galena and stibnite.
3.4.4.Boston, Sierra Alta (also named Tres
Amigos) and Gongolona mines
These three mines used to exploit a system at maximum
5 m thick, of steeply inclined veins oriented 30° NNE to
70° ENE (Boston, Año Nuevo, San Martín, San Rafael,
La Fortuna Gongolona and others). Millaflor (1979) reported an average grade of 7.41 to 12.6 g/t Au.
3.5.Guacimal
The Guacimal ore district in the eastern sector of the
Juntas map sheet consists of a system of quartz veins
with Pb–Zn–Cu–Au–Ag mineralization. The veins are as
much as 1.5 thick and run NNW–SSE, inclined 66–70°
to ENE.
4.Exploration geochemistry
4.1. Methods of study – sampling and
laboratory treatment of samples
Exploration geochemistry, specifically the heavy mineral
and stream sediment survey, were chosen as the most
efficient methods for assessing the ore potential of the
area under consideration. Mineral occurrences found
and documented during geological mapping were also
investigated for the economic potential of the local mineralization. Lastly, the collected samples were used for
the evaluation of possible environmental pollution with
respect to ecological burdens left after earlier mining
and mineral processing. Particular attention was paid to
mercury that was formerly used for amalgamation and
extraction of gold and also to anomalous contents of
heavy metals in dumped waste rock or to their enhanced
background values in wall rocks.
Altogether 237 heavy mineral concentrates were
collected using panning of 10 litres (about 20 kg) of
sandy material. The density of the sampling sites was
1 sample per 4 km2 depending on the drainage pattern
and accessibility. All the samples were taken from active river beds, mostly with flowing water. The heavy
mineral survey on the Juntas and Chapernal map sheets
concentrated on the Aguacate Group. Areas covered
by young and supposedly sterile volcanic rocks of the
Monteverde Formation and also sedimentary formations
or areas that are poorly accessible or are a subject of
environmental protection were excluded from the heavy
mineral survey.
The development of alluvial sediments varied substantially depending on the topography and stream gradients,
which are steep, particularly in the central and northern
sectors of the Miramar and Juntas map sheets. Each sample was documented using a standard procedure recording
17 parameters, and inserted in a database. The samples
were sieved on the spot using a 2 mm sieve and panned
to obtain a rough concentrate. These were then analyzed
in the mineralogical laboratory of GEOMIN Co. Jihlava,
Czech Republic. The concentrates were first observed
under a binocular microscope and some 16 minerals were
selected to be studied in more detail and to establish their
relative proportions. The heavy fraction was sieved on a
0.16 mm screen and, after magnetic separation of ferromagnetic fraction, the above-screen fraction was studied
under a binocular microscope to identify and count gold
particles. The below-screen fraction was pulverized for
chemical analyses (see below).
With respect to the aforementioned character of the
mineralization, the heavy mineral survey was supplemented by a stream sediment survey, within which a
total of 296 samples were collected. Despite the rough
topography, reflected in the steep river gradients, it was
still possible to take reasonable and representative material for analyses. Stream sediment samples were dried
at the camp sites, sieved on 0.18 mm mesh sieve (~80
mesh), subsequently homogenized in the laboratories of
the Czech Geological Survey in Prague.
These powders, together with the below-screen fraction of heavy mineral concentrates, were analyzed in
ACME Laboratories in Vancouver, Canada using the
ICP MS method (Group ADX analytical procedure),
enabling determination of 46 elements, including Au
with a detection limit of 0.2 ppb. Chemical analysis of
the below-screen fraction of heavy mineral concentrates
was used to identify Au, which is extremely fine-grained
or even of sub-microscopic dimensions. Alternatively it
may be bound in pyrite, arsenopyrite and/or other base
metal sulphides.
4.2.Heavy mineral survey
Gold was found in 55 samples of heavy mineral concentrates of a total of 237 samples. In samples in which
gold was optically identified, its particles overwhelmingly
occur in the above-screen fraction (>0.16 mm). However,
gold was successfully optically detected in the belowscreen fraction. The number of gold particles identified
in the individual samples varied between 1 and 10, only
scarcely reaching tens of particles in a single sample. The
richest heavy mineral concentrates included the following samples (presented with above-screen/below-screen
numbers of gold particles): Ju-Tl-3 (Juntas map sheet,
45/66), Ju-TM-23 (Juntas map sheet, locality Quebrada
Gongolona, 29/53) and Mi-TM-25 (Miramar map sheet
– dextral tributary of the Quebrada Zamora, Río Seco
40/16). The size of the gold particles ranged from 0.1 up
to 1 mm. The largest “nugget” was identified in sample
Ju-Tl-3 from the Río Abangares river, reaching 2 mm in
diameter (Fig. 3a).
The crystal form (habit) of the gold particles often
indicates their short transport (Fig. 3d–h). They mostly
occur as tiny plates or packed plates with sharp edges
or tiny wires; rare is dendritic gold. Gold particles often
enclose rock-forming minerals – quartz and K-feldspar in
particular. Gold from larger rivers (e.g. Río Abangares,
Río Congo, Río Boston) also occurs in the form of wellrounded tiny grains or nuggets with indication of longer
transport (Fig. 3a–c). These particles are often coated with
iron oxides and clay minerals. Silver contained in gold (in
the form of a solid solution) is usually leached out from
the surface of the gold (electrum) grains. In sample JU85
TL-42 (Río Tres Amigos), the identified tiny gold grains
contain surface coatings of clay minerals, which seem to
absorb anthropogenic mercury (due to contamination of
the stream by Hg from amalgamation in the past).
Pyrite, arsenopyrite, chalcopyrite, to lesser extent
galena, sphalerite and cinnabar are primary sulphides
accompanying gold in heavy mineral concentrates; cerussite is probably the most abundant secondary mineral.
Although the majority of anomalous contents of mercury
detected in analyses of heavy mineral concentrates and
stream sediments are ascribed to contamination during
extraction of the gold through amalgamation, the occur-
a)
200 m
d)
b)
50 m
e)
c)
100 m
f)
rence of cinnabar argues for the presence of primary Hg
mineralization in the studied districts.
4.3.Relationships between elements
detected in stream sediments and in
the below-screen fraction of heavy
mineral concentrates
Positive correlations between Au and other elements
emerge from chemical analyses of stream sediments
and the below-screen fraction of heavy mineral concentrates.
100 m
50 m
200 m
g)
m
h)
20 m
i)
10 m
Fig. 3 Morphology, shape and intergrowths of gold particles with gangue minerals. a – JU-TL-3 (Rio Abangares) – the largest tiny and well rounded nugget ever found in studied samples indicating long transport from the primary source; b – JU-TL-42 (Río Tres Amigos) worn tiny nugget
containing more than 50 wt. % Ag, accompanied by a grain of alkaline feldspar; c – CH-TM-10 (Quebrada Barrantes) worn tiny nugget with quartz
grains, 20 wt. % Ag; d – JU-TL-42 (Río Tres Amigos) dendritic gold with Ag < 10 wt. % accompanied by clay minerals concentrated on surface
Hg and pure gold (product of amalgamation); e – CH-TL-2 (Quebrada Vueltas) wrinkled tiny leaf of gold intergrown with quartz, showing marks
of transport; f – JU-TM-4 (Quebrada Castillo) gold wire with 20 wt. % Ag enclosing grains of barite and quartz (BSE image), g – JU-TL-104 tiny
gold nugget with quartz–feldspar gangue, Ag < 30 wt. %, locally coatings of clay minerals and Fe-oxides; h – JU-TL-3 (Río Abangares) close up
photo of Au in quartz gangue, 20 wt. % Ag; i – JU-TL-39 (Río Boston) close up photo of porous surface of worn tiny gold nugget with clay minerals, 20 wt. % Ag.
86
The closest positive correlations were found between
Au on the one hand and Ag, Sb with As on the other,
locally was also correlated Au with Hg. These relationships are also an important criterion in exploration
geochemistry, enabling pinpointing of anomalous and/
or promising areas. Other elements such as W, Mo, Tl
and perhaps even Pb may be used as additional criteria in
the search for Au mineralization (Tabs 1–2). In contrast,
no positive correlation was found between Au and base
metals (Cu, Zn), probably reflecting a simple mineral
assemblage of gold accompanied with minimum base
metal sulphides in the ore. On the other hand, the mineralogy of epithermal veins and their vertical zoning in
ore elements may differ in various areas depending on
the erosion surface at which they are exposed (cf. Mosier
and Sato 1986).
4.4.Results of geochemical exploration and
their interpretation
The following features were used for demarcation of
anomalous areas and deserve further attention: a) geological structure characterized by the occurrence of volcanic
rocks of the Aguacate Group, b) tectonics characterized
by combination of NW–SE and N–S trending faults and
crushed zones, c) occurrence of hydrothermal alterations,
particularly silicification, carbonatization and propylitization, d) results of heavy mineral survey presented in the
form of mono-mineral maps and mono-element maps of
chemical analyses of the below-screen fraction of heavy
mineral concentrates (Fig. 4) and e) the results of the
stream sediment survey.
Altogether 14 areas prospective for the occurrence
of gold mineralization were identified based on geochemical exploration. Among them, four were interpreted as particularly promising for the occurrence of
gold mineralization and were suggested for follow-up
investigation. Although another ten areas show positive indication of gold mineralization, they may be
disqualified for follow-up exploration because of their
small areal extent or problematic origin of the Au
mineralization (possible long transport from primary
Tab. 1 Positive correlations of elements detected in stream sediments
and in the below-screen fraction of heavy mineral concentrates (HMC)
Stream sediments
(296 samples)
Undersize fraction of BSF
(237 samples)
Au, As, Ag, Sb, Tl, Cd, Hg, W
Au, As, Ag, Sb, Pb, Hg
U, Th, Bi, Se, Ga
Zn, Fe, Ga, Sc
Ni, Co, Cr
Ni, Co, Cr
Mo, V, Se
Se, Bi, S
La, U, Th, Al, Ti
La, U, Th, P
sources). One area (No. IV – Baranquilla, NNE sector
of the Miramar map sheet – in Kycl et al. 2010) shows
indications of base-metal mineral assemblage accompanied by a broad spectrum of anomalous elements – Cu,
Mo, As, La, V, Se, Ga, Bi and Sc with no relation to
any gold mineralization. This locality is linked to the
nearby Pleistocene (sub-) volcanic dome of Cerro la
Cruz formed of amphibole–biotite rhyodacites.
The remaining four promising areas all exhibit positive prospecting criteria, including sufficient areal extent
(more on the subject in Kycl et al. 2010).
4.4.1.Area II +XI (Zamora and Estrella)
This anomaly is defined at the intersection of the Juntas,
Miramar and Chapernal map sheets covering an area of
c. 40 km2 in the NW extension of the historical mining
district of La Union. The area is formed of andesites and
pyroclastics of the Aguacate Group. Zones of intense
alteration, characterized chiefly by pyritization, propylitization, silicification and argillization, can be locally
observed. Stream sediments and below-screen fraction of
heavy mineral concentrates showed highly enhanced contents of Au, anomalous concentrations of Ag, Sb and Hg
(thirty times the background values) as well as elevated
higher contents of Cu, Pb, As and Cd. The rod-like shape
of gold particles indicates that the primary source of the
gold is not too far away.
4.4.2.Area IX (Pozo Azul)
The Area IX occurs in the central part of the boundary between the map sheets of Juntas and Chapernal,
c. 40 km2 in size, being formed by thirteen catchments.
The Aguacate Group represents the chief lithology of the
area. A large, hydrothermally altered zone extending in
the NW–SE direction occurs at the southern edge of the
complex. In this area, silicification and propylitization
are the major types of alteration, with which the mineralizations are believed to be linked. Only four samples of
heavy mineral concentrates, representing the catchments,
contained gold, but the contents of elements positively
correlated with gold in other catchments indicate that the
mineralization may cover a much larger area than that
of the above-mentioned four catchments. Some heavy
mineral concentrates also contained cinnabar (Ju TM 4,
Ju-TL 3, Ch-TL-6, Ch-TM 10). Their below-screen fraction and also the stream sediments exhibited anomalous
contents of Hg, Bi, Sb, Cu, Pb, Zn, Ba and Cd. The Veta
Vargas deposit near the village of Pozo Azul, exploited in
the past (Alán and Castillo 1983; Mora 1984), is part of
area IX. However, it is notable that the highest contents
of gold identified during the heavy mineral survey sur87
88
0.110
(0.988, 0.993)
0.991
(−0.072, 0.181)
0.055
1
1
(0.446, 0.626)
0.542
(0.898, 0.938)
0.920
(0.369, 0.567)
0.474
Sb
0.013
(−0.131, 0.124)
−0.004
1
1
(−0.072, 0.181)
0.055
(0.159, 0.394)
0.281
(0.055, 0.301)
0.181
(0.286, 0.500)
0.398
Hg
0.105
1
1
0.103
(0.964, 0.978)
0.972
0.120
(−0.122, 0.132)
0.055
0.088
(0.971, 0.982)
0.977
0.495
(0.392, 0.585)
(0.293, 0.506)
(−0.168, 0.086)
(0.107, 0.348)
0.405
−0.041
(0.469, 0.644)
(0.892, 0.934)
0.231
0.563
(0.915, 0.948)
0.933
(0.224, 0.449)
0.342
(0.978, 0.987)
0.983
(−0.138, 0.116)
−0.011
(−0.220, 0.032)
−0.095
(−0.085, 0.169)
0.042
(0.929, 0.957)
0.944
(0.219, 0.445)
0.336
(0.980, 0.988)
0.985
(−0.058, 0.195) (−0.024, 0.227) (−0.007, 0.243) (−0.039, 0.213)
0.915
(0.024, 0.273)
0.070
(0.390, 0.583)
(0.867, 0.918)
0.151
0.493
0.895
0.013
(−0.018, 0.233)
0.110
(−0.073, 0.181)
0.055
(−0.030, 0.222)
0.098
(−0.064, 0.189)
0.063
Cd
0.055
(0.964, 0.978)
0.972
(0.390, 0.583)
0.493
(0.867, 0.918)
0.895
(0.310, 0.520)
0.421
Mo
0.120
(−0.024, 0.227)
0.103
(−0.058, 0.195)
0.070
(0.024, 0.273)
0.151
(−0.027, 0.225)
0.100
Cu
0.042
(0.978, 0.987)
0.983
(0.469, 0.644)
0.563
(0.892, 0.934)
0.915
(0.406, 0.596)
0.507
Pb
−0.095
(0.224, 0.449)
0.342
(−0.168, 0.086)
−0.041
(0.107, 0.348)
0.231
(−0.149, 0.106)
−0.022
Zn
−0.011
(0.915, 0.948)
0.933
(0.392, 0.585)
0.495
(0.811, 0.882)
0.850
(0.293, 0.506)
0.405
W
(−0.028, 0.224)
0.100
(0.028, 0.276)
0.154
(−0.013, 0.238)
0.114
(0.074, 0.318)
0.199
(−0.011, 0.240)
0.116
1
1
(−0.022, 0.229)
0.105
(0.902, 0.940)
0.924
(0.230, 0.455)
0.347
(0.958, 0.975)
0.968
(−0.047, 0.205)
0.080
1
1
(−0.011, 0.240)
0.116
(0.971, 0.982)
0.977
(0.090, 0.333)
0.215
(−0.057, 0.197)
0.071
(−0.010, 0.241)
0.118
1
1
(−0.047, 0.205)
0.080
(0.074, 0.318)
0.199
(−0.039, 0.213)
0.088
0.071
(0.230, 0.455)
0.347
(0.028, 0.276)
0.154
(0.219, 0.445)
0.336
(0.919, 0.950)
0.936
(0.219, 0.445)
0.337
1
1
(0.149, 0.385)
0.271
1
1
(0.219, 0.445)
0.337
(−0.010, 0.241) (−0.057, 0.197)
0.118
(0.958, 0.975)
0.968
(−0.013, 0.238)
0.114
(0.980, 0.988)
0.985
1
1
(0.149, 0.385)
0.271
(0.919, 0.950)
0.936
(0.090, 0.333)
0.215
(0.902, 0.940)
0.924
(−0.028, 0.224)
0.100
(0.929, 0.957)
0.944
(−0.131, 0.124) (−0.114, 0.140) (−0.122, 0.132) (−0.007, 0.243) (−0.085, 0.169) (−0.220, 0.032) (−0.138, 0.116)
−0.004
(0.988, 0.993)
0.991
(0.422, 0.608)
0.521
(0.878, 0.925)
0.904
(0.328, 0.534)
0.437
Tl
(−0.030, 0.222) (−0.073, 0.181) (−0.018, 0.233) (−0.114, 0.140) (−0.022, 0.229)
0.055
(0.422, 0.608)
(0.878, 0.925)
0.098
0.521
(0.159, 0.394)
(0.055, 0.301)
0.904
0.281
(0.446, 0.626)
(0.898, 0.938)
0.181
0.542
1
(0.480, 0.652)
0.920
1
(0.480, 0.652)
1
0.572
0.572
(0.862, 0.915)
(0.409, 0.598)
1
0.892
0.510
Ag
(95% confidence intervals for individual correlation coefficients are also given. Combinations with estimated correlation coefficient in modulus ranging between 0.4 and 0.7 are depicted in blue, while those ranging between 0.7 and 0.9 are shown in red. Number of treated samples: 237)
W
Zn
Pb
Cu
Mo
Cd
Tl
Hg
Sb
Ag
As
Au
As
Tab. 2 Correlation coefficients of elements essential for mineral prospecting derived from analyses of the below-screen fraction of heavy mineral concentrates
Q
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412000
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411000
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410000
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434000
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TL0026
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407000
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TM0043
TL0025
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405000
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13,3–29,0 ppm
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TL0016
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402000
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403000
TM0022
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(Portones)
TL0012
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TM0002
la r a s
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TM0042
TM0005
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400000
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401000
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414000
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394000
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Fig. 4 A demonstration of a geochemical mono-element map of areal distribution (example of arsenic) on scale 1 : 50 000 (sheet Juntas) in the below-screen fraction of heavy mineral concentrates.
391000
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1130000
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391000
89
prisingly occur west and northwest of the known deposit,
not in its surroundings.
4.4.3.Area X (Santa Rosa)
The area X is located on the Juntas map sheet, comprising
nine source areas and covering c. 20 km2. Its northeastern edge borders on granitoids of the Guacimal Pluton.
Basalts and andesites of the Aguacate Group, accompanied by silicification, cover the investigated area. Heavy
mineral concentrates from six catchments contained one
to six particles of gold per sample. The undersize fraction was characterized by enhanced contents of Au, As
and Sb; locally were also found Cu, Pb, Zn and Hg. The
small and poorly documented ore district of Santa Rosa
(Alán 1981) occurs in this area. Possibly only one vein
was exploited there but the anomalous area following the
contact aureole with the Guacimal Pluton is much larger.
4.4.4.Area VII (Agua Agria – Río Jesús)
This area lies in the SE sector of the Miramar map sheet.
The anomaly covering 20 km2 consists of five catchments,
of which three were found to be positive or promising for
the occurrence of gold. Greatly enhanced contents of As,
Au, Sb and Ag were detected particularly in the stream
sediments. The geological structure is varied, the predominant basalt and andesite lava flows and pyroclastics
of the Aguacate Group were penetrated by several small
rhyolite intrusions. Numerous NE–SW fault zones form
horst structures in local volcanites. Abundant alteration
zones characterized by pyritization and silicification can
be observed with anomalies of the above-mentioned
elements. No historical mine is known in the region but
the abandoned mines Magallanes, Quarenta Leones and
San Gerardo are located to the NW of the surveyed area.
5.Mineralization
5.1.Geological background – veins,
structures and mineralization
Epithermal gold deposits exploited in the past are concentrated in a NW–SE trending zone called the Golden
Belt or Cinturón de Oro (Amos and Rogers 1983; Schulz
et al. 1987), almost 100 km long. This zone is located
in the SW foothills of the Cordillera de Tilarán and also
topographically belongs to the Montes del Aguacate and
Serranías de Abangares subregions.
Epithermal gold-bearing mineralization corresponding
to the SADO type (Mosier and Sato 1986) occurs exclusively in the Aguacate Group. The veins are developed
90
in fault and fracture zones, are steeply inclined and of
varying strikes. They often suffered from brecciation and
remobilization. The principal tectonic zones run mostly
NW–SE, but the direction most of the veins indicates that
the major structures running NE–SW, N–S to NNW–SSE
were affected by local extension tectonics rather than
formed in a homogenous strain field. Even though the
stockwork with two main veins (each of the two as much
as 5 m thick) in the Veta Vargas (Juntas) deposit follow
the prevailing strike fluctuating between NE–SW to
NW–SE, the two veins within the stockwork strike E–W.
Although the main stress field linked with subduction
gave rise to significant NW–SE faults following the main
Cordillera, the quartz veins seem to originate in an en
echelon system of Riedel shears striking N–S to NE–SW.
It is noteworthy that this system does not propagate into
the younger Monteverde Formation.
Gold is bound in quartz veins and stockworks of
varying thickness ranging from a few centimetres up to
7 m (La Unión, Los Angeles vein, Abangares – Tres Hermanos, Recio, Sierra Alta, Veta Vargas, Buena Suerte and
Mina Chassoul; Figs 5–6). The greatest gold enrichment
in quartz veins can be observed in places of intersection
of the main NE–SW striking faults and associated N–S
to NW–SE shear zones. Gold anomalies also occur in
host rocks altered by hydrothermal fluids immediately
adjacent to the quartz veins.
Gangue minerals in gold-bearing veins as well as in
base-metal veins are mostly represented by milky brecciated quartz. While chlorite and sericite are also common, barite, calcite, adularia and ankerite are much less
abundant.
Three main generations of quartz suggest a multi-stage
hydrothermal process. The first includes fine-grained
massive quartz of gray-white, gray and gray-black colour
exhibiting greasy to glassy lustre. This type of quartz
forms several metres-thick veins, which are the richest in
gold. The Au contents commonly range in tens ppm and
exceptionally even exceed 100 ppm (Tab. 3). The second
generation is represented by coarse prismatic quartz, often forming druses filled with crystals max. 1 cm in size.
This quartz type is frequently accompanied by base-metal
sulphides exhibiting increased gold contents. The third
generation represents the final stage of the hydrothermal
ore-forming process, producing fine-grained snow-white
quartz of sugary appearance. This generation occurs in
the form of thin (X–X0 cm thick) veins and veinlets,
which are gold-bearing, but the Au contents do not exceed 10 ppm (e.g. Abangares region, Sierra Alta mine
– 0.5–6.8 g/t Au or SE part of the Miramar map sheet,
Moncada area – 0.3–5.2 g/t Au; see Tab. 3).
The mineral assemblage is simple. The most abundant
is pyrite, which is also a common admixture in altered
volcanites, but without any relationship to gold minerali-
acanthite, pyrargyrite, greenockite (primary), covellite,
bornite and cassiterite, mostly in trace amounts, were also
identified. Although cinnabar was identified in several
heavy mineral concentrates, this mineral was not found
in the samples of primary ores. Of earlier investigations
in the Bellavista ore district, Shawe (1910) described
proustite, pyrargyrite, miargyrite and freibergite, while
Roberts and Irving (1957) identified realgar and stibnite
in the Montezuma Mine. USGS et al. (1987) reported
stibnite from the El Recio and Tres Hermanos deposits.
The Cu–Pb–Zn sulphides, which occasionally form
massive vein and nest-like ores (Tab. 3), prevail in the
Guaria–Guacimal district and mina Moncada. The gold
content in this type of mineralization is low or negligible.
The gangue and ore textures are often comb-like,
banded or crustiform, whereas colloform textures are
rare. Many features like brecciation or fracture fillings
suggest that multistage ore-forming processes took place.
Strongly altered and pyritized relics of initial andesite,
forming lenses in the fault zone filled with mylonite, can
also be observed (e.g., at Mina Chassoul adit or Veta
Cajeta – Fig. 6).
5.2.Methods
Fig. 5 An old stope in the Boston vein from which sections rich in gold
are still occasionally illegally extracted by local miners. Sierra Alta ore
district on the Juntas map sheet. Photo by P. Mixa.
Fig. 6 Mina Chassoul – strongly tectonized and altered zone filled with
mylonite clay, brecciated andesite and limonitized rod-like lenses of
quartz. Quartz gangue with pyrite and gold contains over 100 g/t Au.
Photo by P. Mixa.
zation. Gold is always accompanied by pyrite, which is
often rich in arsenic. Other sulphides include common arsenopyrite and marcasite, whereas chalcopyrite, sphalerite and galena are minor to scarce. Anglesite, cerussite,
The electron microprobe study was performed on
(i) samples rich in a variety of sulphides identified by ore
microscopy and (ii) samples which showed high concentrations of gold and other metallic elements established
by chemical analyses. The electron microprobe analyses
(EPMA) were performed using the Cameca SX-100 electron microprobe (Joint Laboratory of Masaryk University
and CGS, Brno, R. Škoda, analyst) in the WDS mode.
The minerals were analyzed with beam diameter of 1–4
µm at 25 kV accelerating voltage and 10–20 nA beam
current. The following standards were used: Ag, Sb, Au,
Bi, Cu, Co, Mn – native elements, S – chalcopyrite, Pb
– PbS, Hg – HgTe, Fe – FeS2, Ni, As – pararammelsbergite, Se – PbSe, Zn – ZnS, Cl – PbCl2. Some minerals
(quartz, calcite, K-feldspar, barite, cassiterite and probable bornite) were not analyzed but only identified in the
energy-dispersive mode (EDX) carried with PGT prism
2000 with LN2 cooled Si : Li detector.
Microthermometric analyses of fluid inclusions entrapped in quartz and sphalerite were undertaken on
doubly polished (200–300 μm thick) sections. The Chaixmeca apparatus (Poty et al. 1976) at the CGS, Prague
was calibrated in the range of –100 °C and +400 °C using
Merck chemical standards, the melting point of distilled
water, and phase transitions in natural pure CO2 inclusions.
The homogenization temperature (Th), temperature of
the first melting (Te) and of the final melting of ice (Tm)
were routinely measured (Roedder 1984). The salinity of
91
Tab. 3 Contents of selected elements in ore samples from the investigated localities
Locality
Sample
Quartz
generation
Au
As
Ag
Sb
Hg
Tl
Mo
Cd
Cu
Pb
Zn
Ba
S
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
wt. %
377.1 2 925.7
7 289
24
3.88
El Recio
REC 5
II
1.372
116.9
19.3
9.9
0.04
0.1
107.5
44.3
El Recio
REC P2
I
7.885
40.9
18.8
11.5
0.10
0.1
185
0.2
3.9
6.8
4
15
0.05
Sierra Alta
SAN 4
I
1.466
98.6
1.6
2.2
0.02
0.2
27.9
0.1
35.3
11.0
42
68
0.71
Sierra Alta
SAN 6B
I
7.826 1 225.2
1.6
39.9
2.14
3.2
425.5
1.3
57.2
61.8
57
62
2.28
Sierra Alta
SAP 1
III
65.083
611.8
48.1
54.8
0.29
0.6
194.8
5.9
95.1
497.7
177
282
0.42
Sierra Alta
SAP 2
III
5.869
42.8
4.2
10.6
0.08
0.1
68.4
0.1
8.2
14.9
13
29
0.05
Tres Hermanos TH 5a
I
5.781
104.2
4.6
40.1
0.19
0.5
77.0
0.1
8.9
22.2
16
439
0.39
Tres Hermanos TH 5b
I
61.113
25.1
57.3
9.8
0.75
0.2
83.1
1.2
224.2
41.0
77
293
0.26
Mina Moncada L 2
II
4.900
421.5
20.9
8.8
0.24
<0.1
3.0
Mina Moncada L 4F
II
0.587
73.0
7.6
1.2
0.04
<0.1
<0.1
Buena Suerte
I
0.210
361.4
<0.1
11.7
2.55
0.4
0.5
<0.1
30.6
5.5
Mina Macacona L 17 B
III
3.980
429.4
1.0
19.8
3.78
<0.1
0.1
<0.1
10.7
Mina Macacona L 17 C
III
1.119
325.6
0.2
11.6
3.20
<0.1
0.1
<0.1
Mina Macacona L 17 G
III
5.265
406.0
1.4
35.6
6.30
<0.1
0.1
Mina Chassoul L 18 BA
I
5.660
952.9
2.3
19.6
0.20
<0.1
Mina Chassoul L 18 BB
I
>100.00 2 269.6 >100.0
36.6
0.33
<0.1
L9
the fluids was calculated according to Bodnar and Vityk
(1995) and the salt composition determined following
Borisenko (1977). The reproducibility of the Th values
was ± 3 °C, of Tm values ± 0.2 °C.
The powder XRD data of altered volcanics were
acquired by the Philips X’Pert System difractometer;
secondary monochromator producing CuK α, radiation
40 kV/40 mA, graphite monochromator, angle interval
3–65° 2α, step 0.05 2α and 3 s exposure/step were used.
The XRD record of some samples was made in their
natural state after saturation with ethylene glycol.
5.3.Ore mineralogy
Mineralogical samples from three main ore veins in the
Juntas District were studied in detail: Recio, Tres Hermanos and Sierra Alta – San Martín veins. Three samples
from the much smaller and less important ore district of
Guacimal were also investigated. They were found on the
dump of an abandoned adit of the Guacimal mine (Tab. 4).
Pyrite is, in general, the most abundant sulphide in the
studied samples. The mineral in sample TH 5b exhibits
concentric oscillatory zoning with zones that contain up
to 2.2 wt. % As that are lighter in the BSE image (see
Fig. 7a). Sample SAP 1 contains older pyrite poor in As,
which is overgrown by younger pyrite rich in As (1.8–8.8
92
207.5 6 980.5 >10000 >10000
131.4 2 183.5
459.3 >10000
16 >10.00
5
5.96
66
<0.05
24.8
9 1 781
0.05
8.8
13.9
11 3 499
<0.05
<0.1
26.6
42.0
22 1 632
<0.05
3.6
0.3
31.1
36.9
56
66
<0.05
7.8
1.0
77.0
118.3
113
192
<0.05
1
wt %, Fig. 7b). Pyrite from the Guacimal district is poor
in As, whereas some pyrites have enhanced contents of
Cu (up to 0.25 wt. %). The concentrations of Ni and Co
were mostly below their detection limits (Tab. 5).
Gold (virtually electrum) was identified in two
polished sections of gangue from the Tres Hermanos
(TH 5b) and Recio (Qtz 4b) veins. Gold forms anhedral,
mostly rounded inclusions in As-bearing pyrite but it
is also found in quartz. Gold inclusions do not exceed
25 µm (Fig. 7a); however, in some samples, regardless
of their high Au content, no native gold was found in
polished sections, most likely due to the small size (below 1 µm).
The chemistry of gold was found to vary only slightly,
with major Ag and traces of Cu.
Other elements like Bi, Sb, Ni, Mn and Hg were also
analysed but not detected (Tab. 9). The empirical formulae are Au0.43–0.45Ag0.54–0.57Cu0.00–0.01 (TH5b) and Au0.47–0.51
Ag0.49–0.53 (Qtz4b). The gold from the Juntas ore district
corresponds to electrum with Ag content mostly slightly
exceeding that of Au: Au0.43–0.51Ag0.47–0.57Cu0.00–0.01.
Sphalerite is generally minor or accessory in the
studied samples, but was found to be abundant in
the Guacimal district, whereas it is rare in the Juntas
district. It mostly occurs as anhedral grains and aggregates, but tiny inclusions of sphalerite in pyrite can
Tab. 4 Localization and mineral assemblages of the mineralogical samples
locality
sample
gangue
Racio vein
Qtz 3
Qtz, Cal,
Py, Ccp, Cas
ore minerals
Cas in inclusions in Py
notice
Qtz 4
Qtz, Kfs
Au, Py, Acn, Prg, Pst, Ag–Sb–Au min
bonanza-like mineralization
Tres Hermanos
TH 5b
Qtz
Au, Py, Ccp, Bn
Au in tiny inclusions
Sierra Alta
SAN 6b
Qtz
Py, Bn
SAP 1
Qtz
Py, Sp, Gn, Grn, Ccp
Qtz 7
Qtz
Py, Py, Gn, Sp, Fe-Sp, Mrc
Gu 3
Qtz
Ag-Py, Sp, Gn
Qtz II dominant
Ju100b
Qtz
Py, Sp, Gn, Ccp, Ang
Qtz II dominant
Ju100c
Qtz
Sp, Py, Gn, Ccp
Qtz II dominant
Guacimal
content of Au 65 ppm
Qtz – quartz, Cal – calcite, Kfs – K-feldspar, Py – pyrite, Ccp – chalcopyrite, Gn – galena, Apy – arsenopyrite, Sp – sphalerite, Cas – cassiterite,
Acn – acanthite, Prg – pyrargirite, Pst – proustite, Ang – anglesite, Mrc – marcasite, Grn – greenockite, Bn – bornite
Tab. 5 Chemical compositions of pyrite and marcasite* (wt. % and apfu)
sample
TH 5B
TH 5B
SAP1
SAP1
SAP1
SAP1
Ju100b
Ju100b
Ju100c
Qtz7
Qtz7*
Fe
46.48
45.94
45.46
42.43
44.11
44.15
45.90
45.78
46.22
45.80
45.28
Co
0.00
0.00
0.08
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Ni
0.00
0.02
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Cu
0.00
0.04
0.08
0.25
0.15
0.18
0.00
0.00
0.15
0.00
0.03
Zn
0.01
0.01
0.00
0.01
0.97
0.02
0.00
0.00
0.00
0.02
0.00
Cd
0.04
0.00
0.01
0.05
0.06
0.00
0.04
0.02
0.04
0.04
0.02
As
0.00
2.20
0.16
8.81
0.21
1.75
0.00
0.44
0.00
0.00
0.00
S
53.99
52.01
53.83
47.43
53.75
52.13
53.77
53.32
54.08
53.64
53.47
Se
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
100.51
100.22
99.62
98.97
99.24
98.22
99.73
99.56
100.50
99.50
98.79
Total
Sb
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
Fe
0.992
0.997
0.978
0.965
0.953
0.971
0.986
0.988
0.986
0.986
0.981
Co
0.000
0.000
0.002
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
Ni
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
Cu
0.000
0.001
0.002
0.005
0.003
0.003
0.000
0.000
0.003
0.000
0.001
As
0.000
0.036
0.003
0.149
0.003
0.029
0.000
0.007
0.000
0.000
0.000
S
2.007
1.966
2.017
1.880
2.022
1.997
2.013
2.005
2.010
2.013
2.018
Se
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
Total
3.000
3.000
3.000
3.000
3.000
3.000
3.000
3.000
3.000
3.000
3.000
calculated on the basis of 3 atoms per formula unit
be euhedral or rounded (Fig. 7c). Its chemistry is rather
simple, being poor in both Fe (0.74–2.21 wt. %) and Cd
(0.45–1.04 wt. %), but a single analysis yielded even
3.34 wt. % Cd. The Mn contents range between 0.02 and
0.18 wt. %. Brown sphalerite from the Guacimal mine
was slightly richer in Fe and Cd and poorer in Mn than
the green variety from the same locality. Tiny inclusions
of rare dark Fe-rich and Cd-poor sphalerite were found to
contain 8.42 wt. % Fe and 0.22 wt. % Cd. This sphalerite
grows on pyrite grains in sample Qtz 7. The EMPA of
sphalerite are summarized in Tab. 6.
Galena is, in general, minor to rare or accessory but
was found to be abundant in the Guacimal ore district,
where it forms aggregates a few cm in size. This mineral
in the Juntas district is also common but tiny, occurring
as euhedral to subhedral grains ~200 µm in size. It is
often intimately intergrown with pyrite and sphalerite
(Fig. 7c). Tiny blebs in pyrite are only a few µm across.
Galena from the Juntas district (samples SAP 1, Qtz 7)
was found to contain 0.18–0.85 wt. % Fe and varying
concentrations of Se (0.0–0.84 wt. %). The contents of
Bi, Ag, Sb, Hg and As are mostly below the respective
detection limits. Only one tiny grain (sample Ju100c) of
galena 100 µm in size was found to have higher contents
of Bi (3.37 wt. %), Ag (1.63 wt. %) and Se (1.84 wt. %).
The higher zinc contents (up to 1.18 wt. %) are ascribed
to intergrowths of galena with sphalerite (Tab. 7)
Chalcopyrite is a common accessory mineral that is
less abundant than sphalerite and galena. It occurs as tiny
grains less than 1 mm in size and was found only in sam93
a
b
Py-II
Au
Ccp
Py-II
Au
Mrc
Sp
Py-I
Py-I
Gn
Py
Ccp
Py-II
Sp
Py-I
Py
200 m
100 m
c
d
Apy
Sp
Py
Gn
Py
100 m
100 m
e
Apy
f
Sp
Gn
Grn
Mrc
Mrc
Py
Ang
100 m
g
50 m
94
100 m
Fig. 7 Back-scattered electron (BSE) photomicrographs of ore minerals from the
Juntas district. a – Gold inclusions (Au) in zoned As-bearing pyrite (Py). Lighter
zones in pyrite contain up to 2.2 wt. % As. Small aggregates of anhedral chalcopyrite (Ccp) also occur in the rim of pyrite and elsewhere in the matrix. Sample
Th 5b, Tres Hermanos vein. b – Subhedral pyrite I (Py-I) is overgrown by a thin
irregular rim of As-bearing pyrite II (Py-II). Other minerals are galena (Gn) and
sphalerite (Sp). Sample SAP 1, Sierra Alta. c –Subhedral pyrite (Py) grain with
inclusions of sphalerite (Sp) and galena (Gn) in quartz (black). Sample Qtz 7, Sierra Alta. d – Pyrite (Py) rimmed by skeletal arsenopyrite (Apy). The same sample. e – Cluster of prismatic crystals of probable marcasite (Mrc). Tiny arsenopyrite (Apy) grains occur nearby. The same sample. f – Mode of occurrence of
primary greenockite (Grn) in the sample Sap1. Greenockite forms intimate intergrowths with galena (Gn) and also subhedral grains enclosed in pyrite (Py) partly transformed for marcasite (Mrc). Other minerals on the picture are sphalerite
(Sp) and tiny anglesite (Ang). Sample SAP 1, Sierra Alta. g – Aggregates of unnamed Ag–Sb–Au sulphide (of acanthite affiliation) grown in the quartz. Sample
Qtz 4b, Recio vein. All BSE photomicrographs by R. Škoda.
Tab. 6 Chemical compositions of sphalerite (Sp) and greenockite (Grn) (wt. % and apfu)
mineral
sample
comment
Fe
Co
Cu
Mn
Zn
Cd
As
S
Total
Fe
Co
Cu
Mn
Zn
Cd
As
S
Total
Sp
SAP1
Sp
SAP1
Sp
Qtz7
2.01
0.00
0.07
0.05
63.89
0.94
0.00
32.78
99.73
0.035
0.000
0.001
0.001
0.955
0.008
0.000
0.999
2.000
0.85
0.00
0.00
0.02
64.73
1.04
0.00
32.80
99.43
0.015
0.000
0.000
0.000
0.972
0.009
0.000
1.004
2.000
1.60
0.00
0.04
0.05
61.00
3.34
0.00
33.01
99.03
0.028
0.000
0.001
0.001
0.923
0.029
0.000
1.018
2.000
Sp
Qtz7
Fe-rich
8.42
0.00
0.00
0.10
56.72
0.22
0.00
34.01
99.48
0.145
0.000
0.000
0.002
0.833
0.002
0.000
1.019
2.000
Sp
Ju100b
1.68
0.00
0.00
0.10
64.70
0.72
0.00
33.37
100.56
0.029
0.000
0.000
0.002
0.957
0.006
0.000
1.006
2.000
brown Sp
Ju100b
centre
1.21
0.02
0.09
0.05
63.97
0.74
0.00
33.60
99.67
0.021
0.000
0.001
0.001
0.951
0.006
0.000
1.019
2.000
brown Sp
Ju100b
rim
1.55
0.03
0.02
0.08
63.88
0.84
0.00
33.49
99.89
0.027
0.000
0.000
0.001
0.949
0.007
0.000
1.015
2.000
green Sp
Ju100c
centre
0.74
0.02
0.00
0.18
65.02
0.45
0.00
33.82
100.23
0.013
0.000
0.000
0.003
0.961
0.004
0.000
1.019
2.000
green Sp
Ju100c
rim
0.78
0.02
0.00
0.13
65.10
0.46
0.00
33.92
100.40
0.013
0.000
0.000
0.002
0.960
0.004
0.000
1.020
2.000
Grn
SAP1
Grn
SAP1
2.06
0.00
0.25
0.00
0.99
72.28
0.53
22.74
98.85
0.052
0.000
0.006
0.000
0.021
0.902
0.010
0.995
2.000
0.42
0.00
0.06
0.00
1.08
75.04
0.18
22.47
99.25
0.011
0.000
0.001
0.000
0.024
0.957
0.003
1.004
2.000
Ni, In, Se were also analysed but not detected; calculated on the basis of 2 atoms per formula unit
ples from the Guacimal mine. Its chemistry corresponds
to a pure formula with no detectable admixtures (Tab. 8).
Arsenopyrite is much less abundant than pyrite,
mostly forming skeletal build-ups on pyrite or isolated
subhedral to euhedral grains or aggregates up to 100 µm
in size enclosed in quartz gangue (Fig. 7d). The mineral
was identified in samples from Sierra Alta (Qtz 7) exhibiting slightly higher Cu content (0.18 wt. %, Tab. 8).
Marcasite is rare, identified in samples Qtz 7 and
SAP 1 from Sierra Alta (Alto Bochinche Hill) as clusters of prismatic or irregular crystals max. 0.3 mm long
embedded in quartz, mostly isolated from other sulphides
(Fig. 7e). Textures found in sample SAP1 indicate that
arsenopyrite sporadically replaced marcasite (Fig. 7f). Its
chemistry is simple (Tab. 5).
Acanthite is rare but was found in sample Qtz 4b as
relatively abundant anhedral aggregates 10–30 µm in
size, closely associated with pyrite. Its chemistry is close
to the ideal formula (Ag1.84Fe0.06As0.01)(S1.04Se0.05) with
slightly enhanced contents of Fe (1.31 wt. %), As (0.39
wt. %) and Se (1.57 wt. %) (Tab. 7).
Pyrargyrite (Ag3SbS3) or its monoclinic polymorph
pyrostilpnite is a very rare mineral in the studied samples,
identified only in sample Qtz 4b as anhedral aggregates
30–50 µm in size associated with acanthite and Ag–
Sb–Au sulphide. It exhibits higher contents of As (0.41
wt. %) and Se (0.52 wt. %, Tab. 7). Its formula corresponds to Ag3.00Sb0.99(S2.95Se0.04).
Covellite is again a very rare mineral that was actually identified only in sample Ju100b from the Guacimal
mine. It occurs as a single grain 50 µm across embedded
in quartz. It exhibits a higher content of Ag (0.93 wt. %)
and its formula, based on EMPA, is Cu 1.02Ag 0.08S 0.97
(Tab. 8).
Bornite (possible) was found as an isolated single
grain in sample SAN 6b (Recio vein) at a contact of
banded wall rock consisting of fine-grained silicified
mylonite with quartz gangue containing randomly disseminated tiny blebs of pyrite. The grain of supposed
bornite is anhedral to subhedral and 50 µm in size, with
an inclusion of pyrite (10 µm) in its centre.
Greenockite (or cubic polymorph of CdS hawleite)
is rare, being associated with other sulphides in sample
SAP1 from Sierra Alta. The mineral occurs in graphic
intergrowths with galena, forming an aggregate 70 × 30
µm in size, or occurs as subhedral grains enclosed in
pyrite–marcasite aggregate. The structural criteria clearly
indicate that greenockite is a primary ore mineral. Other
minerals associated with greenockite include sphalerite
and anglesite (Tab. 6, Fig. 7f).
Cassiterite was found in sample Qtz 3 from the Recio
vein as a single anhedral inclusion 15 × 8 µm in size,
identified using the EDX mode.
Ag–Sb–Au sulphide, most likely a new mineral species, was found in the Recio vein (sample Qtz4) as a
few anhedral corroded aggregates up to 70 × 25 µm in
95
Tab. 7 Chemical compositions of galena (Gn), acanthite (Acn) and pyrargyrite (wt. % and apfu)
mineral
sample
Pb
Bi
Ag
Sb
Fe
Zn
Cd
Hg
As
S
Se
Total
Pb
Bi
Ag
Sb
Fe
Zn
Cd
Hg
As
S
Se
Total
Gn
SAP1
86.69
0.00
0.00
0.00
0.18
0.03
0.01
0.00
0.00
13.51
0.00
100.42
0.992
0.000
0.000
0.000
0.008
0.001
0.000
0.000
0.000
0.999
0.000
2.000
Gn
SAP1
86.11
0.00
0.00
0.00
0.34
0.50
0.05
0.00
0.20
13.56
0.00
100.76
0.970
0.000
0.000
0.000
0.014
0.018
0.001
0.000
0.006
0.988
0.000
2.000
Gn
SAP1
85.65
0.00
0.00
0.00
0.66
0.00
0.00
0.00
0.00
13.44
0.69
100.44
0.969
0.000
0.000
0.000
0.028
0.000
0.000
0.000
0.000
0.983
0.020
2.000
Gn
SAP1
85.57
0.00
0.00
0.00
0.85
0.00
0.00
0.00
0.00
13.21
0.84
100.46
0.971
0.000
0.000
0.000
0.036
0.000
0.000
0.000
0.000
0.968
0.025
2.000
Gn
Ju100c
78.70
3.37
1.63
0.00
0.00
1.18
0.00
0.00
0.00
12.93
1.84
99.65
0.888
0.038
0.035
0.000
0.000
0.042
0.000
0.000
0.000
0.943
0.054
2.000
Gn
Qtz7
85.97
0.00
0.00
0.00
0.27
0.38
0.00
0.00
0.00
13.49
0.00
100.12
0.980
0.000
0.000
0.000
0.012
0.014
0.000
0.000
0.000
0.994
0.000
2.000
Acn
Qtz4b
0.00
0.00
82.14
0.00
1.31
0.00
0.00
0.00
0.39
13.84
1.57
99.24
0.000
0.000
1.839
0.000
0.057
0.000
0.000
0.000
0.012
1.043
0.048
2.999
*pyrargyrite
Qtz4b
0.00
0.00
58.68
21.81
0.00
0.03
0.00
0.00
0.42
17.11
0.61
98.65
0.000
0.000
2.997
0.987
0.000
0.003
0.000
0.000
0.031
2.941
0.042
7.000
*or pyrostilpnite
Tab. 8 Chemical composition of chalcopyrite (Ccp), covellite (Cov) and arsenopyrite (Apy) (wt. % and apfu)
mineral
sample
Pb
Ag
Fe
Co
Ni
Cu
Zn
Cd
As
S
Total
Pb
Ag
Fe
Co
Ni
Cu
Zn
Cd
As
S
Total
Ccp
TH5B
0.00
0.00
28.97
0.00
0.00
36.05
0.02
0.00
0.00
35.20
100.24
0.000
0.000
0.950
0.000
0.000
1.039
0.000
0.000
0.000
2.010
4.000
Ccp
Ju100b
0.07
0.00
30.17
0.00
0.03
34.27
0.32
0.00
0.00
34.96
99.83
0.001
0.000
0.993
0.000
0.001
0.992
0.009
0.000
0.000
2.005
4.000
Ccp
Ju100c
0.00
0.00
30.15
0.00
0.00
34.41
0.03
0.06
0.00
34.86
99.52
0.000
0.000
0.995
0.000
0.000
0.998
0.001
0.001
0.000
2.004
4.000
Cov
Ju100b
0.00
0.93
0.00
0.00
0.00
66.18
0.04
0.00
0.00
31.84
98.98
0.000
0.008
0.000
0.000
0.000
1.019
0.001
0.000
0.000
0.972
2.000
Apy
Qtz7
n.a.
0.00
34.94
0.00
0.00
0.10
0.01
0.03
40.52
23.90
99.49
–
0.000
0.981
0.000
0.000
0.002
0.000
0.000
0.848
1.169
3.000
Bi, Sb, Mn and Se were also analysed but not detected
Numbers of atoms calculated on the basis of a total of 4 for chalcopyrite, 2 for covellite and 3 for arsenopyrite
96
Apy
Qtz7
n.a.
0.00
33.27
0.02
0.02
0.18
0.01
0.02
43.87
21.91
99.28
–
0.000
0.956
0.000
0.000
0.005
0.000
0.000
0.940
1.097
3.000
Apy
Qtz7
n.a.
0.00
33.70
0.00
0.00
0.08
0.00
0.02
44.57
22.04
100.42
–
0.000
0.959
0.000
0.000
0.002
0.000
0.000
0.946
1.093
3.000
Tab. 9 Chemical compositions of gold (electrum) (wt. % and apfu)
TH 5B
TH 5B
TH 5B
TH 5B
TH 5B
Qtz4b
Qtz4b
Qtz4b
Ag
sample
41.32
42.15
39.30
40.04
39.61
34.44
37.62
37.86
Au
59.61
57.87
60.16
59.59
60.27
66.00
61.14
61.48
Cu
0.38
0.20
0.00
0.05
0.00
0.00
0.00
0.00
101.31
100.22
99.47
99.67
99.88
100.45
98.75
99.34
Total
Ag
0.554
0.568
0.544
0.550
0.545
0.488
0.529
0.529
Au
0.438
0.427
0.456
0.449
0.455
0.512
0.471
0.471
Cu
0.009
0.005
0.000
0.001
0.000
0.000
0.000
0.000
Total
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
Bi, Sb, Ni, Mn, Hg were also analysed but not detected; calculated on the basis of 1 atom per formula unit
size, enclosed in quartz (Fig. 7g). The mineral appears
to be homogeneous and the microprobe analyses yielded
72.52–73.33 wt. % Ag, 3.46–3.64 wt. % Au, 6.22–8.05
wt. % Sb, 1.10–2.13 wt. % As, 12.76–13.46 wt. % S and
1.69–1.79 wt. % Se. The Ag/Au ratios in the empirical
formula range between 39 and 40. Alternative formulae
derived from microprobe analyses correspond to
Tab. 10 Chemical composition of Ag–Sb–Au–S phase from the sample
Qtz4b (wt. % and apfu).
Ag
73.33
72.84
72.52
Au
3.46
3.65
3.64
Sb
6.22
7.08
8.05
Ni
0.00
0.02
0.02
As
2.13
1.69
1.10
Ag38.9–40.0Au1.03–1.07Sb2.99–3.83As0.85–1.67S23.15–23.95Se1.25–1.31
(total 70 atoms) or
S
12.76
13.46
13.24
1.69
1.72
1.79
Total
99.59
100.45
100.34
Ag1.67–1.71Au0.04–0.05Sb0.13–0.16As0.04–0.07S1.00–1.04Se0.05–0.06
(total 3 atoms),
Ag
39.781
which are close to the formula of acanthite, Ag2S (Tab. 10).
The most likely formula is: (Ag39Au1)40(SbAs)5(S24Se1)25
or Ag1.7Au0.05Sb0.15As0.05S1.0Se0.05.
Anglesite is rare and was only found in samples from
Sierra Alta (SAP 1) and Guacimal (Ju100b), forming subhedral to euhedral grains max. 60 µm across, associated
with pyrite and galena. It seems to form pseudomorphs
after galena.
Barite is rare and was found as inclusions up to
100 µm in sample SAP 1 (Sierra Alta) and 10 µm in
TH 5b (Tres Hermanos vein).
5.4.Fluid inclusion study
Fluid inclusions of the H2O type were found in various generations of quartz at several gold occurrences,
Moncada base-metal deposit and also in sphalerite from
the Guacimal mine. Fluid inclusions with a gaseous
phase, such as CO2 or CH4, were not found. The fluid
inclusions show different patterns in individual generations of quartz and also in sphalerite, but are more or less
uniform at any given locality.
Quartz I is found in numerous crystal forms and habits.
It is colloform or comb-like or occurs as small euhedral
crystals rimming pieces of rock (breccia-type texture) and
passes into coarse-grained quartz, in which the growth
Se
38.860
38.981
1.705
1.665
1.671
Au
1.028
1.066
1.070
0.044
0.046
0.046
Sb
2.988
3.347
3.833
0.128
0.143
0.164
Ni
0.000
0.022
0.018
0.000
0.001
0.001
As
1.666
1.301
0.848
0.071
0.056
0.036
S
23.286
24.153
23.936
0.998
1.035
1.026
Se
Total
1.251
1.252
1.314
0.054
0.054
0.056
70.000
70.000
70.000
3.000
3.000
3.000
Pb, Bi, Fe, Co, Mn, Zn, Hg were also analysed but not detected
numbers of atoms calculated assuming 70 and 3 atoms per formula unit
zones are filled with dark inclusions forming chevron-like
textures (Fig. 8c–d). Drusy quartz can also be observed
in the investigated samples. Dark-coloured “empty voids”
were often seen in the intergranular space of the quartz
crystals (Fig. 9a). Inclusions in quartz I are of very irregular shape, less than 20 µm in diameter, and exhibit
very variable liquid/vapour ratios (LVR). Varying LVR are
believed to reflect a long maturation of fluid inclusion at
relatively lower temperatures and nucleation of the vapour
phase (Bodnar et al. 1985) rather than be due to the boiling of the fluid. Measurable primary and pseudosecondary inclusions occur mostly in the apical parts of small
euhedral quartz crystals. These inclusions are of oval to
irregular shape, from 5 to 40 µm in diameter, and show
variable LVR ranging from 0.5 to 0.95. Liquid-only or
vapour-only inclusions were also observed in some of the
studied samples. Due to the variable LVR homogenization
temperatures (Th), the inclusions in clusters with LVR of
97
a
1 cm
b
c
1 cm
d
1 cm
1 cm
Fig. 8 Scans of selected doubly polished sections used for fluid inclusion study. a – Breccia-type structure of quartz I, sample TH 5b; b – Colloform and fine-grained quartz I which overgrows drusy coarse-grained chevron quartz II, sample REC 6; c – Fine-grained quartz I on rock pieces overgrowths to chevron quartz, sample BPA 2; d Prismatic quartz II which is coated by later fine-grained crystals of quartz III, sample BV 1.
Photos by Petr Dobeš.
0.9 were measured, and the values of Th fluctuated from
156 to 248 °C (Tab. 11, Figs 10, 12). The salinity of an
aqueous solution varied from 0.2 to 2.9 wt. % NaCl equiv.
(Figs 11–12). NaCl is assumed to be the major compound
of aqueous solutions (Te = –22.1 °C).
Coarse-grained zoned prismatic quartz II (Figs. 8d)
from gold deposits contains several types of fluid inclusions. Some quartz crystals enclose primary fibrous
inclusions of very dark colour up to a few hundreds of
µm long. It was difficult to obtain any reasonable fluid
inclusion parameters of these. The growth zones are
also characterized by primary and pseudosecondary H2O
inclusions. The inclusions are of negative crystal to oval
shape, from 5 to 80 µm in diameter, and mostly with
consistent LVR varying from 0.7 to 0.8. Rare accidental
solids can be found in these inclusions. The temperatures
of homogenization (Th) range between 182 and 288 °C,
and the salinity of an aqueous solution is very low, not
exceeding 2.1 wt. % NaCl equiv. The eutectic temperature (Te = –23.4 to –31.0 °C) indicates an H 2O–NaCl
98
type of solutions with possible small admixture of K ±
Fe ± Mg chlorides.
Fine-grained euhedral crystals of quartz III rim the
coarse-grained quartz II (Fig. 8d). The primary and
pseudosecondary inclusions of H2O type are of oval to
irregular shape, from 5 to 50 µm in diameter, and show
variable LVR (0.5–0.95). Liquid-only or vapour-only
inclusions were also found in the investigated samples.
Due to the variable LVR homogenization temperatures
(Th) the inclusions in clusters with LVR = 0.80 to 0.95
were measured. The obtained Th values range from 146
to 248 °C, and the salinity of an aqueous solution from
0.2 to 2.7 wt. % NaCl equiv.
Round to irregular grains of sphalerite from the Guacimal gold deposit are enclosed in quartz I. Secondary H2O
inclusions (Fig. 9f) with consistent LVR (0.7–0.8) were
observed along healed microfractures in sphalerite. The
Th values of these inclusions range from 214 to 282 °C.
The salinity of an aqueous solution is low (0.7–3.1 wt. %
NaCl equiv.), but slightly higher than that in inclusions
a
b
c
d
e
f
Fig. 9 Photographs of fluid inclusions in various quartz types and sphalerite. a – Dark-coloured “empty voids” in the intergranular spaces, sample REC 6; b – Primary H2O inclusions with variable LVR in quartz I, sample REC 2; c – Growth zones decorated by primary fluid inclusions vs.
micro cracks and trails of secondary fluid inclusions in quartz II, sample BV 1; d – Primary H2O inclusions with consistent LVR in quartz II, sample BV 1; e – Primary H2O inclusions with variable LVR along growth zones in quartz III, sample BV 1; f – Secondary vapour-rich H2O inclusions in sphalerite, sample Gu 7. Photos by Petr Dobeš.
99
Tab. 11 Fluid inclusion data from various quartz types and sphalerite from the Au-bearing and base-metal veins of the Aguacate Group volcanic arc
Locality
Sample
Generation of quartz
FIA
Th (°C)
Tm (°C)
Salinity
Te (°C)
(wt. % NaCl eq.)
Sierra Alta
Tres Hermanos
Veta Vargas
REC 1
Quartz II
primary
212–268
–0.2 to –0.9
0.4–1.6
REC 2
Quartz I
primary
174–246
–0.1 to –0.9
0.2–1.6
REC 5
Quartz I
primary
156–172
–0.1 to –0.5
0.2–0.9
REC 6
Quartz I
pseudosecondary
172–202
–0.1 to –0.7
0.2–1.2
REC P-2
Quartz I
primary
188–210
–0.6 to –1.7
1.1–2.9
SAP 1
Quartz I
pseudosecondary
174–224
–0.1 to –1.4
0.2–2.4
TH 5
Quartz I
pseudosecondary
219–232
–0.2 to –0.7
0.4–1.2
TH 5b
Quartz I
pseudosecondary
182–240
–0.4 to –0.6
0.7–1.1
BV 1
BV 1B
Quartz II (root)
primary
218–250
–0.4 to –0.6
0.7–1.1
Quartz II (central part)
primary
218–247
–0.1 to –0.6
0.2–1.1
Quartz II (peripheral p.)
primary
182–275
–0.2 to –0.4
0.4–0.7
Quartz III
primary
192–248
–0.1 to –1.6
0.2–2.7
Quartz III
pseudosecondary
146–178
–0.1 to –0.4
0.2–0.7
Quartz II
primary
236–274
–0.1 to –0.7
0.2–1.2
Quartz II
pseudosecondary
236–276
–0.1 to –0.3
0.2–0.5
BV 1C
Quartz II
primary
254–288
–0.1 to –0.8
0.2–1.4
BV 4
Quartz I
primary
192–248
–0.3 to –0.5
0.3–0.9
Veta Pozo
BPA 2
Quartz II
primary
246–276
–0.1 to –1.2
0.2–2.1
Guacimal
Gu 7
Sphalerite
secondary
214–262
–0.4 to –1.8
0.7–3.1
Gu PL-1
Sphalerite
secondary
262–282
–0.6 to –1.6
1.1–2.7
Juntas
Moncada
–22.1
–23.4
–22.7
–31.0
–24.2
TM 40A
Quartz II
pseudosecondary
232–274
–0.2 to –0.6
0.4–1.1
TM 40B
Quartz II
primary
245–278
–0.2 to –1.0
0.4–1.7
1
Quartz II
primary
248–262
–0.3 to –0.6
0.5–1.1
–29.4
4B
Quartz II
primary
264–278
–1.4 to –2.6
2.4–4.3
–25.5
trapped in quartz. The homogenization temperatures of
secondary inclusions correspond to those of the primary
inclusions measured in quartz II.
Coarse-grained prismatic quartz and quartz aggregates
from the Moncada base-metal mine were also found to
contain H2O-type inclusions. Primary H2O inclusions occur
in the growth zones of quartz. They are of oval shape, up
to 80 µm in diameter, and exhibit persistent LVR around
0.7. The Th values range between 248 and 278 °C, and the
salinity of an aqueous solution is slightly higher, ranging
from 0.5 to 4.3 wt. % NaCl equiv. The eutectic temperatures (Te = –25.5 to –29.4 °C) indicate H2O–NaCl type of
solution, with a small admixture of K ± Fe ± Mg chlorides.
5.5.Hydrothermal alteration
All the studied epithermal gold mineralizations are accompanied by a variety of intense and characteristic hypogene
alterations (Laguna 1983, 1984), which affect the wall
100
–24.5
rocks to a distance of a few metres from the ore veins
(Tres Hermanos c. 5–6 m, Sierra Alta as much as 10 m).
Fragments of strongly altered volcanic rocks are often an
integral and common part of quartz veins, due to multistage syn-mineralization brecciation and also build-up of
quartz veins through hydraulic fracturing of volcanites.
Besides, effects of regional-scale alterations with no direct
relation to the ore mineralization, are also observed.
The main alteration processes included:
1)Silicification, carbonatization – resulting in the formation of quartz veinlets and small stockworks sometimes
without any mineralization. These are processes accompanying the formation of gold-bearing quartz veins,
and represent a useful tool in mineral exploration. The
quartz usually occurs in several generations, of which
the older are of gray-black colour, whereas the younger
are light-coloured to white. Drusy quartz can be found
in the central parts of the ore veins. Carbonates (calcite, less abundant dolomite and rhodochrosite) are also
10
300
Quartz II-Moncada
5
250
10
Th (°C)
0
Sphalerite
5
0
10
200
150
Quartz III
Quartz I
0
0
15
10
5
0
20
Frequency
Sphalerite
Quartz II-Moncada
1
2
3
4
5
Fig. 12 Salinity vs. homogenization temperatures of fluid inclusions in
various types of quartz and sphalerite of the Au-bearing and base-metal
veins of the Aguacate Group volcanic arc.
Quartz II
20
Quartz I
15
10
5
0
140
160
180
200
220
240
260
280
300
Temperature of homogenization (°C)
Fig. 10 Histogram of homogenization temperatures for fluid inclusions
in various types of quartz and sphalerite of the Au-bearing and base-metal veins of the Aguacate Group volcanic arc.
80
Quartz I
Quartz II
Quartz III
Sphalerite
Quartz II-Moncada
60
Frequency
Quartz III
Salinity (wt. % NaCl equiv.)
25
40
20
0
1
2
3
4
5
Salinity (wt. % NaCl equiv.)
Fig. 11 Histogram of aqueous solution salinities for fluid inclusions in
various types of quartz and sphalerite.
common alteration zones on a regional scale. Carbonates form abundant tiny veinlets of millimetre dimensions, little stockworks, but mostly occur as several-centimetres-thick schlieren in the andesites of the Agua
Quartz II
100
5
cate Group. Although this type of alteration is not linked to gold mineralization, it is still an important prospecting tool to distinguish, on a regional scale, andesites of the Aguacate Group from those of the Monteverde Formation, the latter of which are barren.
2)Pyritization – always accompanies gold mineralization, in which pyrite occurs in quartz veins. However,
massive pyrite ores or disseminated pyrite crystals in
volcanic rocks also form separate extensive alteration
zones, which show no relationship to gold mineralization, so that this kind of alteration has no straight forward use in prospecting for gold.
3)Propylitization – pervasive alteration characterized by
the formation of chlorite, epidote, calcite, sericite, kaolinite, illite, beidellite and pyrite. It is the most common type of alteration affecting volcanic rocks, giving
them characteristic greenish to ochre colour. The most
intense propylitization affected host volcanites up to
a few metres away from the quartz veins with gold
mineralization. The XRD analyses of propylitized
samples from the historical stopes of the Recio, Tres
Hermanos, Guaria–Guacimal and Sierra Alta mineral
deposits (see Tab. 12) revealed that the original andesites were altered into mixtures consisting of quartz,
kaolinite, illite/smectite, pyrite and epidote, where
also dickite, anatase, gypsum, jarosite and diaspore
were identified. Relics of plagioclase, K-feldspar and
muscovite are preserved. Although this type of alteration accompanies the gold mineralization, it can also
be observed in areas lacking it. Similar to pyritization,
propylitization cannot be considered as a direct prospecting criterion in search for gold deposits.
4)Argillization – an alteration producing kaolinite, illite,
beidellite, alunite and pyrophyllite was found in only
one sample from the Bellavista deposit.
5)Sericitization of feldspars in volcanic rocks combined with silicification is a common alteration accompanying mineralizing processes.
101
A distinct metasomatic aureole can be seen around the
Guacimal Pluton, but these alterations are not spatially
related to metallic mineral assemblages.
Monteverde Formation show no signs of hydrothermal
alteration or ore mineralization, contact metamorphism or
structural patterns similar to those observed in volcanites
of the Aguacate Group.
The gold mineralization is linked only to young N–S
5.6.Genesis of the gold mineralization
to NE–SW tectonic movements, which agree with the
final stage of Neogene tectonic development of the magThe gold mineralization of the SADO type (Mosier and
matic arc prior to relatively fast uplift of the area durSato 1986) currently studied in the Cordillera de Tilarán,
ing the Quaternary (Marshall et al. 2000). Brittle shear
Costa Rica, is linked with volcanic rocks of the older,
failures originated during this stage of tectonic developnow extinct Central American volcanic arc. The gold
ment, giving rise to major faults and conjugated fracture
deposits investigated are located in a NW–SE-oriented
systems of NNW–SSE to NW–SE strikes associated with
zone at the NW foothills of the Cordillera de Tilarán.
abundant N–S to ENE–SSW to NE–SW running systems
The mineralization is confined to Miocene–Pliocene
of faults and fractures. This tectonic system can be linked
andesites of the Aguacate Group intruded by the mainly
with extensive ENE–WSW to E–W-oriented transtenmonzogranitic–granodioritic Guacimal Pluton. New LA
sional tectonics (Grygar in Kycl et al. 2010).
ICP-MS dating of zircons from two granite samples
The gangue and ore textures are often comb-like,
yielded U–Pb ages of 6.3 ± 0.5 and 6.0 ± 0.4 Ma (Žáček
banded or crustiform, whereas colloform textures are
et al. this volume). This broadly agrees with the prerare. Many features, such as brecciation or fracture fillexisting K–Ar ages on a monzonite (biotite: 3.9 ± 1.0 Ma,
ings, suggest that multistage ore-formation processes
alkali feldspar: 5.0 ± 0.2 Ma; Schulz et al. 1987) and a
took place. The mineralogy and geochemistry of the ore
quartz diorite (whole rock: 7.2 ± 1.4 Ma; Alvarado et al.
deposits document the presence of at least three hydro1992). The exposed part of the pluton covers an area of
thermal pulses defined by distinct generations of quartz
c. 70 km2 but, judging from the results of a gravimetric
and also by separation of gold mineralization from the
survey carried out by USGS et al. (1987), the hidden part
base-metal mineral assemblage.
of the pluton is elongated in the NW–SE direction and
According to the fluid inclusions study, gold and base~100 km long. Based on the hornblende igneous barommetal mineralization events took place at temperatures
etry (Žáček et al. this volume), the pluton intruded the
ranging from 150 to 290 °C from H2O–NaCl fluids of
Aguacate Group at shallow depths of ~3 km.
low salinities (0.2 and 4.3 wt. % NaCl equiv.). These data
The gold mineralization as well as the whole Aguacate
confirmed the epithermal nature of the studied mineralGroup is discordantly covered by an Early Pleistocene
ization. Assuming that the trapping temperatures of inMonteverde andesite lava sheet up to 500 m thick, whose
clusions were close to the homogenization temperatures,
age was established at 1.1–2.1 Ma (K–Ar method, mostly
then the depth can be estimated from the course of the
whole rock, e.g. Alvarado et al. 1992). Lavas of the
two-phase equilibrium curves. In this
case, the depth of mineral precipitaTab. 12 Qualitative XRD analysis of altered volcanics adjacent to the quartz veins of selected
tion is thought to have varied between
gold deposits
500 and 1200 m below the surface (see
Haas 1971).
Sierra Alta, Nivel 8 Tres Hermanos El Recio
Guaría – Guacimal
As follows from the above observations, the gold-bearing quartz origimajor minerals quartz
quartz
quartz
quartz
nated as a product of shallow hydroplagioclase
kaolinite
mica/smectite mica/smectite
thermal circulation of meteoric waters
K-feldspar
chlorite
whose motion was most probably trigillite/smectite
dickite
gered by the thermal gradient around
K-feldspar
Guacimal Pluton granitic intrusion.
minor minerals albite
pyrite
alunite
jarosite
Mineralizations occurred, including
chlorite
anatase
anatase
diaspore
large regional alterations, in the Late
K-feldspar
hematite
Pliocene–Early Pleistocene in an intrace minerals
pyrite
gypsum
pyrite
pyrite
terval between c. 6.0 ± 0.4 Ma (U–Pb
gypsum
anatase
age of the Guacimal Pluton, Žáček et
anatase
goethite
al. this volume) and the 2.1 Ma (the
markasite
Fe-dolomite
oldest of the K–Ar whole-rock ages
micas
from the barren Monteverde Formation,
Alvarado et al. 1992).
102
6.Conclusions
Exploration geochemistry (chiefly heavy mineral and
stream sediment surveys) was carried out in the map
sheets of Juntas, Chapernal and Miramar, Costa Rica, on
a scale of 1 : 50 000. The study was intended to identify
and outline new areas promising for the occurrence of
epithermal gold mineralization; samples from historical
mining districts were also studied in order to asses its
likely genesis. The field-work (2006–2009) was focused
predominantly on the area formed by the volcanic rocks
of the Aguacate Group. The results can be summarized
as follows:
• Altogether 237 heavy mineral concentrates, 297 stream sediments and 98 lithogeochemical samples were
collected over an area of c. 950 km2.
• The gold mineralization is confined exclusively to the
Pliocene Aguacate Group and is always accompanied
by intense hydrothermal alteration.
• A positive correlation in heavy mineral concentrates
was found between the presence of gold and arsenopyrite, cerussite, cinnabar and base-metal sulphides.
• A positive correlation was also found in stream sediments and the below-screen fraction of heavy mineral
concentrates between Au + Ag on the one hand, and
As, Sb, Hg, Pb, Tl, Mo with W on the other.
• The shapes of gold chips in sediments (sharp plates,
wires and dendritic forms) indicate that the primary
source for most of the defined anomalies is very close
to the sampling sites. The gold occurs in the form of
fine particles mostly several tenths of µm across, occasionally attaining a size of 1–2 mm.
• On three map sheets, 14 areas promising for the occurrence of gold mineralization were defined. Of them,
four were suggested for follow-up exploration.
• Recent mineralogical investigation identified the following ore minerals: gold (electrum, 30–42 wt. % Ag),
pyrite, sphalerite, galena, chalcopyrite, arsenopyrite,
marcasite, acanthite, freibergite, pyrargyrite, greenockite, covellite, cassiterite, cerussite and an unidentified
Ag–Sb–Au sulphide. Gold in primary ores is always
very fine-grained (< 1–25 µm) and mostly enclosed in
pyrite and arsenopyrite.
• Fluid inclusions study showed that Au-mineralization originated during a multi-stage hydrothermal process at 150 to 290 °C from H2O–NaCl fluids with very
low salinity (0.2–4.3 wt. % NaCl equiv). The epithermal nature of these veins was ascertained. The depth
of the mineral precipitation is estimated to have varied between 500 and 1 200 m below the palaeosurface.
• Gold mineralization is interpreted as being the product of shallow hydrothermal circulation of dominantly meteoric waters, whose motion was triggered
by the thermal gradient around the Guacimal Pluton.
At least three pulses of ascending ore fluids gave rise
to quartz veins with rich gold and base-metal mineralization, which are confined to brittle structures of
N–S to ENE–WSW strikes. The age of mineralization
and intense regional hydrothermal alteration falls in
the Late Pliocene–Early Pleistocene (c. 6.0–2.1 Ma).
Acknowledgements. This study was made possible thanks
to the Program of Development and Cooperation (project
No. RP/6/2006) between the Czech Republic and Costa
Rica, specifically between the Ministry of Environment
of the Czech Republic and the Department of Geology
and Mines (DGM) of MINAET, Costa Rica, carried out
during 2006–2009. Petr Hradecký and Petr Kycl were
in charge of the project and the authors are obliged to
them and the whole team for their assistance in the field.
We are very grateful to Mr. José Francisco Castro Muñoz, Ms. Marlene Salazar Alvarado and Ms. Sofia Huapaya for their support, cooperation and friendship. We
are also indebted to Radek Škoda of Masaryk University Brno for carrying out microprobe analyses, František
Laufek and Irena Haladová for XRD analyses, Jan Malec for photomicrographs of gold particles and Zdeněk
Táborský (all CGS) for valuable suggestions regarding
ore microscopy. The reviewers, Jiří Zachariáš and Peter
Koděra, as well as handling editor Miroslav Štemprok,
provided helpful comments on the manuscript that improved its quality.
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Epithermal gold mineralization in Costa Rica, Cordillera de Tilarán

Transcription

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